Thioester Tissue

An important drug in the present context is the mineralocorticoid receptor antagonist spironolactone (7.74, Fig. 7.12). Among its many metabolic reactions, spironolactone is readily hydrolyzed at the thioester bond (Fig. 7.12, Reaction a) to form deacetyl-spironolactone (7.75, Fig. 7.12), a metabolite found in a variety of tissues [155 -157]. This thiol compound, which is also a potent mineralocorticoid antagonist, promotes the mechanism-based inactivation of hepatic, adrenal, and testicular cytochrome P450 isozymes. There is now good evidence to indicate that this behavior is the result of microsomal 5-oxidation (see Chapt. 7 in [7]). When spironolactone was incubated with liver microsomes from rats pretreated with dexamethasone (an inducer of CYP3A), the sulfinic and sulfonic acid derivatives were characterized [158]. Perhaps the importance of the 5-deacetylation of spironolactone... 

Bach R, Konigsberg W, Nemerson Y Human tissue factor contains thioester linked palmitate and stearate on the cytoplasmic half cystine. Biochemistry 1988 27 4227-423 I. 

Biotin is the coenzyme in a small number of carboxylation reactions in mammalian metabolism and some decarboxylation and transcarboxylation reactions in bacteria. Although the biotin-dependent enzymes are cytosolic and mitochondrial, about 25% of tissue biotin is found in the nucleus, much of it bound as thioesters to histones. Biotin has two noncoenzyme functions induction of enzyme synthesis and regulation of the cell cycle. 

An unique example of stereoisomer selective tissue uptake of stereoisomers is noted with NSAIDs. The R enantiomer of ibuprofen shows preferential uptake into fat following the administration of the racemate and individual isomersJ However, this apparent difference in fat distribution is probably a consequence of the selective metabolic uptake and formation of the coenzyme A thioester of the R isomer, which does not occur with the S isomer. 

Tanaka, T., McRae, B.J., Cho, K. et al. (1983). Mammalian tissue trypsin-like enzymes. Comparative reactivities of human skin tryptase, human lung tryptase, and bovine trypsin with peptide 4-nitroanilide and thioester substrates. J. Biol. Chem. 258, 13552-13557. 

Since MIC is highly reactive, it is not metabolized in the classical sense. Conjugation of MIC with glutathione (GSH) forming an adduct S-(N-methyl-carbamoyl) glutathione, and corresponding cysteine adduct, S-(N-methylcarbamoyl) cysteine appears to represent an important pathway of biotransformation of MIC in the rats exposed to MIC intraper-itoneally. The reaction of MIC with GSH and with cysteine is reversible, and can provide a source of free MIC in the tissues. It is speculated that these carbamate thioester conjugates of MIC may actually contribute to toxic effects of MIC. Similar studies in experimental animals exposed to MIC by the inhalation route have not been reported. 

Fatty acids released from adipose tissue are the source of ketone bodies (Figure 22-22). Fasting, high levels of glucagon and catecholamine, and a low level of insulin result in rapid lipolysis and ready availability of fatty acids. Fatty acids, after being converted to CoA thioesters, are oxidized in the mitochondria. The rate-limiting step in the oxidation process is the transport of fatty acyl-CoA... 

Presently, other captopril analogs, hke zofenopril (a prodrug of which the cleavage of the thioester led the active compound zofenoprilat) are still marketed. They are able to coordinate Zn(II) and act as free radical scavenger. Their lipophihcity allows them to attain high lever in heart tissue and then they can be used as cardioprotective drugs (Figure 20.42). ... 

This mitochondrial peroxisome enzyme, alpha-meth-ylacyl-CoA racemase (AMACR) (encoded by the gene P504S) catalyzes the racemization of alpha-methyl branched carboxylic coenzyme A thioesters and is present in prostate tissue and a wide variety of carcinomas (colorectal, ovarian, breast, bladder, lung, renal cell), melanoma, and lymphoma.AMACR is useful in prostate needle biopsies when the differential diagnosis... 

It is unclear how the synthesis of predominantly short-chain fatty acids occurs, especially as the fatty acid synthase from the uropygial gland appears to have the same characteristics as in other tissues. During the biosynthesis of fatty acids from malonyl-CoA and methylmalonyl-CoA by the fatty add synthase, the growing chain is attached to the pantotheine group of the multienzyme polypeptide by a thioester link. One of the enzyme activities associated with a domain of the multienzyme polypeptide is the thioesterase (reaction 4.4), which has a specificity for cleaving thioesters of longer chain fatty acids esterified to the acyl carrier protein (ACP). 

These results were recently confirmed by Roughan and Slack (1977). The wide distribution of acetyl-CoA synthetase in leaf, seed, and tuber tissue coupled with the presence of an acetyl-CoA hydrolase suggests that these two enzymes may be involved in the transport of acetate from a site of its synthesis (namely, the mitochondria) in a form that is essentially metaboli-cally inert (as acetate) to a site where it is effectively converted and utilized as a thioester derivative. 

All tissues are capable of forming coenzyme A from pantothenic acid. CoA functions as the carrier of fatty acids, as thioesters, in mitochondrial P-oxidation (section 5.5.2). The resultant two-carbon fragments, as acetyl CoA, then undergo oxidation in the citric acid cycle (section 5.4.4). CoA also functions as a carrier in the transfer of acetyl (and other fatty acyl) moieties in a variety of biosynthetic and catabolic reactions, including ... 

Because of the importance of coenzyme A and its acyl thioesters in metabolic control, several enzymatic assays in conjunction with spectrometric or fluorometric monitoring have been developed to determine their total content in tissues (5). 

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