Journal of Clinical Investigationlol. 43, No. S, 1964

Metabolism of Vitamin D. I. Preparation of RadioactiveVitamin D and Its Intestinal Absorption in the Rat *

DAVID SCHACHTER,t JAMES D. FINKELSTEIN,4 ANDSZLOMAKOWARSKI(Front the Department of Medicine, Columbia University College of Physicians & Surgeons,

and the Presbyterian Hospital, New York, N. Y.)

Vitamin D has been called "the most dominatingsingle factor in the regulation of the absorption ofcalcium" (1). Although experiments with seg-ments of small intestine in vitro have demonstratedthat the vitamin is required to maintain an activetransport mechanism for calcium in the mucosa(2-4), the molecular events underlying the actionremain unknown. To define them, detailed in-formation on the metabolism of vitamin D, nowlacking, is needed.

The metabolic fate of the vitamins D can bestudied most effectively with radioactive com-pounds, for chemical methods of estimation arerelatively insensitive or nonspecific, and biologicalassay is tedious and difficult. C14-vitamin D3 la-beled at C3 (5) and randomly labeled C14-vitaminDe (6) have been prepared by prior investigators,and with the latter material Kodicek and collabo-rators have provided much of the available meta-bolic data (7-10). The information remains lim-ited, however, partly because it is difficult to pre-pare sufficient quantities of material of relativelyhigh specific activity. Accordingly, the presentstudies were begun to develop more efficient meth-ods for the preparation of radioactive vitamin Dfor metabolic studies. Tritium-labeled vitamin D3and C14-labeled vitamin D, have been prepared,and the major pathways of transport and metabo-

* Submitted for publication October 28, 1963; acceptedDecember 26, 1963.

Supported by U. S. Public Health Service grant AM-01483.

Portions of this work were presented at the 55th An-nual Meeting of the American Society for Clinical In-vestigation, Atlantic City, N. J., April 1963.

f Investigator, New York City Health Research Coun-cil (contract I-183). Supported in part by a NationalInstitutes of Health general research support grant.

t U. S. Public Health Service trainee. Present address:National Institute of Arthritis and Metabolic Diseases,Bethesda, Md.

lism in the rat identified (11). Vitamin D is ab-sorbed chiefly into lymph, distributed widely inthe tissues but accumulated in highest concentra-tion in liver, and excreted mainly into bile. Thepresent report describes in detail the preparationof the radioactive compounds and their intestinalabsorption in the rat.

Materials and Methods

Preparation of H'-vitamnin D,. Initial experimentsdemonstrated persistence of biological activity in a solu-tion of vitamin D, in 80% acetic acid after 16 hours at230 C in an atmosphere of N2. Therefore 1 g of crystal-line vitamin D, under N, 1 was labeled 2 by catalytic ex-change in tritiated acetic acid under identical conditions(schedule TR-1). The product, a brownish material ina vial under N,, was applied for thin-layer chromatog-raphy on plates of activated silicic acid gel 3 0.5 mmthick. Reference vitamin D, was applied to the outermargins. The plates were developed in the dark for 45minutes with CHC13 as the solvent, dried briefly at roomtemperature, sprayed lightly with 0.001%o Rhodamine 6 Gin acetone, and viewed under an ultraviolet lamp (maxi-mal emission, 253.7 mA&) to localize the dark, absorbingband of vitamin D3. The bands of gel were scrapedfrom the plates and eluted repeatedly with CHC13. Vi-tamin D in the eluate, and generally in the present stud-ies, was estimated chemically with an SbCl, reagent(12). (Dried samples were treated with the SbCl3 re-agent at room temperature for exactly 1 minute and theoptical density at 500 m/A determined immediately in aBeckman model DU spectrophotometer in cuvettes of 10mmlight path). The combined CHC13 eluates, contain-ing 294 mg of SbCl3-reactive material, were evaporatedto dryness at 25 to 30° C under a stream of N,, and theresidue was dissolved in 8 ml of dry pyridine. To esterifythe vitamin, 350 mg of powdered 3,5-dinitrobenzoyl chlo-ride was added and the mixture kept under N,, in thedark, at room temperature. After 20 hours another 100mg of dinitrobenzoyl chloride was added and the mix-ture kept for 2 more days. It was then added dropwise

1 Mann Research Laboratories, New York, N. Y.2 By the Nuclear-Chicago Corp., Des Plaines, Ill.3 Silica Gel G, Brinkmann Instruments, Great Neck,

N. Y.

787

DAVID SCHACHTER,JAMES D. FINKELSTEIN, AND SZLOMAKOWARSKI

to 20 ml of 0.1 N NaHCO,, and the resulting yellow pre-cipitate was washed three times with water to removeall pyridine odor, dried under a N, stream, dissolved in4 ml of warm acetone, and filtered. After addition of 4ml of methanol and cooling, yellow crystals of dinitro-benzoyl-vitamin D, were harvested and recrystallized fromacetone/methanol (1/1, vol/vol). The dried product, 102mg of ester, and ester prepared similarly from crystal-line vitamin D, gave identical melting points: 128 to129° C (cloudy melt) (13) and 1330 C (clear melt).Melting points were determined with a Fisher-Johnsapparatus 4 and corrected with crystalline reference com-pounds. The tritium content of the ester, and H' and C14generally in these studies, were measured in a PackardTri-Carb liquid scintillation spectrometer in glass vialscontaining 15 ml of a standard toluene scintillator solution(14), or Bray's solution (15) for water-soluble samples.All counts were corrected for self-absorption by additionof an internal standard: H'-toluene, C4-toluene, orH'-H,0. The specific activity of the H'-vitamin D, dini-trobenzoyl ester was 6,650 dpm per ,ug, corresponding to247 dpm per IU vitamin D.

