Biochemistry of Liver and Kidney

8/11/2019 Biochemistry of Liver and Kidney

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Biochemistry ofthe liver andkidney.

Urine formation.


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Role of the liver in carbohydrate metabolism.

From intestine glucose pass into the liver, wheremost part of it undergone the phosphorillation.Glucose-6-phosphate formed in result of thisreaction, which catalyzed by two enzymeshexokinase and glucokinase.

Glucose-6-phosphate is a key product ofcarbohydrates metabolism. In the liver thissubstance can metabolized into different waysdepend of livers and whole organismsnecessity.


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1. Synthesis of glicogen. Content in the liver70-100g

2. Glucose-6-phosphatase catalize dephosphorillation of glucose-6-phosphate and free glucose formed

3. Excess of glucose-6-phosphate, which not used for synthesis ofglicogen and forming of free glucose

4. Glucose-6-phosphate decomposites for H2O and CO2, and freeenergy for hepatocytes formed.

5. Part of glucose-6-phosphate oxidized in pentosophosphate cycle.

6. Hepatocytes content full set of gluconeogenesis necessary

enzymes. So, in liver glucose can be formed from lactate,pyruvate, amino acids, glycerine. Gluconegenesis from lactatetakes place during intensive muscular work. Lactate formed fromglucose in muscles, transported to the liver, new glucose formedand transported to the muscles


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Role of the liver in protein metabo l ism.

Liver has full set of enzymes, which are necessary foramino acids metabolism. Amino acids from food used inthe liver for following pathways:

1. Protein synthesis.

2. Decomposition for the final products.

3. Transformation to the carbohydrates and lipids.

4. Interaction between amino acids.5. Transformation to the different substances with amino

group.

6. Release to the blood and transport to another organsand tissues.


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Liver syntesizes 100 % of albumines, 90 % of

1-globulines, 75 % of 2-globulines, 50 % of -

globulines, blood clotting factors, fibrinogen,protein part of blood lipoproteins, such enzyme

as cholinesterase.

Liver can synthesize non-essential amino acids.

Liver synthesizes purine and pyrimidine

nucleotides, hem, creatin, nicotinic acid, cholin,

carnitin, polyamines.


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Role of the liver in detoxi f icat ion pro cesses.

A xenobiotics is a compound that is foreign to the body. The principalclasses of xenobiotics of medical relevance are drugs, chemical

cancerogens, and various compounds that have found their way intoour environment by one route or another (insecticides, herbicides,pesticides, food additions, cosmetics, domestic chemicalsubstances).

Some internal substances also have toxic properties (for example,bilirubin, free ammonia, bioactive amines, products of amino acidsdecay in the intestine).

Moreover, all hormones and mediatores must be inactivated.

Reactions of detoxification take place in the liver.

Big molecules like bilirubin excreted with the bile to intestine andleaded out with feces. Small molecules go to the blood and excretedvia kidney with urine.


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The metabolism of xenobiotics has 2 phases:

In phase 1, the major reaction involved is

hydroxylation, catalyzed by members of a class

of enzymes referred to as monooxygenases or

cytochrome P-450 species. These enzymes can

also catalyze deamination, dehalogenation,desulfuration, epoxidation, peroxidation and

reduction reaction. Hydrolysis reactions and

non-P-450-catalyzed reactions also occur in

phase 2.


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In phase 2, the hydroxylated or other compounds produced in phase 1are converted by specific enzymes to various polar metabolites byconjugation with glucuronic acid, sulfate, acetate, glutathione, orcertain amino acids, or by methylation.

In certain cases, phase 1 metabolic reaction convert xenobiotics frominactive to biologically active compounds. In these instances, theoriginal xenobiotics are referred to as prodrugs or procarcinogens. Inother cases, additional phase 1 reactions convert the activecompounds to less active or inactive forms prior to conjugation. Inyet other cases, it is the conjugation reactions themselves that

convert the active product of phase 1 to less active or inactivespecies, which are subsequently excreted in the urine or bile. In avery few cases, conjugation may actually increase the biologicactivity of a xenobiotics.


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There are at least 5 types of phase 2 reactions:

1. Glucuronidation.UDP-glucuronic acid is theglucuronyl donor, and a variety of glucuronyl

transferases, present in both the ER and cytosol, arethe catalysts. Molecules such as bilirubin, thyroxin, 2-acetylaminofluorene (a carcinogen), aniline, benzoicacid, meprobromate (a tranquilizer), phenol, crezol,indol and skatol, and many steroids are excreted as

glucuronides. The glucuronide may be attached tooxygen, nitrogen, or sulfur groups of substrates.

