INTESTINAL ABSORPTION OF PROTEIN HYDROLYSIS PRODUCTS: A REVIEW’

K. E. Webb, Jr.2*3

Virginia Polytechnic Institute and State University, Blacksburg 24061

ABSTRACT

Many experimental techniques have allowed researchers to probe the fate of hydrolysis products from proteins in the small intestine. An overview of amino acid and peptide absorption from the small intestine is presented with attention given to historical perspectives that have led to current concepts. Speculation about nutritional significance of these processes is offered. Species differences exist in site of amino acid absorption. Numerous mechanisms are available for the transport of amino acids, including Na+- dependent carriers (energy-requiring), Natindependent carriers and diffusion. The relative contribution each transport system makes to absorption is dependent on substrate concentration. Individual amino acids are not absorbed with equal efficiency; methionine usually is absorbed in the greatest proportion. There are interactions among amino acids for transport by specific transport systems. Small peptides (mostly di- and tripeptides) are absorbed from the small intestine more rapidly than are free amino acids, peptides are transported by systems independent of those responsible for transporting free amino acids. Evidence exists that the active transport of these peptides is via a proton gradient. Although the concept that peptides are absorbed intact into the circulation is not universally accepted, evidence supporting the possibility of tissue utilization of these small peptides is accumulating. Nutritional implications imposed by peptide absorption remain largely unexplored. (Key Words: Amino Acids, Peptides, Small Intestine, Absorption, Transport, Carriers.)

J. Anim. Sci. 1990. 68:3011-3022

Introduction

Digestion of proteins to free amino acids in the intestinal lumen was recognized as early as 1865 by Kollinker and Muller, as cited by Matthews (1975b). The discovery of trypsin and erepsin (known now as different intestinal peptidases) in the early 1900s made it clear that proteins were at least partially hydrolyzed

lPresented at a sympsium titled “Regulation of Diges- tive Function in Domestic Livestock Intestinal Absorption and Exocrine Secretions” at the Annu. Mtg., Southern Sec- ti0 Am Soc. of Anim. Sci., Nashville, TN.

3The author’s interests in and knowledge about the areas reviewed in this paper have been stimulated over the past 20 years largely by the research efforts of his students and post- doctorals. Sincere appreciation is expressed to all his stu- dents, his postdoctorals, his technicians and his secretary.

%mi. Anim. ki.

Received June 14, 1989. Accepted January 9,1990.

to amino acids (Matthews, 1975b). Even though this information was available, many early scientists believed that proteins were absorbed primarily as intact molecules (Van Slyke, 1917). Van Slyke (1917) also suggested that the inability of early analytical procedures to detect amino acids in the hepatic portal circulation provided support for ideas such as complete destruction of amino acids in the intestinal wall or synthesis of tissue proteins by the intestine.

As we have progressed from these early thoughts, much information has accumulated and has necessitated a continual reshaping of our description of protein metabolism in domestic animals. New concepts concerning digestive and absorptive processes and their impact on nutrient supply to the animal are emerging. The concept that significant quanti- ties of amino acids reach the circulatory system as intact di- and tripeptides remains the

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most controversial. The materials presented in this review provide an overview of the current concepts of amino acid and peptide absorption. Attention is given to historical development of these concepts. Numerous reviews have been published on these topic areas; readers are directed to these for adt ltional information (Adibi, 1975, 1976; Matthews, 1975a,b; Adibi and Kim, 1981; Munck, 1981; Webb, 1986; Alpers, 1987; Hopfer, 1987).

Amino Acid Absorption

Techniques of Measuring Amino Acid Ab- sorption. Thiry-Vella loops, which date to the 1800s, were used to study enzymatic activity (Johnston, 1932) and absorption (Raudin et al., 1933; Newman and Taylor, 1958). Problems with maintenance of a functional epithelium in the absence of a constant exposure of the mucosa to chyme led researchers to develop other cannulation procedures such as reentry cannulas (Phillipson, 1952; Harrison and Hill, 1962; Goodall and Kay, 1965) and double reentry cannulas (Homey et al., 1972, 1973; Phillips et al., 1978). Also, widespread use of cannulas installed in a variety of locations along the gastrointestinal tract, designed sim- ply to sample digesta and intended to be used in conjunction with inert digesta markers, came into use and continue to be widely used.

In situ procedures have been utilized in which absorption has been studied using ligated intestinal segments of the rat (Gibson and Wiseman, 1951) and sheep (Williams, 1969). Perfusion of the intestine of rats (Jacobs and Lang, 1965) or intestinal segments of sheep (Phillips et al., 1979) also has provided useful information. Tubes with double or triple lumens that are passed through the nasal cavity, down the esophagus, through the stomach and into the small intestine have been especially useful in measuring amino acid absorption in the intact intestine of the human (Cummins, 1952; FleshIer et al., 1966; Adibi and Gray, 1967).

A major role, as we have progressed in our understanding of amino acid absorption, has been played by in vitro procedures. Perfusion of the isolated intestine also has proved to be a useful technique (Fisher and Parsons, 1949). Wilson and Wiseman (1954) were credited with the development of the everted sac technique. By using sacs formed from everted intestine, researchers were able to measure

simultaneously amino acid disappearance from mucosal fluids, appearance in serosal fluids and accumulation in tissue. Sheep small intestine has been studied using this procedure (Phillips et al., 1976). During the same time period that the everted sac was used, the usefulness of intestinal rings in measuring amino acid absorption became known (Agar et al., 1954). Studies of amino acid absorption from the smal l intestine of sheep utilizing this technique were reported by Johns and Bergen (1973).

Whole tissue preparations have provided much useful information; however, they have been criticized for their complexity and inabil- ity to differentiate among the wide variety of absorptive as well as metabolic activities that must be occurring in these tissues (Munck, 1972). pioneering work demonstrating the transport of amino acids into ghost bacterial cells proved that fractured membranes would reform in a vesicular nature, free from cytoplasmic components, and still retain their transport capabilities (Kabach, 1960). Miller and Crane (1961) were the first to apply these techniques to epithelial cells of the intestine.

Researchers have not only isolated the plasma membranes of enterocytes, but they also have been able to separate this heter- ogeneous membrane into its fractions due to differing compositions of individual mem- branes. Isolation of brush border and basolateral membranes and the formation of vesicles from these thus is possible (Murer and Kinne, 1980). Amino acid transport across these membranes can be studied under condi- tions in which the composition of the fluids on both sides of the membrane is carefully controlled. Furthermore, the absence of cyto- solic components reduces the potential for metabolic processes to interfere with the interpretation of data relative to transport processes.

Site of Amino Acid Absorption. Anatomical changes along the small intestine give rise to the thought that the capacity for amino acid absorption may vary with location in the small intestine. Even though considerable informa- tion is available concerning details of intestinal microstructure and the everchanging nature of this anatomy (Trier, 1968; Ito, 1981; Madara and Trier, 1987). a completely defined rela- tionship between anatomy and absorption is not available. Enterocytes certainly are the predominant cell type associated with amino

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acid absorption from the small intestine, but such fundamental questions as how maturity of the enterocyte alters absorption remain to be answered. The distribution of individual trans- port systems and their respective activities may vary relative to intestinal site.