The radioactive ester was hydrolyzed before metabolicstudies by crushing approximately 300 Itg of crystals in0.3 to 0.4 ml of 2%o KOHin 98% ethanol. The tube wasflushed with N, and incubated in the dark at 470 C for2 hours. The resulting hydrolysate was adjusted to pH8 with glacial acetic acid, the free vitamin extracted with10 vol of CHCI,, and the extract applied to a silicic acidgel plate for thin-layer chromatography as describedabove. The H'-vitamin'D, band was eluted with CHC1,,assayed for radioactivity, and the CHClI solution storedunder N, at - 15' C in the dark when not used immedi-ately. (Batches of the free vitamin were usually usedwithin 2 to 4 days of their preparation. When the vita-min was stored for 7 or more days, traces of breakdownproducts appeared on thin-layer chromatography, andthe material was rechromatographed before use.) TheH3-vitamin D, prepared in this manner was tested at threedosage levels for biological activity with the duodenalslice assay (4, 16) and crystalline vitamin D3 as referencestandard. The data in Figure 1 indicate that the radio-active vitamin is fully active. Chemical purity of theH'-vitamin D, was assessed further by gas-liquid chro-matography, using a Barber-Colman model 15 apparatuswith an argon ionization detector and radium cathode.The 6-foot X 4--mm column was packed with 1% SE-30silicone on 110 to 120 mesh Anakron ABS 5 and main-tained at 230' C (flash heater, 2750 C; detector cell,265' C; 30 pounds per square inch). The radioactivevitamin and a crystalline reference standard gave identi-cal results, the characteristic two-peaked curves (presum-ably pyro- and isopyro-vitamin D,) reported previously(17).

Preparation of C`-vitamin D_. Uniformly labeled C`4-vitamin D. was prepared biosynthetically by the method of

4 Fisher Scientific Co., New York, N. Y.5 Analabs, Hamden, Conn.

a

-

0

z0

< 0.3-j

Mo.0 0

. w 0.2zww .2a ZzwEX cr

W ..0.1I :;CM0

4- C

0.2 2.0 20.0IU per RAT

FIG. 1. BIOASSAY OF H'-vITAMiNX D,. The H3-vitaminD, and crystalline vitamin D, were tested at three doselevels, each dose fed to each of 8 rats depleted of vitaminD (16). Duodenal slices were prepared and tested foraccumulation of Ca4" in vitro 48 hours later, as previ-ously described (4). C = value observed with depletedrats fed no vitamin D.

Kodicek (6) modified as follows. A small inoculum of S.cerevisiae 6 was incubated aerobically in 25 ml of mediumof the following composition: 2% glucose, 0.1 Mpotassiumphosphate of pH 5.3, 0.01 M sodium acetate of pH 5.3,0.1%o NH4Cl, 0.1%o yeast extract,7 0.03% MgSO4 7 H0,0.005%0 ZnSO4 7 HO, and 0.005% FeSO4 7 HO. After24 hours at room temperature the incubation mixture was

added to 400 ml of identical medium containing 0.5 mc ofacetate-2-C4.8 A stream of 0° was passed through themixture, CO2 was trapped in alkali, and the culture was

incubated for 24 hours at 32' C. After the yeast was

harvested by centrifugation, the precipitate was washedthree times with water and then triturated with 12 ml ofglacial acetic acid for 1 hour at 70' to increase the yieldof C'4-ergosterol (18). After lyophilization, the driedyeast was extracted for 36 hours in a Soxhlet apparatus,using a fresh charge of 150 ml acetone every 12 hours.The acetone extract was evaporated, the residue saponifiedin 6%7 ethanolic KOH (18 hours, 23' C, under N,, in thedark), and the nonsaponifiable fraction extracted intopetroleum ether. Af ter evaporation of the petroleumether, the residue was dissolved in a small volume ofethanol, 2.0 mg of carrier ergosterol added, and the C'4-ergosterol precipitated as the digitonide. The digitonidewas decomposed with pyridine (19) and the free sterol

6 Weare indebted to Dr. Margarita Silva for the pure

culture of S. cerevisiae.7 Difco Laboratories, Detroit, Mich.8 New England Nuclear Corp., Boston, Mass.

_

* H3- Vit D3o Cryst. Vit.D3

788

VITAMIN D METABOLISM

extracted into ethyl ether. Dry, peroxide-free solutionsof C14-ergosterol in ethyl ether were irradiated in aquartz cuvette, under N2, using a mercury arc lamp 9 ata distance of 10 to 12 cm and an interference filter 10with maximal transmission at 290 m/A. After 40 to 50%conversion of the ergosterol, as judged by the SbCl3 re-action, the ether was evaporated under N2; the residuewas dissolved in benzene and maintained at 600 C for 2hours under N2 to convert previtamin D2 to vitamin D2(20). The solution was then applied to silicic acid gelplates for thin-layer chromatography as described above,except that benzene/ethyl acetate (5/1) was the develop-ing solvent (21). Ergosterol, vitamin D2, and otherproducts of irradiation were clearly separated as ab-sorbing bands on the plates, the individual bands elutedwith CHC13, and the C'4-ergosterol reirradiated. Finally,the combined C`4-vitamin D2 eluates were evaporated, andthe residue was dissolved in dry pyridine and esterifiedwith 3,5-dinitrobenzoyl chloride as described above. Sub-sequently 13 mg of carrier, crystalline dinitrobenzoyl-vi-tamin D2 was added and the ester crystallized from ace-tone/methanol (1/1, vol/vol). The recrystallized driedproduct, 10 mg of material, melted sharply at 1430 C(6), as did the authentic ester. The specific activity was1.1 dpm per IU vitamin D. In a second preparation C14-ergosterol isolated from two batches of yeast was com-bined, and the specific activity of the resulting C4-vita-min D2 was 5.3 dpm per IU. The values are comparableto several reported by Kodicek, 0.5 dpm per IU (6) and5.5 dpm per IU (22).