2. Sulfation.Some alcohols, arylamines, and phenols aresulfated. The sulfate donor in these and other biologicsulfation reactions is adenosine 3-phosphate-5-

phosphosulfate (PAPS); this compound is called activesulfate


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3. Conjugation with Glutathione.Glutathione (-glutamylcysteinylglycine) is a tripeptide consisting of

glutamic acid, cysteine, and glycine. Glutathione iscommonly abbreviated to GSH; the SH indicates thesulfhydryl group of its cysteine and is the business partof the molecule. A number of potentially toxicelectrophilic xenobiotics (such as certain carcinogens)

are conjugated to the nucleophilic GSH. The enzymescatalyzing these reactions are called glutathione S-transferases and are present in high amounts in livercytosol and in lower amounts in other tissues.


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Acetylation.These reactions is represented by X + Acetyl-CoA Acetyl-X +CoA, where X represents a xenobiotic. These reactions are catalyzed byacetyltransferases present in the cytosol of various tissues, particularly liver.The different aromatic amines, aromatic amino acids, such drug asisoniazid, used in the treatment of tuberculosis, and sulfanylamides aresubjects to acetylation. Polymorphic types of acetyltransferases exist,resulting in individuals who are classified as slow or fast acetylators, andinfluence the rate of clearance of drugs such as isoniazid from blood.

5. Methylation.A few xenobiotics (amines, phenol, tio-substances, inorganiccompounds of sulphur, selen, mercury, arsenic) are subject to methylationby methyltransferases, employing S-adenosylmethionine as methyl donor.Also catecholamines and nicotinic acid amid (active form of vitamin PP) areinactivated due to methylation.Very important way of detoxification is ureogenes (urea synthesis). Free

ammonia, which formed due to metabolism of amino acids, amides andamines, removed from organism in shape of urea.


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Kidneythe couple organ, which is responsible for excriting of finalproducts of metabolism and for homeostasis. They regulate water

and mineral metabolism, acid-base balance, excriting of nitrogenousslags, osmotic pressure. Also they regulate arterial pressure anderhythropoesis.

Nephronis the structural and functional unit of kidney.

Urinefluid with different organic and inorganic compouds, whichmust be excreted (excess of water, final products of nitrogen

metabolism, xenobiotics, products of proteins decay, hormones,vitamins and their derivates). Most of them present in urine in abigger amount than in blood plasma. So, urine formationis notpassive process (filtration and diffusion only).

In basis of urine formation lay 3 processes: filtration, reabsorbtion andsecretion.


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Glomerulal filtration.Water and low weight molecules go to the urinewith help of following powers: blood hydrostatic pressure inglomerulas (near 70 mm Hg), oncotic pressure of blood plasma

proteins (near 30 mm Hg) and hydrostatic pressure of plasmaultrafiltrate in glomerulal capsule (near 20 mm Hg). In normalconditions, as You see, effective filtration pressure is about 20 mmHg.

Primary urine formed in result of filtration (about 200 L per day).Between all blood plasma substances only proteins dont present ina primary urine. Most of these substances are undergone to the

following reabsorbtion. Only urea, uric acid, creatinin, and other finalproducts of different metabolic pathways arent undergone to thereabsorbt

For evaluate of filtration used clearance(clearance for somesubstanceit is a amount of blood plasma in ml, which is cleanedfrom this substance after 1 minute passing through kidney).


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Reabsorbtion.Lenght of renal tubules is about 100 km. So, all important for our organism arereabsorbed during passing these tubules. Epitelium of renal tubules reabsorb per day 179 L ofwater, 1 kg of NaCl, 500 g of NaHCO3, 250 g of glucose, 100 g of free amino acids.

All substances can be divided into 3 group:

1. Actively reabsorbed substances.

2. Substances, which are reabsorbed in a little amount.3. Non-reabsorbed substances.

To the first group belong Na+, Cl-, Mg2+, Ca2+, H2O, glucose and other monosaccharides, aminoacids, inorganic phosphates, hydrocarbonates, low-weight proteins, etc.

Na+reabsorbed by active transport to the epitelium cell, theninto the extracellular matrix. Cl-and HCO3

-following Na+according to the electroneutrality principle, wateraccording to theosmotic gradient. From extracellular matrix substances go to the blood vessels. Mg2+and Ca2+arereabsorbed with help of special transport ATPases. Glucose and amino acids use the energy ofNa+ gradient and special carriers. Proteins are reabsorbed by endocytosis.

Urea and uric acid are little reabsorbable substances.Creatinin, mannitol, inulin and some other substances are non-reabsorbable.