Many studies have addressed the question of site of absorption. Schedl and Clifton (1963) and Schedl et al. (1968) reported that percentage of methionine absorption was greatest in the proximal intestine of the human. Later they reported that the middle intestine of humans was most active in the absorption of alpha-aminoisobutyric acid. Most believe that the middle region of the small intestine has the greatest capacity for amino acid absorption in laboratory species such as the rat and hamster (Spencer and Samiy, 1961; Cohen and Haung, 1964; Larsen et al., 1964; Matthews and Laster, 1965; Baker and George, 1971).

Only a few studies using ruminant small intestine have been reported. Williams (1969) ranked amino acids in the order in which they were absorbed from intestinal loops in the sheep and observed that this order varied with intestinal location, indicating that affhities for the transport systems differed among sites. Lysine absorption by sheep intestinal rings increased with distance fiom the pylorus, with maximal absorption occurring in the ileum (Johns and Bergen, 1973). but the authors felt that this was an artifact of their in vitro system. Phillips et al. (1976) used everted sacs of sheep small intestine and reported that the greatest amount of threonine and valine ab- sorption occurred in the ileum, and that the micromoles of methionine absorbed were about equal from both the jejunum and ileum. These amino acids were not absorbed from the duodenum. They later confirmed, using per- fused intestinal segments of sheep, that the ileum is the dominant site of amino acid absorption (Phillips et al., 1979). Wilson and Webb (1990) reported that total methionine and lysine uptake was higher by ileal brush border membrane vesicles than by jejunal brush border membrane vesicles. Also, methi- onine transporters had lower affinities and higher capacities for methionine than the corresponding lysine transporter; had for ly- sine in both ileal and jejunal tissue.

The significance of these site differences in amino acid absorption is not clear. Species of animal seems to be quite important in deter- mining what the predominant site of amino

acid absorption is. Impacts of variables such as diet and physiological state on the site of amino acid absorption remain largely un- known. The optimal pH for most intestinal proteases is in the range of pH 7.2 to 9 (Bondi, 1987). The contents of the ruminant small intestine usually do not reach this pH range until near the latter one-half to one-third of the small intestine (Ben-Ghedalia et al., 1976). Because the luminal environment of the ileal region of the small intsstine of the ruminant is more compatible with optimal proteolytic activity, one might expect transport capabilities to be concentrated there. Little is known about the ability of intestinal transport systems to adapt to transitions in dietary intake and content or physiological status of the animal (Karasov and Diamond, 1987).

Transport Systems and Mechanisms of Absorption. Numerous proposals are present in the literature describing a wide variety of potential systems for the transport of amino acids. Confusion exists because procedures differ. Also, there is considerable overlap in substrate specificity among amino acid trans- porters. Nevertheless, several carriers are in- volved in the movement of amino acids across the intestine.

Regardless of the specific nature of a transporter, all carriers require the presence of an amino or imino group and a carboxyl group (Spencer et al., 1962; Schultz et al., 1972). Generally speaking, small molecular weight hydrophylic neutral amino acids are taken up less readily than the larger hydrophobic amino acids, the transport of basic amino acids is intermediate (Sepulveda and Smith, 1978, 1979).

The precise number of transporters for amino acids present in the small intestine is not known at present. Several different carriers are present; some are located specifically on the brush border, some on the basolateral membrane, and some on both (Bender, 1985; Hopfer, 1987). Classification of amino acid transport systems is based on substrate prefer- ence and is determined by kinetic and inhibi- tion analysis measurements (Christensen, 1984). Transport by a particular system seems to be determined by size, charge and(or) configuration of amino acid side chains. However, amino acids with quite diverse structures often share a transport system. Christensen (1984) listed 12 amino acid transport systems that are suggested to occur in

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animal tissue. The A, ASC, L and Y+ systems have been identified in both epithelial and nonepithelial tissues. The Gly, N, B, L1, T, X- AG, X-A and X-G systems have been identified in nonepithelial tissue but not in epithelial tissues to date. Stevens et al. (1984) identified three additional transport systems (NBB, PHE and IMINO) in the intestinal mucosa. The carrier proteins responsible for transporting amino acids across cell membranes have not been isolated; their presence is presumptive based on transport inhibition studies.

Some of these carriers require energy to function and are said to be active transporters and Na+dependent. The fist explanation for the mechanisms associated with Na+depen- dent active transport was presented by Crane et al. (1961). They explained the active transport of glucose; this was later extended to amino acids by Curran et al. (1967). Presumably the accumulation of amino acids against a concen- tration gradient was driven by the energy potential of a Na+ gradient. For this to occur, the carrier complex of the active transport system must co-transport a Na+ ion and an amino acid. The Na+ gradient, high Na+ extracellularly and low Na+ intracellularly is maintained by Na+iK+ ATPase located in the basolateral membrane of the enterocyte (Quigley and Gotterer, 1969; Fujita et al., 1972). Transport systems not coupled with Na+ are called Na+-independent and do not require metabolic energy to transport amino acids against a concentration gradient.

Relative Contributions of Amino Acid Transport Systems. The transport of amino acids by intestinal enterocytes occurs by simple diffusion, facilitated diffusion (Na+- independent) and active transport (Na+depen- dent; Stevens et al., 1984). The relative significance of each route is highly dependent on the concentration of substrate present. Active transport of phenylalanine in brush border membrane vesicles generally was quan- titatively the most important route of amino acid uptake when amino acid concentration was low. Diffusion made a progressively larger contribution to total uptake as substrate con- centration increased.

Wilson and Webb (1990) reported that the relative contribution of transport systems for methionine and lysine in bovine brush border membrane vesicles from both jejunal and ileal tissues was influenced dramatically by sub- strate concentration. Only at very low substrate

concentrations ( 4 . 0 mM) did facilitated trans- port systems account for more transport than diffusion. In the case of methionine transport by ileal tissue, diffusion was the predominant form of transport throughout the range of substrates studied. Moe et al. (1987) reported that when bovine ileal brush border membrane vesicles were incubated in buffer containing .1 mM substrate, Na+dependent, Na+-indepen- dent and diffusion systems accounted for 14, 37 and 49% and 9,53 and 38% of methionine and lysine transport, respectively. Also in the bovine, Guerino and Baumrucker (1987) re- ported that the Na+-independent systems made the greatest contribution to total uptake for both lysine and methionine at a substrate concentration of 1.0 mM.

The relative importance of Na+dependent, Na+-independent and diffusion transport to total amino acid transport clearly differs depending on which initial amino acid concen- tration was used to measure transport. There- fore, caution must be exercised when drawing conclusions concerning intestinal amino acid transport when uptake of amino acids has been measured at only a single substrate concentra- tion.

Relative Absorption of Amino Acidr. The diverse nature of the network of transport systems present in animals attests to the importance of this process for maintenance, growth and even survival of the animal. The net result of these combined transport systems seems to serve the animal well when individ- ual amino acids or groups of amino acids are considered.