Animal experiments. Male albino Sherman rats wereused, except in some of the lymph collection experimentswhere Sprague-Dawley animals gave similar results.For cannulation of the thoracic lymph duct (23) or com-mon bile duct, rats weighing 350 to 400 g were anesthe-tized with ether and restrained postoperatively in cageswith free access to food 11 and water. Lymph or bilewas collected in vessels chilled in ice; a control collectionperiod of 24 hours preceded each experiment. For oraladministration the radioactive sterols were dissolved ina small volume of ethanol and diluted with 5 vol ofpropylene glycol. Each rat was anesthetized for 2 to 3minutes with ether and given 0.5 ml by gastric tube.Radioactivity in lymph, feces, and tissues was estimatedby homogenizing each sample with at least 25 vol ofethanol/acetone (1/1, vol/vol) for 2 minutes in a Vortishomogenizer. The mixture was filtered, the extractevaporated, and the residue dissolved in the toluenescintillator solution for counting. After decanting thetoluene solution, any remaining residue was shaken with0.5 ml of water and then 15 ml of Bray's solution tocount water-soluble radioactivity. (Water-soluble ra-dioactivity generally did not exceed 5 to 10%o of the

9 Model QA-450, Burdick Corp., Milton, Wis.10Type A-11, Baird-Atomic Inc., Boston, Mass.11 Rockland rat diet, A. E. Staley Mfg. Co., Decatur,

Ill.

30 A I/ 34~~~~

10 ~ HUS fe DINSRT0

z

FIG. 2. CUMULATIVE RECOVERY OF RADIOACTIVITY INLYMPH AFTER ORAL ADMINISTRATION- OF RADIOACTIVE VI-TAMIN D. Each solid plot represents one rat given 1.4mg of uniformly labeled C14_vitamin D,, in ethanol/propylene glycol. The broken plots represent a thirdrat given 0.16 mg of H3-vitamin D,, (A) plus 0.16 mg ofC14_vitamin D, (B). Samples were usually collected at i,1, 2, 5, 10, 18, 24, and approximately 40 to 50 hours.

total counts recovered in the present experiments). Therecovery of radioactive vitamin D added to lymph, feces,or tissue averaged 939o' with this method.

To prepare loops of intestine for absorption studies,rats were fasted for 18 hours in metabolic cages with-out access to feces. They were then anesthetized withether for 10 to 12 minutes, and in this time intestinalsegments approximately 8 cm long were rinsed with 10 mlof isotonic saline and then filled and tied as previouslydescribed (24). Each loop was filled with 0.5 ml ofsolution prepared by dissolving radioactive vitamin in a

small volume of ethanol and adding the ethanol througha no. 26 needle, with rapid stirring, to the appropriatevolume of 0.02 M sodium taurocholate in 0.8517o NaCl.The resulting solutions were grossly clear. After ex-

cision of the loops, the unabsorbed radioactivity was esti-mated as described above. In each experiment at leasttwo control loops were excised immediately after filling,and their radioactivity was taken as the zero-time value.When duodenal loops were tested, the common bile duct,which enters this segment, was ligated.

Bioassay for vitamin D activity was performed as

previously described (4, 16). The rats were depletedon the U. S. P. rachitogenic diet 2 12 for 4 to 5 weeksbefore use. Test and reference standard substances w-ereadministered in ethanol/propylene glycol (1/5, vol/vol)by gastric tube, and 48 hours later duodenal slices weretested in sitro for accumulation of Ca47 or Ca45.

Materials. Cholesterol-4-C"' was obtained commer-

12 Nutritional Biochemicals Corp., Cleveland, Ohio.

789

9 DAVID SCHACHTER,JAMES D. FINKELSTEIN, AND SZLOMAKOWARSKI

TABLE I

Fraction of absorbed H3-vitamin D3recovered in lymph*

Fractionof absorbed

Fraction Hs-vitaminof dose Ds in

Rat Dose absorbed lymph

Ag % %1 42 49.7 86.72 46 54.3 81.93 46 56.3 80.54 1,039 45.8 96.75 1,041 46.3 81.2

* Rats weighed 350 to 400 g and were fed the dose inethanol/propylene glycol by gastric tube as described inMethods. Fraction of the dose absorbed in 16 to 18 hoursis indicated; other conditions are described in the text.

cially.13 Samples of purified sodium taurocholate, so-dium taurodeoxycholate, and sodium taurochenodeoxy-cholate were donated.14 When not specified, the tauro-cholate used was commercial bile salt, which appeared tobe as effective as the purified compound, as indicatedbelow.

Results

Absorption into lymph. Vitamin D is absorbedchiefly into lymph; representative results of thecumulative recovery in lymph after administrationby gastric tube are illustrated in Figure 2. Eachof two rats received 1.4 mg of C14-vitamin D2 inethanol/propylene glycol, and thereafter C14 wasdetectable in lymph within 1 hour. Absorptionremained relatively slow in the first 2 hours,thereafter became more rapid and linear with timeup to 8 to 18 hours, and essentially ceased after26 hours. A third rat was given 0.16 mg of C14-vitamin D2 plus 0.16 mg of H3-vitamin D3 as asingle oral dose, and both isotopes were measuredin the lymph (Figure 2). Vitamin D3 was ab-sorbed into lymph more rapidly than vitamin D2,although the curves with time appeared generallysimilar. The results were confirmed with twoother lymph-fistula rats and in the following loopexperiment. One jejunal loop was tied in eachof 10 rats (weight range, 120 to 165 g) and filledwith the taurocholate-saline medium containing9 pg of H3-vitamin D3 plus 9 Pg of C14-vitaminD2. After 6 hours the loops were excised, andestimation of the residual radioactivity indicated

13 New England Nuclear Corp., Boston, Mass.14 By Dr. Alan F. Hofmann, Rockefeller Institute,

New York, N. Y.

that in each instance more vitamin D3 had beenabsorbed than vitamin D2. Expressed as a per-centage of the initial dose, the mean values forabsorption of D3 and D2 were 42.0 and 20.0%o,respectively.