Some substances (K+, ammonia and other) are secretedinto urine in the distal part of tubules. K+ischanged to Na+by the activity of Na+-K+ATPase.


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Pecul iari t ies o f b iochem ical pro cesses in kidney.

Kidney have a very high level of metabolic processes. Theyuse about 10 % of all O2, which used in organism.

During 24 hours through kidney pass 700-900 L of blood.The main fuel for kidney are carbohydrates. Glycolysis,ketolysis, aerobic oxidation and phophorillation are veryintensive in kidney. A lot of ATP formed in result.

Kidney have plenty of different enzymes: LDG (1, 2, 3, 5),AsAT, AlAT. Specific for kidney is alanine aminopeptidase, 3rd isoform.


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Regu lat ion o f ur ine formation.

Na-uretic hormone (produced in heart) decreasereabsorbtion of Na+, and quantity of urine increased.

Aldosteron and some other hormones (vasopressin, renin,angiotensin II) increase Na-reabsorbtion and decreasequantity of urine.

Role of kidney in acid-base balance regulation.

Kidney have some mechanisms for maintaining acid-base

balance. Na+reabsorbtion and H+secretion play veryimportant role.

1. Primary urine has a lot of Na2HPO4(in dissociated form).When Na+reabsorbed, H+secreted into urine andNaH2PO4 formed.


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2. Formation of hydrocarbonates. Inside renal cellscarboanhydrase forms from CO2and H2O

H2CO3, which dissociated to H+

and HCO3-

. H+

excreted from cell into urine (antiport with Na+)and leaded with urine. Na+connect with HCO3

-,NaHCO3formed and go to the blood, thereupon

acidity decreased.3. Formation of free ammonia. NH3used for

formation of NH4+(H+ion associted), and

different acid metabolites excreted as ammonia

salts.


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Physical and chemical characteristics of urine.

Urine amount (diures) in healthy people is 1000-2000 ml per day. Day-timediures is in 3-4 times more than night-time.

Normal colour of urine is yellow (like hay or amber), what is due to presence of

urochrom (derivate of urobilin or urobilinogen). Some another coloursubstances are uroerythrin (derivate of melanine), uroporphyrines,rybophlavine and other. Colour depends from urine concentration.

Urine is transparent. This characteristic depends from amount of different salts(oxalates, urates, phosphates), amount of present epitelium cells andleucocytes.

Density of urine depends from concentration of soluble substances. Borders of

variation are from 1002 to 1035 g/l. Near 60-65 g of hard substances areexcreted with urine per day.

In normal conditions urine has acid or weak acid reaction (pH=5,3-6,8). Thisdepends from presence of NaH2PO4and KH2PO4.

Fresh urine has a specific smell, which is due to presence of flying acids. But alot of microorganisms, which are present in urine, split urea and freeammonia formed.


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Indican (potassium salt of indoxylsulfuric acid).Per day excrition of indican is about 10-25 g. Increasing of indicanslevel in urine is due to inrtensification of decay proteins in the intestines and chronic diseases, which areaccompanied by intensive decopmosition of proteins (tuberculosis, for example).

Organic acids.Formic, acetic, butyric, -oxybutyric, acetoacetic and some other organic acids are present in urine in alittle amount.

Vitamines.Almost all vitamines can be excreted via kidney, especially, water-soluble. Approximately 20-30 mg of vit C,0.1-0.3 mg of vit B1, 0.5-0.8 mg of vit B2 and some products of vitamines metabolism. These data can be used

for evaluating of supplying our organism by vitamines.Hormones.Hormones and their derivates are always present in urine. Their amount depends from functional state of

endocrinal glands and liver. There is a very wide used testdetermination of 17-ketosteroids in urine. For healthyman this index is 15-25 g per day.

Urobilin.Present in a little amount, gives to urine yellow colour.

Bilirubin.In normal conditions present in so little amount that cannot be found by routine methods of investigations.

Glucose.In normal conditions present in so little amount that cannot be found by routine methods of investigations.

Galactose.Present in the newborns urine, when digestion of milk or transformation of glalactose into glucose in theliver are violated.

Fructose.It is present in urine very seldom, after eating a lot of fruits, berries and honey. In all other cases it indicatesabout livers disorders, diabetes mellitus.

Pentoses.Pentoses are excreted after eating a lot of fruits, fruit juices, in case of diabetes mellitus and steroiddiabetes, some intoxications.

Ketone bodies.In normal conditions urine contains 20-50 mg of ketone bodies and this amount cannot be found byroutine methods of clinical investigations.

Porphyrines.Urine of healthy people contains a few I type porphyrines (up to 300 mkg per day).


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Biochemistry of Liver and Kidney

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