Numerous examples can be cited enumerat- ing the relative absorption of amino acids. When absorption rates of amino acids from equimolar mixtures of 18 common dietary amino acids or of eight essential amino acids were studied in humans, methionine and the branchedchain amino acids (leucine, isoleu- cine and valine) were absorbed most rapidly (Adibi and Gray, 1967; Adibi et al., 1967). As a group, essential amino acids were absorbed more rapidly than nonessential amino acids. Similar results with human subjects were obtained by Silk et al. (1980). They perfused the upper jejunum with a mixture of 16 amino acids simulating lactalbumin. The top six amino acids in order of absorption, measured by percentage of amino acid absorbed, were methionine, leucine, valine, phenylalanine, ar- ginine and isoleucine. The disappearance, as a

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percentage of amino acid present, was greater for essential than for nonessential amino acids from the small intestine of sheep (Coelho da Silva et al., 1972; MacRae and Ulyatt, 1974; Armstrong et al., 1977; Christiansen and Webb, 1990a) and cattle (Armstrong et al., 1977; Chistiansen and Webb, 199Ob). In all the above studies with sheep and cattle, absorption, as a percentage of that present initially, was greater for methionine than for other amino acids.

When absorption of an individual amino acid is measured as the percentage of amino acid present, dietary essential amino acids tend to be absorbed in greater amounts than nonessential amino acids. Additionally, methi- onine usually is near or at the top of the list. The complex pattern of absorption of amino acids probably is attributable to differences in the affinities of carrier systems for individual amino acids and the fact that competition for transport is greater among amino acids for which a carrier has a greater affinity (Adibi, 1969). Johns and Bergen (1973) reported a 48% reduction in lysine absorption by ovine intestinal rings when leucine was present at three to four times the lysine concentration. More methionine was absorbed than either threonine or valine from everted sacs of ovine intestine (%Kips et al., 1976) and, when different mixtures of methionine. valine and threonine were perfused through isolated seg- ments (Phillips et al., 1979), methionine was absorbed in the greatest amounts and was a strong inhibitor of the absorption of both valine and threonine.

AU the interactions that are observed among amino acids in the absorptive process are not negative. Some amino acids stimulate the transport of others. Examples include the ability of alanine and phenylalanine (Reiser and Christiansen, 1969), leucine (Munck. 1966) and methionine (Reiser and Christian- sen, 1971) to increase the intestinal transport of the basic amino acids arginine, lysine and ornithine. Phillips et al. (1979) also noted that methionine stimulated the absorption of thrm nine when threonine concentrations were low.

Peptide Absorption

Dogma suggests that it is necessary to hydrolyze proteins to free amino acids prior to absorption; this has delayed acceptance of alternative means of absorption. The possibil-

ity of peptide transport was mentioned over 100 years ago (Matthews, 1987). Newey and Smyth (1959, 1960) are credited with provid- ing the fiist convincing evidence that peptides are absorbed. They demonstrated the transport of intact diglycine. General acceptance that peptide absorption is siguficant physiologi- cally was not rapidly forthcoming, however. Interest and enthusiasm for this concept had to await the reports of substantial intestinal peptide absorption (Adibi and Phillips, 1968; Matthews et al., 1968). Even with mounting evidence, both direct and indirect, acceptance of peptide absorption was not to be common- place in the 1970s.

Even in studies with ruminants, the possi- bility of alternative forms of amino acid absorption was suggested. Wolff et al. (1972) mentioned that the amounts of amino acids disappearing from the lumen of the gut were generally 1.5 to 2 times greater than those appearing in the portal plasma of sheep. Phillips et al. (1976) were able to account for only 19 to 63% of amino acids disappearing from the mucosal side of an in vitro system with sheep intestine. Tagari and Bergman (1978) reported that only 30 to 80% of the amino acids leaving the intestinal lumen could be recovered in portal plasma of sheep. In each of these studies, metabolism of amino acids by intestinal mucosa and(or) analytical inaccura- cies were cited as potential reasons for these discrepancies. None of the authors suggested the potential contribution of peptide absorption in these discrepancies. Absorption Rate of Peptides and the Speci-

ficity of Transport Systems. Substantial evi- dence supports the contention that peptides are absorbed more rapidly than free amino acids (Adibi and Phillips, 1968; Craft et al., 1968; Adibi, 1971; Cheng et al., 1971; Burston et al., 1972). Whereas these works clearly demon- strated the advantage that selected peptides exhibited in rate of absorption, their nutritive significance became more apparent when more complex mixtures of peptides were studied.

Absorption of amino acids from rat jejunum in vivo was greater from pancreatic hydroly- sates of four proteins (casein, albumin, lyso- zyme and lactalbumin) than from equivalent mixtures of free amino acids (Matthews, 1972). Both rate and extent of absorption of amino acids from a partial enzymatic hydroly- sate of lactalbumin consisting mainly of small peptides were greater than absorption of amino

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acids from an amino acid mixture simulating lactalbumin (Silk et al., 1980). Hara et al. (1984) computed what they called intensity of absorption for amino acids, which combined both rate and extent of absorption. They compared absorption of amino acids from an enzymatic hydrolysate of egg white protein with a corresponding mixture of amino acids in rats. The absorption intensity of peptide amino acids was 70 to 80% higher than for amino acids from a corresponding amino acid mixture.

The fact that peptides are absorbed more rapidly than free amino acids suggests that independent transport systems for peptides may exist or that there is a competitive advantage by the peptides for carriers. The latter, however, has been largely discounted (Rubino et al., 1971). Studying several differ- ent amino acids and peptides, they demon- strated that free amino acids had no effect on the transport of dipeptides, and vice versa.

The certainty of the independence of pep tide transport, on the other hand, is virtually indisputable, as indicated by intestinal uptake of amino acids despite inborn errors of amino acid transport. In Hattnup disease, in which many neutral amino acids are poorly absorbed, the efficient absorption of phenylalanine as the dipeptide was demonstrated (Asatoor et al., 1970). Likewise, Hellier et al. (1972) demon- strated that lysine, relatively poorly absorbed as free lysine, was well absorbed as a dipeptide in cystinuric patients who have difficulty absorbing cysteine, lysine, ornithine and argi- nine as free amino acids. This advantageous rate of absorption and specificity for transport- ers held by peptides has nutritional importance.

Peptide Configuration and Absorption. Configuration differences among peptides play a determining role in the transport of peptides across intestinal mucosa. Although some ques- tions remain, peptide transport is limited primarily to di- and tripeptides. The early reports of Craft et al. (1968) and Matthews et al. (1968) demonstrated a progressive increase in the rate of transport from free glycine to the tripeptide. Later, Peters et al. (1972) suggested that transport was limited to the dipeptide. Evidence for at least limited transport of tripeptides has been presented by several investigators (Evered and Was, 1970; Vale et al., 1970; Addison et al., 1974a,b; Adibi et al., 1975). The likelihood of large-scale transport of larger peptides seems minimal (Adibi and Morse, 1977).

"he amino acid composition of the peptide influences absorption. Glutamic acid was ab- sorbed at nearly twice the rate from rat small intestine when it was present as glutamin- lytyrosine rather than glutaminylmethionine (Burston et al., 1972). Not only do the particular amino acids present influence ab- sorption, but whether an amino acid is in the N- or C-terminal position also is important. Lysine was absorbed much more rapidly when it was present in the N-terminal position in a dipeptide with glycine than when it was in the C-terminal position. Conversely, lysine was absorbed most rapidly when it was in the C- terminal position in a dipeptide with glutamic acid (Burston et al., 1972).