Table I summarizes the results of experimentsin which the quantity of H3-vitamin D3 absorbedfrom the gastrointestinal tract was compared withthe quantity recovered in the lymph. Each of fivelymph-fistula rats received the dose by gastrictube, and after a collection period of 16 to 18hours, absorption was measured as the differencebetween the dose administered and the radioac-tivity recovered in the feces, intestinal contents,and gastrointestinal tract. The data demonstratethat 80.5 to 96.7% (mean, 85.4) of the radioac-tivity absorbed can be recovered in the lymph.Two dose levels were tested in this experiment,42 to 46 ug and 1.04 mg, and these did not in-fluence significantly either the fraction of thedose absorbed or the percentage of absorbed vita-min recovered in the lymph. Further, no signifi-cant saturation of the absorptive mechanism wasobserved in two loop experiments with C14-vitaminD9 and one experiment with H3-vitamin D3.Jejunal loops in each of five rats were filled witha dose varying from 5 to 500 pg of vitamin in aclear solution of 0.04 M sodium taurocholate and0.85% saline and excised 6 hours thereafter. Thepercentage absorbed was not decreased signifi-cantly at the higher dose levels in this range.

The radioactivity absorbed into lymph was stud-ied further by isolation of the chylomicron frac-tion, thin-layer chromatography, and bioassay.Chylomicrons were isolated by layering 2 ml oflymph under 10 ml of 0.85% NaCl and centri-fuging in a Spinco model L ultracentrifuge (30,-000 rpm, 60 minutes, rotor 40). The chylo-microns contained 62.7 to 81.2%o (mean, 72.3) ofthe total radioactivity in four lymph samples afteroral H3-vitamin D3 and 72.7 to 79.2%o (mean,77.3) in three samples after C14-vitamin D9.Most of the vitamin D in lymph is thereforetransported in the chylomicron fraction.15

15 The H3-vitamin D3 was not present as free lipid inthe lymph, since radioactivity could not be extracted di-rectly into hexane. The radioactivity could be extractedonly after treatment of the lymph with ethanol-acetone(1/1) to denature the proteins.

790

VITAMIN D METABOLISM

For thin-layer chromatography ethanol-acetoneextracts were made of five lymph samples col-lected from three rats fed H3-vitamin D3. Theextracts were evaporated to-dryness under N2 at

230 C and the residues dissolved in CHC13 andapplied to silicic acid gel plates. After 45 min-utes in the dark with petroleum ether/diethylether/glacial acetic acid (80/20/1, vol/vol/vol)as developing solvent to separate the major classesof lipids (25), the plates were dried thoroughlyat room temperature for 2 hours. Successivebands of gel were scraped directly into glasscounting vials, pulverized finely with glass rods,and shaken vigorously with the toluene scintillatorsolution for at least 3 minutes before measurement

of the radioactivity in the liquid scintillationcounter. Recovery of the radioactivity appliedaveraged 81%o with this method. The free sterolzone (Rf, 0.25 to 0.3) contained 56.7 to 80.2%(mean, 69.4) of the radioactivity, and a zone cor-

responding to Rf 0.95 contained 12.7 to 28.0%(mean, 17.5). The latter material has not yet

been identified, but that this zone contains sterolesters is interesting. The free sterol bands were

eluted with CHC13 and rechromatographed on

silicic acid gel plates with CHC13 as developingsolvent (Methods). The vitamin D3 zone con-

tained 70.6 to 93.8% (mean, 81.7) of the radio-activity, with no other defined band.

Lymph samples were collected for biologicalassay from each of two rats, both before and af-ter administration of H3-vitamin D3. The ani-mals were fasted throughout the experiment.Vitamin D activity was estimated by the duodenalslice method (4, 16), modified only in that eachtest animal was given 0.1 ml of lymph or refer-ence standard subcutaneously. The lymph sam-

ples collected after H3-vitamin D3 showed a

marked increment in vitamin D activity, corre-

sponding to 78 and 108%o, respectively, of the vi-tamin content estimated from the measured ra-

dioactivity. Thus most of the vitamin D ab-sorbed into lymph was biologically active andpresent as the free sterol.

Requirement for bile. Indirect evidence pro-

vided by several groups of investigators (26-28)indicates that bile is required for adequate ab-sorption of vitamin D. The following experi-ments demonstrate the requirement directly. Rats

TABLE IX

Effect of a bile fistula on absorption ofH3-vitamin D3 in the rat*

H3-vitamin D3absorbed (mean

No. of and range ofGroup rats initial dose)

Controls 7 48.6 (43.6-56.3)Bile fistula 4 3.8 (0-7.3)

* Each operated or control rat was given approximately47 jug of H3-vitamin D3 in ethanol/propylene glycol bygastric tube. Absorption was measured as the differencebetween the oral dose and the radioactivity recovered inthe feces plus intestinal contents, as described in the text.

with draining bile fistulas and intact controlswere restrained for approximately 24 hours andthen given H3-vitamin D3 by gastric tube. Six-teen hours later they were sacrificed; the residualradioactivity in feces, intestinal contents, and gas-trointestinal tract was estimated and absorptioncalculated as the difference between dose adminis-tered and residual radioactivity. The resultsshown in Table II demonstrate that the bile fistulaanimals absorbed an average of 3.8% of the ad-ministered dose, whereas the controls absorbed48.6% (p < 0.001).

