Cysteine Conjugate -Lyase in .NET

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7.3.6 Cysteine Conjugate -Lyase
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This enzyme uses cysteine conjugates as substrates, releasing the thiol of the xenobiotic, pyruvic acid, and ammonia, with subsequent methylation giving rise to the methylthio derivative. The enzyme from the cytosolic fraction of rat liver is pyridoxal phosphate requiring protein of about 175,000 daltons. Cysteine conjugates of aromatic compounds are the best substrates, and it is necessary for the cysteine amino and carboxyl groups to be unsubstituted for enzyme activity.
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7.3.7 Acylation
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Acylation reactions are of two general types, the rst involving an activated conjugation agent, coenzyme A (CoA), and the second involving activation of the foreign
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METABOLISM OF TOXICANTS
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O H2N Benzidine (a) Acetylation COOH + Benzoic acid C(O)SCoA + Glycine C(O)NHCH2COOH + Hippuric acid (b) Amino acid conjugation + CoASH C(O)SCoA + + AMP NH2 + CoA CH3CHN NH2
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NH2CH2COOH
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CoASH
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Examples of acylation reactions.
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compounds and subsequent acylation of an amino acid. This type of conjugation is commonly undergone by exogenous carboxylic acids and amides, and although the products are often less water soluble than the parent compound, they are usually less toxic. Examples of acylation reactions are shown in Figure 7.21.
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Acetylation. Acetylated derivatives of foreign exogenous amines are acetylated by N -acetyl transferase, the acetyl donor being CoA. This enzyme is cytosolic, has been puri ed from rat liver, and is known to occur in several other organs. Evidence exists for the existence of multiple forms of this enzyme. Although endogenous amino, hydroxy, and thiol compounds are acetylated in vivo, the acetylation of exogenous hydroxy and thiol groups is presently unknown. Acetylation of foreign compounds is in uenced by both development and genetics. Newborn mammals generally have a low level of the transferase, whereas due to the different genes involved, fast and slow acetylators have been identi ed in both rabbit and human populations. Slow acetylators are more susceptible to the effects of compounds detoxi ed by acetylation. N,O-Acyltransferase. The N -acyltransferase enzyme is believed to be involved in the carcinogenicity of arylamines. These compounds are rst N -oxidized, and then, in species capable of their N -acetylation, acetylated to arylhydroxamic acids. The effect of N ,O-transacetylation is shown in Figure 7.22. The N -acyl group of the hydroxamic acid is rst removed and is then transferred, either to an amine to yield a stable amide or to the oxygen of the hydroxylamine to yield a reactive N -acyloxyarylamine. These compounds are highly reactive in the formation of adducts with both proteins and nucleic acids, and N ,O-acyltransferase, added to the medium in the Ames test, increases the mutagenicity of compounds such as N -hydroxy-2-acetylamino uorine.
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PHASE II REACTIONS
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O ArNCCH3 OH
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Acyltransferase
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O ArNH + CH3C-enzyme OH O [ ArNHOCCH 3] [ ArNH +]
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Ar NH2
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Ar NHCCH3
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reaction with cellular nucleophiles
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N-, O-Acyltransferase reactions of arylhydroxamic acid. Ar = aryl group.
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Despite its great instability this enzyme has been puri ed from the cytosolic fraction of the rat liver.
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Amino Acid Conjugation. In the second type of acylation reaction, exogenous carboxylic acids are activated to form S-CoA derivative in a reaction involving ATP and CoA. These CoA derivatives then acylate the amino group of a variety of amino acids. Glycine and glutamate appear to be the most common acceptor of amino acids in mammals; in other organisms, other amino acids are involved. These include ornithine in reptiles and birds and taurine in sh. The activating enzyme occurs in the mitochondria and belongs to a class of enzymes known as the ATP-dependent acid: CoA ligases (AMP) but has also been known as acyl CoA synthetase and acid-activating enzyme. It appears to be identical to the intermediate chain length fatty acyl-CoA-synthetase. Two acyl-CoA: amino acid N -acyltransferases have been puri ed from liver mitochondria of cattle, Rhesus monkeys, and humans. One is a benzoyltransferase CoA that utilizes benzyl-CoA, isovaleryl-CoA, and tiglyl-CoA, but not phenylacetyl CoA, malonyl-CoA, or indolacetyl-CoA. The other is a phenylacetyl transferase that utilizes phenylacetyl-CoA and indolacetyl-CoA but is inactive toward benzoyl-CoA. Neither is speci c for glycine, as had been supposed from studies using less de ned systems; both also utilize asparagine and glutamine, although at lesser rates than glycine. Bile acids are also conjugated by a similar sequence of reactions involving a microsomal bile acid: CoA ligase and a soluble bile acid N -acyl-transferase. The latter has been extensively puri ed, and differences in acceptor amino acids, of which taurine is the most common, have been related to the evolutionary history of the species. Deacetylation. Deacetylation occurs in a number of species, but there is a large difference between species, strains, and individuals in the extent to which the reaction occurs. Because acetylation and deacetylation are catalyzed by different enzymes, the levels of which vary independently in different species, the importance of deacetylation as a xenobiotic metabolizing mechanism also varies between species. This can be seen in a comparison of the rabbit and the dog. The rabbit, which has high acetyltransferase activity and low deacetylase, excretes signi cant amounts of acetylated amines. The dog, in which the opposite situation obtains, does not. A typical substrate for the aromatic deacetylases of the liver and kidney is acetanilide, which is deacylated to yield aniline.