Peptides compete with one another for transport (Matthews et al., 1979; Taylor et al., 1980). In addition to inhibition, there may be some stimulatory effects of one peptide on the absorption of another. Addison et al. (1974) showed a quantitatively greater absorption of carnosine from hamster jejunum in the pres- ence of glutaminylglutamate. They also showed that methionylmethionine and glycyl- glycine were potent inhibitors of carnosine absorption. Carrier specificity, therefore exists in peptide transport; this will influence the nature of absorption from mixtures of peptides.

Mechanisms of Peptide Transport. Peptides are transported against a concentration gradient by a system(s) requiring energy (Addison et aI., 1972, 1975; Matthews et al., 1974; Nutzenadel and Scriver, 1976). These workers studied peptides that were resistant to hydroly- sis by cytoplasmic peptidases and demon- strated concentrative uptake of peptides in intestinal mucosal cells in the presence of normal cellular metabolism and were able to inhibit this transport by the elimination of the presence of oxygen or by addition of metabolic inhibitors.

Because the role of Na+-dependent transport of amino acids already was well understood, this led researchers to believe that peptide transport also was Na+dependent. In spite of the fact that many were able to demonstrate considerable concentrative transport in the absence of Na+, Na+-dependent transport was thought to be the driving force (Matthews et al., 1969; Rubino et al., 1971; Cheeseman and Parsons, 1974; Himukai and Hoshi, 1980; Cheeseman and Johnston, 1982; Himukai, 1985). Thus, skepticism existed, and it was not until the work of Ganapthy et al. (1981) that a clearer understanding became available. They

AMINO ACIDS AND m E S 3017

used isolated brush border membrane vesicles and showed clearly the Na+-independent nature of peptide transport. Others subsequently con- firmed these observations (Berteloot et al.. 1981, 1982; Ganapathy and Leibach, 1982; Himdcai et al., 1983; Ganapathy et al., 19W Rajendran et aL, 1984).

Ganapathy and Leibach (1985), based on studies of pH effects on peptide transport, proposed that protons may be co-transported with peptides and that an electrochemical proton gradient may be the driving force for the concentrative transport of peptides. Takuwa et al. (1985) were the first to demonstrate the active accumulation of a peptide in brush border membrane vesicles in the presence of a H+ gradient. Unequivocal evidence was provided for direct coupling between peptide and H+ during transport and for energization of peptide transport by electri- cal as well as chemical components of the proton-motive force when a membrane poten- tial and a H+ gradient were employed simulta- neously (Miyamoto et al., 1985).

Transport of free amino acids and peptides appear to be independent. The presence of different caniers as well as different driving forces for active transport probably contributes to a more efficient absorption of total amino acids. This likely offers alternatives that allow the animal to adapt to a wider range of physiological and dietary conditions.

Mesenteric Appearance of Peptides There appears to be little doubt that peptide absop tion occurs across brush border membranes of the intestine. However, the presence of di- and tripeptidase activity in the cytosol of entero- cytes is well documented (Donlon and Fottrell. 1972; Kim et al., 1972, 1979; Noren et al., 1973). Di- and hipeptides that cross the brush border membrane may not leave the cell via the basolateral membrane intact due to the presence of substantial peptide hydrolyase activity in the enterocyte. This suggests that only the hydrolysis products of peptides, amino acids, would reach the mesenteric circulation, rather than intact peptides.

Attempts to measure the mesenteric or portal appearance of peptides have not been successful in many studies. However, in some studies, intact peptides have been detected. In 1922, Folin and Berglund reported a large rise in plasma non-protein, non-amino acid, non- urea N in humans consuming gelatin. London and Kotschreff (1934) maintained that poly-

peptides entered the portal blood on a large scale during protein absorption. Methodologi- cal limitations and common belief, however, prevented general acceptance of the possibility that intact peptides might enter the portal circulation.

Peptides that generally are resistant to dipeptidase activity, such as hydroxyproline- containing peptides following a gelatin meal (Prockop et al., 1%2) and carnosine and anserine following the consumption of chicken breast (Perry et al., 1967), have been detected in elevated levels in plasma. Glycylglycine has been detected in peripheral plasma following intestinal administration of this dipeptide (Adi- bi, 1971).

Plasma concentrations of amino acids in guinea pigs generally were lower following the intestinal infusion of a partial enzymatic hydrolysate of casein than when an amino acid mixture simulating casein was infused (Sleisenger et al., 1977). The authors inter- preted the results to indicate that the lower concentrations of amino acids associated with the partial hydrolysate were the result of entry of amino acids into the portal blood in peptide form. Earlier, Gardner (1975) reported that, in the isolated small intestine of the rat, peptide- bound amino acids accounted for about 36% of amino acids appearing on the serosal surface.

Work from our laboratory with Holstein calves revealed that about 70% of the amino acids appearing in portal blood were associated with peptides (Koeln, 1982). To define the size of the peptides appearing in the portal plasma, we separated the peptides on a gel filtration column using Sephadex G-15 (Schlagheck and Webb, 1984). Most of the peptides had molecular weights of 300 to 500 daltons.

The magnitude of intact peptide transfer from the gut lumen to mesenteric blood still is not certain. Portal appearance of peptide amino acids may reflect transfer from the intestinal lumen or they may be degradation products associated with intestinal protein turnover. Further investigation of t h i s area will be necessary before a definitive conclusion can be drawn.

Tissue Incorporation and Nutritional Signif- icance of Peptides. The fact that peptides may be absorbed intact raises the question of the fate of these in the animal. Are peptides a source of amino acids for tissues? If so, to what extent are amino acid needs supplied by peptides and what influences the extent to

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which peptides supply tissue needs for amino acids?

Evidence that circulating peptides might be utilized by tissues was gained through study of the fate of i.v.- administered dipeptides (Adibi et al., 1977). They showed that both glycyl-L- leucine and glycyl-glycine were cleared very rapidly (within minutes) from plasma follow- ing injection. They also showed that these peptides were not excreted in the urine and that neither could be recovered from intracellu- lar spaces of several tissues.

Hydrolysis of peptides in plasma was suspected, but subsequently it was determined that no hydrolase activity against glycyl- glycine was present in plasma (Krzysik and Adibi, 1977). They observed only modest hydrolase activity against glycyl-L-leucine. The possibility that these peptides might be transported into tissue cells, and then hydre lyzed, dso was confirmed. They determined that the cytosol of all tissues investigated contained substantial hydrolyase activity to- ward both peptides. Therefore, peptides ab- sorbed intact may not be hydrolyzed in plasma, but are transported into cells where they are hydrolyzed and their constituent amino acids are made available for cell needs. This possibility was further confirmed when incor- poration of radioactive label in tissue proteins was studied following injection of 14c-iabeied glycine or glycyl-glycine (Kmysik and Adibi, 1979). Accumulation of label into proteins was similar whether from glycine or glycyl-glycine. The only exception was muscle, in which the free amino acid was incorporated more readily.