The influence of lipid-solubilizing agents inrats with bile fistulas was then studied. Initialexperiments, in which the animals were fed asingle dose of H3-vitamin D3 with or without1.0 ml of fresh rat bile, yielded inconsistent re-sults, perhaps because a continuous supply of bileis required at the sites of intestinal absorption.More consistent effects were obtained with thefollowing procedure. After a control period of24 hours with bile draining freely through thefistula, each rat was anesthetized with ether, andtwo adjacent, mid-jejunal loops were tied. Oneloop was filled with a control suspension (H3-vita-min D3 dissolved in 0.1 ml ethanol and added to5 to 10 ml 0.85%o saline through a no. 26 needle,with vigorous mixing throughout) and the adja-cent loop with the test solution (prepared exactlylike the control, except that the ethanolic H3-vita-min D3 was added to 0.85% saline containing0.04 M bile salt or other compound, with the re-sulting solutions clear). The loops were excisedafter 5 hours, and the results are listed in TableIII. Sodium taurocholate increased the absorp-tion approximately twofold, and the pure and

791

DAVID SCHACHTER,JAMES D. FINKELSTEIN, AND SZLOMAKOWARSKI

40-

30

0 20 40 60 80 00

DISTANCE fROM PYLORUS

(IX. of pylor ic -cecol length )

FIG. 3. ABSORPTION OF H3-VITAMIN D3 ANDC'HOLESTEROL FROM LOOPS OF INTESTINE. Each solid plot

represents one rat tested with H3-vitamin D3, and each

broken plot one rat tested with C54-cholesterol. Distance

from the pylorus is measured to the mid-point of the ap-

propriate 8-cm ioop. Each loop was filled initially with 0.5

ml of taurocholate-saline medium (Methods) containing

approximately Fg of the appropriate sterol. Loops

wvere excised after 5 hours and residual radioactivity

estimated. The rats weighed 165 to 190 g.

commercial preparations were equally effective.

Sodium taurodeoxycholate and sodium taurocheno-

deoxycholate were less effective, yielding insig-

nificant increases in the smaller group tested.

Sodium lauryl sulfate, 0).04 M, markedly decreased

the absorption. The three purified bile salts were

tested further at a concentration of 0.0)1 M in je-

junal loops prepared in rats with intact common

bile ducts. A single loop was tied in at least eight

rats to test each bile salt. The relative effective-

ness of the three salts in promoting absorption

was 1.36/1.19/1.00 for taurocholate/taurocheno-

deoxycholate/taurodeoxycholate (p < 0.00)1 for

taurocholate-taurodeoxycholate difference; p <

0.01 for taurochenodeoxycholate-taurodeoxycho-

late; p < 0.05 for taurocholate-taurochenodeoxy-

cholate) .

Segmenet of inltestine. The absorption of H3-

vitamin Dz from loops in situ prepared from vari-

ous segments of the small intestine was examined

in each of four rats. The results illustrated in

Figure 3 demonstrate that in each animal ab-

sorption occurred throughout the proximal three-

fourths of the small intestine, was maximal in the

mid-jejunal segmnent, and decreased distally to

minimal values in the terminal ileum. Loop ex-periments with C&4-vitamin D2 confirmed thispattern of absorption. For comparison with vi-tamin D3 similar experiments were performedwith C14-cholesterol; representative plots areshown in Figure 3. Cholesterol, too, was ab-sorbed most effectively from loops in the proxi-mal half of the small intestine, and absorption de-creased progressively in the distal half. In thejejunum the absorption of labeled cholesterol wasgenerally less than that of vitamin D3 (Figure 3),but the data do not permit a comparison of trueabsorption rates. C14-cholesterol is diluted by en-dogenous cholesterol in the gut wall (29), whereasH3-vitamin D3 is not significantly diluted in thequantities used.

Absorption of the labeled sterols was comparedfurther in three lymph fistula rats. Each animalreceived 130 jg of H3-vitamin D3 and of C14-cho-lesterol in ethanol/propylene glycol, and both iso-topes were estimated in successive lymph samples.The initial lag period in absorption observed withvitamin D (Figure 2) was longer for C'4-cho-lesterol. In one animal, for example, the maximalrate of absorption of H3-vitamin D3, i.e., 4.6%of the initial dose per hour, was observed in the2- to 4-hour collection period. The correspondingmaximal rate for C14-cholesterol, 3.5% per hour,was observed in the 4- to 6-hour period. A lagperiod for cholesterol of this magnitude has beennoted by prior investigators (30, 31).

Sequential steps. The absorption of vitamin Dfrom intestinal loops can be resolved into at leasttwo steps, defined as follows. Step 1 is uptake oflabeled vitamin from the lumen so that the radio-

TABLE III

Effect of various solubilizing agents on absorption of H3-vitamin D3 from loops of jejunum

H'-vitamin Ds absorbed(mean %of initial dose)

No. of Saline CompoundCompound tested rats vehicle added

% %o % %Na taurocholate 10 14.0 29.0*

Pure 4 13.2 26.8Commercial 6 14.5 28.8

Na taurodeoxycholate 4 13.1 16.7Na taurochenodeoxy-

cholate 2 14.5 17.2Na lauryl sulfate 3 17.5 6.8

* p < 0.001 for the effect of taurocholate. Each loop was filled ini-tially with approximately 5 jyg of Hs-vitamin Di.