High concentrations of circulating peptides were observed in the plasma of calves fed a conventional diet (McCormick and Webb, 1982) and in calves fed purified diets (Danil- son et al., 1987). In the former case, peptide amino acids appeared to be removed from plasma by the tissues of the hindlimbs. In the latter case, when purified diets were fed this removal was not uniformly observed. When soy protein supplied the N in the diet, peptides appeared to be removed from plasma by hindlimb tissues. When urea supplied the N in the diet, peptide removal by hindlimb tissues was negligible. Dietary status may influence peptide utilization by tissues.

Peptides can serve as a source of amino acids for tissues. Additional research will reveal the details of the quantitative nature of peptides as a source of amino acids for tissues

as well as physiological influences on or control of this process.

Epilogue

Processes involved with absorption of free amino acids and peptides by intestinal mucosa are complex and are mediated by a number of important factors. Much is known about these processes, yet much remains unclear. Because peptides may play a dominant role in the absorption of amino acids, our approaches to supplying supplemental dietary protein and(or) amino acids to animals may need to be examined closely. Forms of supplemental amino acids, such as specific peptides or mixtures of specific peptides, may improve animal productivity. More information also is needed on how tissue needs for amino acids are met. If di- and tripeptides are important sources of amino acids for tissues, we may need to reevaluate the methods we use to measure muscle protein accretion, turnover and metabolism. Animal tissues are dynamic; pro- cesses employed in the future to gain addi- tional insight into the mechanisms by which animal tissues are supplied with amino acids also must be dynamic.

lmpllcatlons

Small peptides (primarily di- and tripep- tides) and amino acids are absorbed from the small intestine. Both peptides and amino acids can be absorbed against a concentration gra- dient. The energy-dependent transport of amino acids is linked with the co-transport of Na+; for peptides, energy-dependent transport is linked with the co-transport of protons. Peptides are absorbed more rapidly than amino acids. Tissues appear to be able to utilize peptides as a source of amino acids. The nutritional significance of peptide absorption is incompletely understood.

Llterature Cited

Addison, J. M., D. Burston and D. M. MatIhews. 1972. Evidence for active trimsport of the dipeptide glycyl- sarcosine by hamster jejunum in vitro. Clin. Sci. 4 3 907.

Addison, J. M., D. Burston and D. M. Matthews. 1974a. Transport of the tripeptide Beta-alanyl-glycyl-glycine by hamster jejunum in vitro. Clin Sci. Mol. Med. 46: 5P.

Addison, J. M., D. Burston, D. M. Matthews, J. W. R i p e and S. Wilkinson. 1974b. Evidence for active baosport

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of the tripeptide glycylsarcosylsarcosine by hamster jejunum in vitro. Clin. Sci. Mol. Med. 46:30P.

Addison. J. M., D. Burston. J. W. Payne, S. Wilkioson and D. M. Matthews. 1975. Evidence for active transport of tripeptides by hamster jejunum in vitro. Clin. Sci. Mol. Med. 49:305.

Addison, J. M., D. M. Matthews and D. Burston. 1974c. Competition between carnosine and other peptides for transport by hamster jejunum in vitro. Clin. Sci. Mol. Med. M707.

Adibi, S. A. 1969. The influence of molecular structure of neutral amino acids on their absorption kinetics in the jejunum and ileum of hmnan intestine in vivo. Gastroenterology 5 6 9 3 .

Adibi, S. A. 1971. Intestinal transport of dipeptides in man: Relative importance of hydrolysis and intact absorp- tion. J. Clii. Invest. 502266.

Adibi, S. A. 1975. Dipeptide absorption and hydrolysis in human small intestine. In: D. M. Matthews and J. W. Payne (Ed.) Peptide Transport in Protein Nutrition. pp 147-166. American Elsevier, New York.

Adibi, S . A. 1976. Intestinal phase of protein assimilation in man. Am. J. Clin. Nutr. 293205.

Adibi, S. A. and S. J. Gray. 1967. Lntestiaal absorption of essential amino acids in man. Gastroenterology 5 2 837.

Adibi, S. A., S. J. Gray and E. Menden. 1967. The kinetics of amino acid absorption and alteration of plasma composition of free amino acids after intestinal perfusion of amino acid mixtures. Am. J. Clin Nutr. 2 0 24.

Adibi, S. A. and Y. S. Kim. 1981. Peptide absorption and hydrolysis. In: L. R. Johnson (Ed.) Physiology of the Gastrointestinal Tract. pp 1073-1095. Raven Press, New York.

Adibi, S. A., B. A. Knysik and A. L. Drash. 1977. Metabolism of inmvenously administered dipeptides inrats: Effects onamino acid pools, glucose concentra- tion and insulin and glucagon secretion. Clin. Sci. Mol. Mcd. 52193.

Adibi, S. A. and E. L. Morse. 1977. The number of glycine residues which limits intact absorption of glycine oligopeptides in human jejunum. J. Clin. Invest. 60: 1008.

Adibi, S. A., E. L. Morse, S. S. Masilamani and P. M. Anim. 1975. Evidence for two different modes of tripeptide disappearance in human intestine. J. Clin. Invest. 56: 1355.

Adibi, S. A. and E. Phillips. 1968. Evidence for greater absorption of amino acids from peptide than from free form in human intestine. Clin. Res. 164.46.

Agar, W. T., F.J.R. Hird and G. S. Sidhu. 1954. The uptake of amino acids by the intestine. Biochim. Biophys. Acta 14:80.

Alpers. D. H. 1987. Digestion and absorption of carbohy- drates and proteins. In: L. R. Johnson (Ed.) Physiology of the Gastrointestinal Tract (2nd Ed.). pp 146S1487. Raven Press, New Yo&

Armstrong, D. G., G. P. Savage and D. G. Harrison. 1977. Digestion of nitrogenous substances entering the small intestine with particular reference to amino acids in ruminant livestock. In: Roc. 2nd Int. Symp. onfiotein Metab. and Nun. pp 55-60. The Netherlands.

Asatoor. A. M., B. Cheng, K.D.G. Edwards. A. P. Lant, D. M. Matthews, M. D. Milne, P. Navab and A. J. Richards. 1970. Intestinal absorption of two dipeptides in Harmup disease. Gut 11:380.

Baker, R. D. and M. J. George. 1971. Pattern of neutral amino acid uptake along rat small intestine. Biochim. Biophys. Acta 225315.

Ben-Ghedalia, D., H. Tagari and A. B o d . 1976. The ileum of the sheep as a site of protein digestion. Br. J. Nutr. 36:211.

Bender, D. A. 1985. Amino Acid Metabolism. John Wiley & Sons, New Yo*.

Berteloot, A., A. H. Kahn and K. Ramaswamy. 1982. Characteristics of dipeptide transport in normal and papain-treated brush-border membrane vesicles from mouse intestine. II. Uptake of glycyl-Gleucine. Biochim. Biophys. Acta 686:47.

Berteloot, A., A. H. Kahn and K. Ramaswamy. 1981. Characteristics of dipeptide transport in normal and papain-treated brush-border membrane vesicles from mouse intesthe. I. Uptake of glycyl-1-phenylalanine. Biochim. Biophys. Acta 649179.

Bondi, A. A. 1987. Animal Nutrition. John Wiley and Sons, New York.

Burston, D., J. M. Addison and D. M. Matthews. 1972. Uptake of dipeptides containing basic and acidic amino acids by rat small intestine in vitro. Clin Sci. 43: 823.

cheese ma^ C. I. and G. Johnston. 1982. Glycyl-L-leucine transport in the rat small intestine. Can. J. Physiol. pharmacal. 60: 1177.

Cheeseman, C. I. and D. S. Parsons. 1974. Intestinal absorption of peptides. Peptides uptake by small intestine of Raua pipiens. Biochim. Biophys. Acta 373: 523.

Chcng. B., F. Navab, N. T. Lis, T. N. Miller and D. M Matthews. 1971. Mechanisms of dipeptide uptake by rat small intestine in vitro. Clin Sci. 40:247.

Christensen. H. N. 1984. Organic ion transport during the seven decades: The amino acids. Biochim. Biophys. Acta 7792%.

Christiansen, M. L. and K. E. Webb, Jr. 1%. Nitrogen utilization and digestibility of amino acids by lambs fed a high-concentrate diet with limestone or magne- sium oxide. J. Anim. Sci. 68:2095.

Christiansen, M. L. and K. E. Webb, Jr. 19%. Intestinal acid flow, dry matter, stsrch and protein digestibility and amino acid absorption in beef cattle fed a high- concentrate diet with defluorinated rock phosphate, limestone or magnesium oxide. J. Anim. Sci. 68:2105.

Coelho da Silva, J. P., R. C. Seeley, D. E. Beever, J.H.D. Prescott and D. G. Armstrong. 1972. The effect in sheep of physical forms and stage of growth on the sites of digestion of a dried grass. 2. Sites of nitrogen digestion. Br. J. Nu@. 28:357.

Cohen, L. L. and K. C. Huang. 1%. Intestinal transport of tryptophan and its derivatives. Am. J. Physiol. 206: 647.

Craft, I. L., D. Geddes, C. W. Hyde, I. J. Wise and D. M. Matthews. 1968. Absorption and malabsorption of glycine and glycine peptides in man. Gut 9:425.

Crane, R K., D. Miller and I. Bihler. 1961. The restrictions on possible mechanisms of active transport of sugars. In: A. Kleinzeller and A. Kotyk (Ed.) Membrane Transport and Metabolism. pp 163-181. Czech. Acad. Sci., Prague.

Cummins, A. J. 1952. Absorption of glucose and methionine from the human intestine. The influence of the glucose concentration in the blood and in the intestinal lumen. J. Clin. Invest. 31:928.

Curran, P. T., S. G. Schultz, R. A. Chez and R. E. Fuiazi. 1967. Kinetic relations of the Na+-amino acid

3020 WEBB

interaction at the mucosal border of intestine. J. Gen. Physiol. 501261.

Daailson, D. A., K. E. Webb, Jr. and J. H. Herbein. 1987. Transport and hindlimb exchange of plasma and blood-cell amino acids in calves fed soy- or urea-based purified diets. J. Anim. Sci. 64:1842.

Donlon, J. and P. F. Fottxell. 1972. Studies on substrate specitkation and subcellular location of multiple forms of peptide hydrolases in guinea pig intestinal mucosa. Comp. Biochem. Physiol. 41B:181.

Evered, D. F. and M. Wass. 1970. Transport of glutathione across the small intestine of the rat in vitro. J. Physiol. 209:4P.

Fisher, R. B. and D. S. Parsons. 1949. A preparation of surviving rat small intestine for the study of absorption. J. Physiol. 11036.

Fleshler, B., J. H. Butt and J. D. Wisemau. 1966. Absorption of glycine and L-alanine by the human jejunum. J. Clin. Invest. 45:1433.

Folin, 0. and H. Berglund, 1922. The retention and distribution of amino acids with special reference to the u r c ~ formation. J. Biol. Chem. 51:395.

Fujita, M., H. Ohta, K. Kawai. H. Matsui and M. Nakao. 1972. Differential isolation of microvillous and basolatd plasma membranes from intestinal mucosa: Mutually exclusive distribution of digestive enzyme and ouabain-sensitive ATPase. Biochim. Biophys. Acta 274336.

Ganapathy, V., G. Burckhardt and F. H. Leibach. 1984. Characteristics of glycylsarcosine transport in rabbit intestinal brush-border membrane vesicles. J. Biol. Chem. 259:8954.

Ganapathy, V. and F. H. Leibach. 1985. Is intestinal peptide transport energized by a proton gradient’? Am. J. Physiol. 249G153.

Ganapathy, V. and F. H. Leibach. 1982. Peptide transport in rabbit kidney. Studies with L-camosine. Biochim. Biophys. Acta 691:362.

Ganapathy, V.. J. F. Mendicino and F. H. Leibach. 1981. Transport of glycyl-lproline into intestinal and renal brush-border vesicles from rabbit. J. Biol. Chem. 256: 118.

Gardner, MLG. 1975. Absorption of amino acids and peptides from a complex mixhue in the isolated small intestine of the rat. J. Physiol. (Lond.) 253:233.

Gibson, Q. H. and G. Wiseman. 1951. Selective absorption of stereoisomers of amino acids from loops of the small intestine of the rat. Biochem. J. 48:426.

Goodall, E. D. and R.N.B. Kay. 1965. Digestion and absorption in the large intestine of the sheep. J. Physiol. 17612.

Guerino, F. and C. R. Baurmucker. 1987. Identification of methionine and lysine trausport systems in cattle small intesthe. J. Anim. Sci. 65:630.

Hara, H. R. Funabiki, M. Iwata and K. Y m 1984. Portal absorption of small peptides in rats under unrestrained conditions. J. Nutr. 1141122.

Harrison, F. A. and K. J. Hill. 1%2. Digestive secretions and the flow of digata along the duodenum of the sheep. J. Physiol. 162:225.

Hellier, M. D., C. D. Holdworth, D. Perrett and C. Thimmalai. 1972. Intestinal dipeptide transport in normal and cystinuric subjects. Clin. Sci. 43:659.

Himukai, M. 1985. The characteristics of carnosine transport and carnosine-induced electrical phenomena by the everted intestine. of guinea pig. Jpn. J. Physiol. 3 5 9 5 .

Himukai, M. and T. Hoshi. 1980. Mechanisms of glycyl-l- leucine uptake by guinea pig small intestine: Relative importance of intact peptide transport. J. Physiol. (Lond.) 302: 155.

Himukai, M., A. Kameyama and T. Hoshi. 1983. Interaction of glycylglycine and Na+ at the mucosal border of guinea pig small intestine. Biochim. Biophys. Acta 732:659.

Hopfa; U. 1987. Membrane transport mechanisms for hexoses and amino acids in the small intestine. In: L. R. Johnson (Ed.) Physiology of the Gastrointestinal Tract. (2nd Ed.). pp 1499-1526. Raven Press, New York.

Homey, F. D., D. B. Duncan, P. A. Leadbetter and T. S. N e n d d e r . 1973. Intestinal re-entrant cannulation of the horse. Can. Vet. J. 1443.