792

VITAMIN D METABOLISM

activity cannot be washed from the mucosal sur-face with isotonic saline. Step 2 is transport fromthe gut wall chiefly to the lymph, measured as thedecrease in total radioactivity of the loop. Thefollowing experiment was designed to compare thesteps in various segments of the small intestine.One mid-jejunal and one distal ileal loop weretied in each rat and filled with isotonic saline-taurocholate solution containing H3-vitamin D3.At various intervals thereafter the animals werekilled in groups of five, each loop excised andrinsed with 2.0 ml of isotonic saline, and the je-junal and ileal rinse fluids and tissues pooledseparately. After estimation of the radioactivity,step 1 was calculated as initial radioactivity in theloop minus final radioactivity in the rinse fluid,and step 2 as initial minus final radioactivity in theentire loop. The results illustrated in Figure 4demonstrate that step 1 was more rapid than step2 at both levels of the intestine, indicating a lagin the transport within the mucosa. Jejunum andileum appeared to differ primarily in step 1. Theinitial rate of step 1 was more rapid and the totaluptake by the mucosa more complete in jejunumthan in ileum. Step 2, in absolute units, was lessin ileum than in jejunum, but expressed as a per-centage of step 1, the values at 4j hours for je-junum and ileum, respectively, were 42.5 and40.0%, i.e., not significantly different.

Other factors. The effects of prior vitamin Ddepletion on absorption of H3-vitamin D3 fromjejunal loops were examined with ten rats main-tained on the vitamin D-free regimen (16) for 4weeks. Four days before the experiment half theanimals were given 20,000 IU vitamin D3 sub-cutaneously. No significant difference in absorp-tion between the treated and depleted animals wasfound. In a similar study with cortisone-treated(1.0 mg subcutaneously per day for 6 days) anduntreated control rats, no difference in the subse-quent absorption of H3-vitamin D3 from jejunalloops was noted.

Discussion

The preparation of tritiated vitamin D3 de-scribed in our present report is relatively simpleand convenient, and the specific activity of theproduct is adequate for many metabolic studies.The material was recrystallized repeatedly as the

I00

zia 75

z 50 ILEUM- Steu/I

0

i- 25 - -c. ILEdVUUM, S/up 20

0 I 2 3 4HOURS

FIG. 4. SEQUENTIAL STEPS IN ABSORPTION OF HTAMIN D, FROMINTESTINAL LOOPS. Step 1 is uptake fromthe lumenal contents, and step 2 is transfer out of theloop, as defined in the text. Each loop was filled initiallywith 5 ,ug of H3-vitamin D, in taurocholate-saline medium.Rats weighed approximately 125 g.

3,5-dinitrobenzoyl ester and, after hydrolysis,shown to be biologically fully active and chemi-cally pure on gas-liquid chromatography. More-over, the metabolism of H3-vitamin D3 in the ex-periments thus far has been generally similarto that of uniformly labeled C14-vitamin D2 pre-pared biosynthetically. As demonstrated above,both radioactive compounds are absorbed chieflyinto lymph in the rat, and further studies in ourlaboratory (11) indicate that both are taken upmainly by liver and excreted into bile. The ma-jor pathways of transport and metabolism of thevitamins D are thus similar to those of cholesterol(32).

The present metabolic studies focus on the in-testinal absorption of vitamin D. Our intestinalloop experiments clearly demonstrate that vitaminD can be absorbed throughout the proximalthree-fourths of the rat small intestine, withmaximal transport in the mid-jejunum and mini-mal absorption in the distal ileum. The absorp-tion of cholesterol from loops shows a similar pat-tern, except that it is more restricted to the proxi-mal half of the small intestine and decreases morerapidly in the distal half. The results with cho-lesterol confirm prior observations (30) of itsmaximal absorption in the upper jejunum of therat in vivo. Further, the upper jejunum is the

793

DAVID SCHACHTER,JAMES D. FINKELSTEIN, AND SZLOMAKOWARSKI

site of triglyceride absorption in man (33), andthis segment is particularly effective in transportand esterification of C14-palmitate by everted sacsof hamster intestine (34). The jejunum, there-fore, -appears to be specialized for the absorptionof several lipid substances. The results of ourexperiments do not support Kodicek's conclusion(35) that vitamin D is absorbed chiefly in thedistal half of the rat small intestine. This conclu-sion was based on estimates of the vitamin D con-tent of the intestinal wall after an oral dose. Thecontent of vitamin D need not reflect the rate ofabsorption in a given segment, particularly manyhours after the dose.

From the results of prior investigators (26-28), we had expected the requirement for bilesalt for vitamin D absorption. However, thegreater effectiveness of taurocholate as comparedto taurochenodeoxycholate and taurodeoxycholateis not readily explicable at present, except thatnormal rat bile does contain mainly taurocholate,with smaller quantities of taurochenodeoxycholate(36). Weadded the bile salts in great excess ofvitamin D3, and all yielded clear solutions to betested. Thus optimal absorption of a lipid likevitamin D by rat intestinal mucosa may requireprior solubilization as a micelle containing a spe-cific bile salt. The bile salt requirement may alsoexplain in part the poor absorption of sterols inthe distal ileum. Experiments with loops of ratintestine in vivo (37) and everted gut sacs invitro (38) have demonstrated that bile salts areabsorbed principally in the distal ileum. Thecellular sites responsible for this transport couldstrip micelles of their bile salt, thereby inter-fering with the absorption of sterols.

Although both cholesterol and vitamin D areabsorbed into lymph and found mainly in thechylomicron fraction, the respective mechanismsclearly differ in that the major fraction of lymphcholesterol is esterified (30), whereas that of vi-tamin D is not. Wecompared the transport intolymph of small, equal doses of C14-cholesteroland H3-vitamin D3. The peak rate of transport ofvitamin D exceeded that of C14-cholesterol, ow-ing probably to dilution of the latter by en-dogenous pools in the intestine. In addition, theC14-cholesterol, as compared to vitamin D, re-quired more time to reach its peak transfer rate

to lymph. This lag period could result from slowmixing in the endogenous cholesterol pools, fromslow transfer between pools, or from a slow rateof esterification.

The cellular mechanisms responsible for steroltransport in the intestinal mucosa are poorly un-derstood. Many investigators have emphasizedthe selective nature of the absorption. Gloverand Morton (39) reviewed the evidence that sev-eral mammalian species absorb phytosterols poorlyas compared to cholesterol and 7-dehydrocholes-terol, whereas the compounds differ little in struc-ture or lipid solubility. In our studies vitamin D2was absorbed into lymph less rapidly than vita-min D3. Further evidence that sterol transportis a specialized cellular function is that cholesteroland vitamin D are absorbed more readily in proxi-mal jejunum than in distal ileum and that tauro-cholate, in contrast to taurochenodeoxycholate andtaurodeoxycholate, is required for optimal trans-fer of vitamin D3 in the rat. Prior investigators(e.g., 31, 39, 40) have considered sterol absorptionin two steps: 1) uptake at the mucosal surfaceand 2) transfer into lymph. Experiments with,8-sitosterol (40) and tobacco leaf phytosterols(41) indicate that these sterols are taken up atthe mucosal surface, although less readily thancholesterol, but only insignificant quantities aresubsequently transferred to lymph. Thus bothsteps in sterol absorption appear to be relativelyspecific. Vitamin D absorption can also be re-solved into a rapid mucosal uptake followed bya much slower transfer to the lymph. Wedem-onstrated in the present studies, moreover, thatjejunum and ileum differ in their absorption ofthe vitamin primarily in the first, mucosal uptakestep. In view of such evidence, it is reasonableto suspect that cellular organelles play an activerole in determining the specificity of sterol ab-sorption. Although the specific transport sitesmay involve lipoproteins, as suggested by Gloverand Green (42), there is as yet no direct evi-dence concerning the chemical nature of such sites.

Summary

Methods are described for the chemical prepa-ration of H3-vitamin D3 and of uniformly labeledC14-vitamin D2 from C14-ergosterol synthesizedby yeast. The intestinal absorption of the la-

794

VITAMIN D METABOLISM

beled compounds was studied in vivo in the rat.Vitamin D is absorbed maximally in the jejunumand transferred chiefly into lymph. It was foundmainly in the chylomicron fraction, as the free,biologically active sterol. Bile salt, specificallytaurocholate as compared to taurodeoxycholateand taurochenodeoxycholate, is required for opti-mal absorption. Two steps can be distinguished inthe transport across the intestine: uptake at themucosal surface, relatively rapid, and transferto lymph, relatively slow. Absorption is morerapid from mid-jejunal as compared to distalileal segments primarily because the first, mucosaluptake step is more rapid and complete.

Addendum

Subsequent to the completion of this manuscript Nor-man and DeLuca (43) described a method for the prepa-ration of the tritiated precursors of vitamins D. and D,and their irradiation to the active vitamins. After oraladministration of HW-vitamin D, in Wesson oil to rats,the ileum was found to contain more radioactivity thanthe more proximal small intestine, as observed previouslyby Kodicek (35). As noted in the Discussion, suchobservations do not test absorption directly, need notreflect the capacity of various segments of the intestineto absorb the vitamin, and are not directly comparableto the loop experiments described here.

References1. Nicolaysen, R., N. Eeg-Larsen, and 0. J. Malm.

Physiology of calcium metabolism. Physiol. Rev.1953, 33, 424.

2. Schachter, D., and S. M. Rosen. Active transport ofCa4" by the small intestine and its dependence onvitamin D. Amer. J. Physiol. 1959, 196, 357.

3. Dowdle, E. B., D. Schachter, and H. Schenker.Requirement for vitamin D for the active trans-port of calcium by the intestine. Amer. J. Physiol.1960, 198, 269.

4. Schachter, D., D. V. Kimberg, and H. Schenker.Active transport of calcium by intestine: actionand bio-assay of vitamin D. Amer. J. Physiol.1961, 200, 1263.

5. Havinga, E., and J. P. L. Bots. Vitamin D and re-lated compounds. I. The synthesis of vitamin-D3-3-C'4. Rec. trav. Chim. Pays-Bas 1954, 73, 393.

6. Kodicek, E. The biosynthesis of '4C-labelled ergo-calciferol (abstract). Biochem. J. 1955, 60, 25 P.

7. Kodicek, E. Metabolic studies on vitamin D i),Ciba Foundation Symposium on Bone Structureand Metabolism, G. E. W. Wolstenholme andC. M. O'Connor, Eds. London, J. & A. Churchill,1956, p. 161.

8. Kodicek, E., and D. R. Ashby. The metabolism of14C-labelled vitamin D2 in the rat. Biochem. J.1960, 75, 17 P.

9. Kodicek, E., and D. R. Ashby. The fate of physio-logical doses of '4C-labelled vitamin D. in normaland rachitic rats. Biochem. J. 1960, 76, 14 P.

10. Innes Chalk, K. J., and E. Kodicek. The associationof '4C-labelled vitamin D2 with rat serum proteins.Biochem. J. 1961, 79, 1.

11. Schachter, D., J. D. Finkelstein, and S. Kowarski.Pathways of transport and metabolism of C'4-vitamin D, in the rat. J. clin. Invest. 1963, 42,974.

12. DeWitt, J. B., and M. X. Sullivan. Spectrophoto-metric procedure for quantitative estimation ofvitamins D. J. industr. eng. Chem., analyt. ed.1946, 18, 117.

13. Brockmann, H. Die Isolierung des antirachitischenVitamins aus Thunfischleber6l. Hoppe-Seylers Z.physiol. Chem. 1936, 241, 104.

14. Snyder, F., and N. Stephens. Quantitative carbon-14and tritium assay of thin-layer chromatographyplates. Analyt. Biochem. 1962, 4, 128.