Homey, F. D., P. A. Leadbetter and T. S. Neudoerffer. 1972. Reentrant cannulation and postoperative therapy in cattle. Am. J. Vet. Res. 33:1385.

Ito, S. 1981. Functional gastric morphology. Ia- L. R. Johnson (Ed.) Physiology of the Gastrointestinal Tract. pp 517-550. Raven Press, New York.

Jacobs, F. A. and A. H. Lang. 1965. Dynamics of amino acid transport in the intact intestine. Roc. Soc. Exp. Biol. Med. 118772.

Johns, J. T. and W. G. Bergen. 1973. Studies on amino acid uptake by ovine small intestine. J. Nutr. 103:1581.

Johnston, C. G. 1932. A method for quantitative intestinal studies. Proc. SOC. Exp. Biol. Med. 30193.

Kabach, H. R 1960. Uptake of amino acids by “ghosts” of mutaut stTains of E. coli. Fed. Roc. 19:136.

Karasov, W. H. and J. M. Diamond. 1987. Adaptation of mtestinal nutrient transport. In: L. R. Johnson (Ed.) Physiology of the Gastrointestinal Tract (2nd Ed.). pp 1489-1497. Raven Press, New York.

Kim, Y. S., W. Birtwbistle and Y. W. Kim. 1972. Peptide hydrolases in the brush border and soluble fractions of small intestinal mucosa of rat and man. J. Clin. Invest. 5 1:1419.

Kim, Y. S., Y. W. Kim, H. D. Gaines and M. H. Sleisenger, 1979. Zymogram studies of human intestinal brush border and cytoplasmic peptidases. Gut 20987.

Koeln, L. L. 1982. Movement of plasma free, erythrocyte free, peptide and serum protein amino acids across the gastrointestinal tract and liver of calves. W.D. Dissertation. Virginia Polytechuic Institute and State Univ., Blacksburg.

Knysik, B. A. and S. A. Adibi. 1979. Comparison of metabolism of glycine injected intravenously in free and dipeptide forms. Metabolism 281211.

Knysilr, B. A. and S. A. Adibi. 1977. Cytoplasmic dipeptidase activities of kidney, ileum, jejunum, liver, muscle and blood. Am. J. Physiol. 233:EMO.

M e n , R. R, J. E. Ross and D. F. Tapley .1964. Transport of neumd, dibasic and N-methyl-substituted amiW acids by rat intestine. Biochim. Biophys. Acta 88570.

London, E. S. and N. P. Kotschneff. 1934. In welcher Form wird Nahrungseiweiss resorbiat? 1. Mitteilung. 2. Physiol. Chem. 228:235.

MacRae J. C. and M. J. Uyatt. 1974. Quantitative digestion of fresh herbage by sheep. 11. The sites of digestion of some nitrogenous constituents. J. Agric. Sci. (Camb.) 82:309.

Madam, J. L. and J. S. Trier. 1987. Functional morphology of the mucosa of the small intestine. In: L. R. Johnson (Ed.) Physiology of the Gastrointestinal Tract (2nd Ed.). pp 1209-1249. Raven Press, New York.

Matthews, D. M. 1972. Rates of peptide uptake by smal l intestine. In: K. Elliott and M. O’Connor (Ed.) CIBA

AMINO ACIDS

Symposium: Peptide Transport in Bacteria and Mam- malian Gut. pp 71-88. Associated Scientific Publish- ers, Amsterdam.

Matthews, D. M. 1975a. Absorption of peptides by mammalian intestine. In: D. M. Mafthews and J. W. Payne (Ed.) Peptide Transport in Protein Nutrition. pp 61-16. American Elsevier, New York.

Matthews, D. M 1975b. Intestinal absorption of peptides. physiol. Rev. 55527.

Matthews, D. M. 1987. Mechanisms of peptide h‘ansport. In: S. A. Adibi, W. FekI, P. Furst and M. Oehmke (Ed.) Dipeptides as New Substrates in Nutrition Therapy. pp

Matthews, D. M. J. M. Addison and D. Burston. 1974. Evidence for active transport of the dipeptide carno- sine (beta-alanyl-L-histidine) by hamster jejunum in vitro. CLin. Sci. Mol. Med. 46:693.

Matthews, D. M., I. L. Craft, D. M Geddes, I. J. Wise. and C. W. Hyde. 1968. Absorption of glycine peptides from the small intestine of the rat. Clin Sci. 35:415.

Matthews, D. M., G. H. Gaudy, E. Taylor and D. Burston. 1979. Influx of two dipeptides, glyclysarcosine and L- glumyl-Gglutamic acid, into hamster jejunum in vitro. Clin. Sci. 5615.

Matthews, D. M. and L. Laster. 1965. Competition for intestinal transport among five neutral amino acids. Am. J. Physiol. 208:601.

Matthews, D. M., M. T. Lis, B. Cheng and R. F. Crampton. 1969. Observations on the intestinal absorption of some oligopeptides of methionine and glycine in the rat. Clin. Sci. 37:751.

McCormick, M. E. and K. E. Webb, Jr. 1982. Plasma free, erythrocyte free and plasma peptide amino acid exchange across the hindlimbs of calves in “steady state’’ and fasting metabolism. J. Nutr. 112:276.

Miller, P. and R. K. Crane, 1961. The digestive function of epithelium of the small intestine. Biochim. Biophys. Acta 52:293.

Miyamoto, Y., V. Ganapathy and F. H. Leibach. 1985. Proton gradient-coupled uphill transport of glycylsar- cosine in rabbit renal brush-border membrane vesicles. Biochem. Biophys. Res. Commun. 132946.

Moe, A. J.. P. A. Pocius and C. E. Polan. 1987. Transport of L-amino acids by brush border membrane vesicles from bovine small intestine. I. Diary Sci. 70290.

Munck, B. G. 1981. Intestinal absorption of amino acids. In: L. R Johnson (Ed.) physiology of the Gastrointestinal Tract. pp 1097-1122. Raven Press, New Yo&.

M w k , B. G. 1972. Methodological problems in the study of amino acid transport by the small intestine. In: Transport across the Intestine. pp 16%194. Churchill Livingstone, London.

Munck, B. G. 1966. Amino acid transport by the small intestine of the rat. The existence and specificity of the transport mechanism of amino acid and its relation to the transport of glycine. Biochim. Biophys. Acta 120 97.

Murer, H. and R. Kinne. 1980. The role of sodium in anion transport across renal and intestinal cells. Studies with isolated plasma membrane vesicles. Fed. Roc. 40:630.

Newey, H. and D. H. Smyth. 1960. Intracellular hydrolysis of dipeptides during intestinal absorption. J. physiol. 152:367.

Newey, H. and D. H. Smyth. 1959. The intarinal absorption of some dipeptides. J. Physiol. (Lond.) 145:48.

Newman, H. J. and M. W. Taylor. 1958. A cannulated Thiry- Vella fistula in the chicken. Am. J. Vet. Res. 19:473.

6-53. Karger. London.

AND PEPTIDES 3021

Noren, 0.. H. Sjostrom and L. Josefsson. 1973. Studies on soluble dipeptidase from pig intestinal mucosa. I. purification and specifcity. Biochim. Biophys. Acta 327446.

Nutzenadel, W. and C. R Scriver. 1976. Uptake and metabolism of Beta-alanine and L-carnosine by rat tissues in vitro: Role in nutrition. Am. J. Physiol. 2u): 646.

P q , T. L., S. Hamen, B. Tischler, R. Bunting and K. Berry. 1967. Carnosinemia. A new metabolic disorder associated with neurologic disease and mental defect. N. Engl. J. Med. 277:1219.

Peters, T. J., J. Donlon and P. F. Fottrell. 1972. The subcellular localisation and specificity of intestinal peptidehydro1ases.h W.L.Bur1andandP.D. Samuel (Ed.) Transport across the Intestine. pp 153-167. Churcbill Livingstone, Edinburgh.

Phillips, W. A., K. E. Webb, Jr. and J. P. Fontenot. 1976. In vitro absorption of amino acids by the small intestine of sheep. J. Anim. Sci. 42:201.

Phillips, W. A., K. E. Webb, Jr. and J. P. Fontenot. 1978. Isolation of segments of the jejunum and ileum for absorption studies using double reentrant cannulae in sheep. J. Anim. Sci. 46:726.

Phillips, W. A., K. E. Webb, Jr. and J. P. Fontenot. 1979. Characteristics of threonine, valine and methionine absorption in the jejnnum and ileum of sheep. I. Anim. Sci. 48926.

Phillipson, A. T. 1952. The passage of digesta from the abomasum of sheep. J. Physiol. 11684.

Prockop. D. J., H. R. Keiser and A. Sjoerdsma. 1962. Gastrointestinal absorption and renal excretion of hydroxyproline peptides. Lancet 2527.

Quigley, J. P. and G. S. Gotterer. 1969. Distribution of (Na+K)-stimulated ATPase activity in rat intestinal mucosa. Biochim. Biophys. Acta 173:456.

Rajendran, V. M., A. Berteloot, Y. Ishilrawa, A. H. Kahn and K. Ramaswamy. 1984. Transport of carnosine by mouse intestinal brushborder membrane vesicles. Biochim. Biophys. Acta 778:443.

Raudin, I. S., C. G. Johnson and P. J. Momson. 1933. The absorption of glucose from the intestine. Am. J. Physiol. 104:700.

Reiser, S. and P. A. Christiansen. 1969. A cross-inhibition of basic amino acid transport by neutral amino acids. Biochim. Biophys. Acta 183:611.

Reiser, S. and P. A. Christiausen. 1971. Stimulation of basic amino acid uptake by certain neutral amino acids in isolated intestinal epithelial cells. Biochim. Biophys. Acta 241:102.

Rubino. A., M. Field and H. Shwachman. 1971. Intestinal tramportofaminoacidresiduesofdipeptides. 1.lnflux of the glycine residue of glycyl-L-proline across mucosal border. J. Biol. Chem. 246:3542.

Schedl, H. P. and J. A. Clifton. 1963. Kinetics of L- nrethionine absorption from the human small intestine. J. Lab. Clin. Med. 62:lOll (Abstr.).

Schedl. H. P., C.E. Pierce, A. Rider and J. A. Clifton. 1968. Absorption of L-methionine from the human small intestine. J. Clin. Invest. 47:417.

Schlagheck, T. G. and K. E. Webb, Jr. 1984. Characteriza- tion of peptides from the gastrointestinal eact of calves. Fed. Roc. 43:671 (Abstr.).

Schultz, S. G., L. Yu-Tu and C. K. Strecker. 1972. Influx of neutral amino acids across the brush border of rabbit ileum. Stermspecificity and the roles of the alpha- amino and alpha-carboxylate groups. Biochim. Bi- ophys. Acta 288:367.

3022 WJ2BB

Sepulveda, F. V. and M. W. Smith. 1978. Discrimination between different entry mechanisms for neutral amino acids in rabbit ileal mucosa. J. Physiol. 282:73.

Sepulveda, F. V. and M. W. Smith. 1979. Different mechanisms for neutral amino acid uptake by new- born pig colon. J. Physiol. 286479.

Silk, D.B.A., P. D. Fairclough, M. L. Clark, J. E. Hegarty, T. C. Marrs, J. M. Addison, D. Burston, IC. M. Clegg and D. M. Malthews. 1980. Use of a peptide rather than free amino acid nitrogen source in chemically defined 'elemental' diets. I. Parenter Entexal Nutr. 4548.

Sleisenger, M. H., D. Pelling, D. Burston and D. M. Matthews. 1977. Amino acid c~ncentrations in portal venous plasma during absorption from the small intestine of the guinea pig of an amino acid mixture simulating casein and a partial enzymatic hydrolysate of casein. Clia Sci. Mol. Med. 52259.

Spencer, R. P., T. M. Bow and M. A. Markulis. 1962. Amino group requirement for in vitro intestinal transport of amino acids. Am. J. Physiol. 202271.

Spencer, R and A. H. Samj. 1961. Intestinal absorption of Gphenylalanine in vitro. Am. J. Physiol. 200501.

Stevens, B. R., J. D. Kannitz and E. M. Wright. 1984. Intestinal transport of amino acids and sugars: Advances using membrane vesicles. Annu. Rev. Physiol. 46:417.

Tagari, H. and E. N. Bergman. 1978. Intestinal disappear- ance and portal blood appearance of amino acids in sheep. J. Nutr. 108:790.

Takuwa, N., T. Shimada, H. Matsumoto and T. Hoshi. 1985. Proton-coupled transport of glyclyglycine in rabbit

renal brush-border membraue vesicles. Biochim. Biophys. Acta 814186.

Taylor, E., D. Burston and D. M. Malthews. 1980. Influx of glyclysarcosine and GLysyl-Llysine into hamster jejunum in vitro. Clin Sci. 58:221.

Trier, J. S. 1968. Morphology of the epithelium of the small intestine. In: W. Heidel (Ed.) Handbook of Physiology. Vol. 3. pp 1125-1 175. Americau Physiological Socie-

Vale, W., R Burgus, T. F. Dunn and R. Guillemin. 1970. Release of TSH by oral administration of synthetic peptide derivatives with TRF activity. J. Clin. Endocri- nol. Metab. 30:148.

Van Slyke, D. D. 1917. The present sisnificance of the amino acid in physiology and pathology. Arch. Intern. Med. 1956.

Webb, Jr., K. E. 1986. Amino acid and peptide absorption fhm the gastrointestinal tract. Fed. h c . 452268.

Williams, V. J. 1969. The relative rates of absorption of amino acids from the small intestine of the sheep. Comp. Biochem. Physiol. 29:865.

Wilson. J. W. and K. E. Webb, Jr. 1990. Lysine and methionine bransport by bovine jejunal and ileal brush border membrane vesicles. J. Auim. Sci. 68:504.

Wilson, T. H. and G. Wiseman. 1954. Intestinal absorption The use of sacs of everted small intestine for the study of the transference of substances from the mucosal to the serosal surface. I. Physiol. 123116.

Wolfl. J. E., E. N. Bergman arad H. H. Williams. 1972. Net metabolism of plasma amino acids by liver and portal- drained viscera of fed sheep. Am. J. Physiol. 223438.

ty, washington, Dc.