15. Bray, G. A. A simple efficient liquid scintillatorfor counting aqueous solutions in a liquid scintil-lation counter. Analyt. Biochem. 1960, 1, 279.

16. Gordan, G. S., and D. Schachter. Vitamin D ac-tivity of normal and neoplastic breast tissue. Proc.Soc. exp. Biol. (N. Y.) 1963, 113, 760.

17. Ziffer, H., W. J. A. VandenHeuvel, E. 0. A. Haahti,and E. C. Horning. Gas chromatographic be-havior of vitamins D. and D,. J. Amer. chem.Soc. 1960, 82, 6411.

18. Klosty, M., and W. Bergmann. Sterols of algae.III. The occurrence of ergosterol in Chlorellapyranoidosa. J. Amer. chem. Soc. 1952, 74, 1601.

19. Schoenheimer, R., and H. Dam. Ueber die Spalt-barkeit und Loeslichkeit von Sterindigittoniden.Hoppe-Seylers Z. physiol. Chem. 1933, 215, 59.

20. Velluz, L., A. Petit, G. Michel, and G. Rousseau.Sur un stade non photochimique dans la formationdes calciferols. C. R. Acad. Sci. (Paris) 1948,226, 1287.

21. Avigan, J., DeW. S. Goodman, and D. Steinberg.Thin-layer chromatography of sterols and steroids.J. Lipid Res. 1963, 4, 100.

22. Kodicek, E., E. M. Cruickshank, and D. R. Ashby.The metabolism of 14C-labelled vitamin D_ injectedintracardially into rats. Biochem. J. 1960, 76,15 P.

23. Bollman, J. L., J. C. Cain, and J. H. Grindlay.Techniques for the collection of lymph from theliver, small intestine, or thoracic duct of the rat.J. Lab. clin. Med. 1948, 33, 1349.

24. Manis, J. G., and D. Schachter. Active transport ofiron by intestine: features of the two-step mecha-nism. Amer. J. Physiol. 1962, 203, 73.

25. Mangold, H. K. Thin-layer chromatography oflipids. J. oil chem. Soc. 1961, 38, 708.

795

DAVID SCHACHTER,JAMES D. FINKELSTEIN, AND SZLOMAKOWARSKI

26. Greaves, J. D., and C. L. A. Schmidt. The roleplayed by bile in the absorption of vitamin D inthe rat. J. biol. Chem. 1933, 102, 101.

27. Taylor, N. B., C. B. Weld, and J. F. Sykes. The re-lation of bile to the absorption of vitamin D. Brit.J. exp. Path. 1935, 16, 302.

28. Heymann, W. Metabolism and mode of action ofvitamin D. IV. Importance of bile in the ab-sorption and excretion of vitamin D. J. biol.Chem. 1937, 122, 249.

29. Swell, L., E. C. Trout, Jr., J. R. Hopper, H. Field,Jr., and C. R. Treadwell. Mechanism of choles-terol absorption. II. Changes in free and esteri-fied cholesterol pools of mucosa after feedingcholesterol-4-C'. J. biol. Chem. 1958, 233, 49.

30. Swell, L., E. C. Trout, Jr., J. R. Hopper, H. Field,Jr., and C. R. Treadwell. The mechanism of cho-lesterol absorption. Ann. N. Y. Acad. Sci. 1959,72, 813.

31. Borgstr6m, B., B-A. Lindhe, and P. Wlodawer.Absorption and distribution of cholesterol-4-C' inthe rat. Proc. Soc. exp. Biol. (N. Y.) 1958, 99,365.

32. Gould, R. G., and R. P. Cook. The metabolism ofcholesterol and other sterols in the animal organismin Cholesterol, Chemistry, Biochemistry, and Pa-thology, R. P. Cook, Ed. New York, AcademicPress, 1958, p. 237.

33. Borgstr6m, B., A. Dahlqvist, G. Lundh, and J.Sj6vall. Studies of intestinal digestion and ab-sorption in the human. J. clin. Invest. 1957, 36,1521.

34. Johnston, J. M. Site of fatty acid absorption. Proc.Soc. exp. Biol. (N. Y.) 1959, 100, 669.

35. Kodicek, E. The metabolism of vitamin D in Vi-tamin Metabolism, Fourth International Congressof Biochemistry. London, Pergamon, 1958, vol.

1, p. 198.36. Bergstrom, S., and J. Sj6vall. Occurrence and me-

tabolism of chenodesoxycholic acid in the rat. Bileacids and steroids 13. Acfa chem. scand. 1954, 8,611.

37. Baker, R. D., and G. W. Searle. Bile salt absorp-tion at various levels of rat small intestine. Proc.Soc. exp. Biol. (N. Y.) 1960, 105, 521.

38. Lack, L., and I. M. Weiner. In vitro absorption ofbile salts by small intestine of rats and guinea pigs.Amer. J. Physiol. 1961, 200, 313.

39. Glover, J., and R. A. Morton. The absorption andmetabolism of stero!s. Brit. med. Bull. 1958, 14,226.

40. Swell, L., E. C. Trout, Jr., H. Fields, Jr., and C. R.Treadwell. Absorption of H'-p-sitosterol in thelymph fistula rat. Proc. Soc. exp. Biol. (N. Y.)1959, 100, 140.

41. Swell, L., E. C. Trout, Jr., H. Field, Jr., and C. R.Treadwell. Intestinal metabolism of C14-phytos-terols. J. biol. Chem. 1959, 234, 2286.

42. Glover, J., and C. Green. Sterol metabolism. 3.The distribution and transport of sterols across theintestinal mucosa of the guinea pig. Biochem.J. 1957, 67, 308.

43. Norman, A. W., and H. F. DeLuca. The prepara-tion of H3-vitamins D, and D. and their localizationin the rat. Biochemistry 1963, 2, 1160.

796