Cysteine Conjugate -Lyase

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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CYSTATHIONINE /3-LYASE CYSTATHIONINE /3-SYNTHASE CYSTEINE CONJUGATE /3-LYASE CYSTEINE LYASE... 

MacFarlane M, Foster JR, Gibson GG, et al. 1989. Cysteine conjugate [3-lyase of rat kidney cytosol Characterization, immunocytochemical localization, and correlation with hexachlorobutadiene nephrotoxicity. Toxicol AppI Pharmacol 98 185-197. 

Cysteine conjugate /3-lyases cleave the carbon-sulfur bond of S-substituted L-cysteine conjugates via a -elimination reaction to produce pyruvate, ammonia, and the corresponding thiols. These enzymes are found in kidney, liver... 

Anders and co-workers proposed a-thiolactones (thiiranones) as intermediates in the cysteine conjugate, /3-lyase-catalyzed bioactivation of bromine-containing cysteine S-conjugates <1995JA9590>. The proposed intermediates... 

Kharash, E. D., G. M. Hoffman, D. Thoming, D. C. Hankins, and C. G. Kilty. 1998. Role of renal cysteine conjugate (3-lyase pathway in inhaled compound A nephrotoxicity in rats. Anesthesiology 88 1624-1633. 

Dekant W, Vamvakas S, Berthold K, et al. 1986c. Bacterial P-lyase mediated cleavage and mutagenicity of cysteine conjugates derived from the nephrocarcinogenic alkenes trichloroethylene, tetrachloroethylene, and hexachlorobutadiene. Chem Biol Interact 60 31-45. 

Cysteine conjugates can also be degraded by the enzyme cysteine conjugate 0-lyase, which involves pyridoxal as a cofactor as shown in Figure 7.19 (26). 

FIGURE 7.19 Metabolism of a cysteine conjugate mediated by cysteine conjugate 0-lyase. 

Jones TW, Chen Q, Schaeffer V, et al. 1988. Immunohistochemical localization of glutamine transaminase K, a rat kidney cysteine conjugate p-lyase, and the relationship to the segment specificity of cysteine conjugate nephrotoxicity. Mai Pharm 34 621-627. 

Pentachlorothiophenol was formed Iji vitro from S-(PCP)Cys by a C-S lyase enzyme from onion root. This enzyme was active with S-(PCP)Cys, S-(2,M-dinitrophenyl)cysteine, and the cysteine conjugate of propachlor. A C-S lyase from Alblzzla lophanta was previously shown to utilize a broad range of cysteine derivatives (2U). 

Evidence was also provided that Insoluble residues may be produced from GSH conjugates via cysteine conjugate or thiol Intermediates. These studies also suggested that certain reactions should be studied in greater detail to assess their importance in pesticide metabolism l.e., the C-S lyase reaction, the methyl transferase reaction, and the transamination reaction. 

As in the case of propachlor mercapturic acid sulfoxide, the biological significance of xenobiotic mercapturic acids that contain oxidized sulfur is not known. Casida et al. (39) have reported that sulfoxidation of some thiocarbamate herbicides is a beneficial step in the detoxication process. However, cysteine conjugates can exhibit adverse biological activities. Smith (40) has reviewed work on the metabolism of the toxic principle in kale and has shown that C-S lyase action on S-methylcysteine sulfoxide produces the toxic principle. Virtanen ( ) has reviewed the processes in other plants that lead to the production of compounds with biological activity from -substituted cysteine sulfoxides. 

Furfiirylthiol is a key flavour especially for coffee, beef and roast-like food aromas. It was synthesised in concentrations of up to 1 g L using /1-lyase activity of whole bacterial cells, e.g. Enterobacter cloacae or Eubacterium limosum [256] (Scheme 23.21). Resting cells were used to cleave the sulfur-carbon bond of a furfural-cysteine conjugate and an XAD-4 resin connected to the gas outlet... 

Figure 4.66 Metabolism of a cysteine conjugate by CS lyase (p-lyase). The cysteine conjugate is shown arising from a glutathione conjugate after biliary excretion. The thiol product, which may be toxic (see text) can also be methylated and further oxidized as shown.

Thus, the nephrotoxicity and covalent binding of metabolites to renal protein can be reduced by treating animals with the organic acid probenecid, which competes with the cysteine conjugate for the active uptake system. The cytotoxicity of the GSH conjugate of HCBD in vitro is reduced by inhibitors of (3-glutamyltransferase and (3-lyase, indicating that both these enzymes are essential for the toxicity. 

Hexachlorobutadiene is a nephrotoxic industrial chemical, damaging the pars recta of the proximal tubule. Initial conjugation with GSH is necessary, followed by biliary secretion and catabolism resulting in a cysteine conjugate. The conjugate is reabsorbed and transported to the kidney where it can be concentrated and becomes a substrate for the enzyme p-lyase. This metabolizes it into a reactive thiol, which may react with proteins and other critical macro molecules with mitochondria as the ultimate target. The kidney is sensitive because the metabolite is concentrated by active uptake processes (e.g., OAT 1), which reabsorb the metabolite into the tubular cells. 

Other halogenated compounds such as trichloroethylene may be metabolized to similar cysteine conjugates, which are also nephrotoxic but may not all require p-lyase activation. 

Tetrafluoroethylene is metabolized by hepatic glutathione. S -transferase and the resulting cysteine conjugate is further metabolized by renal P-lyase. This pathway results in the formation of a reactive thiol that causes kidney toxicity in rats. 

Dekant, W., Berthold, K., Vamavkas, S., Henschler, D. Anders, M.W. (1988) Thioacylating intermediates as metabolites of S-(l, 2-dichlorovinyl)-L-cysteine and 6 -(l,2,2-trichlorovinyl)-L-cysteine formed by cysteine conjugate P-lyase. Chem. Res. Toxicol., 1, 175-178... 

The nephrotoxic mode of action of tetrafluoroethylene is similar to that of hexachloro-butadiene. It is first metabolized to a cysteine conjugate, which is metabolized by cysteine conjugate /l-lyase to a reactive product that can bind to cellular macromolecules. 

This activity has been demonstrated in mammalian liver and kidney, as well as in intestinal bacteria. Glutathione S-transle rases catalyze the formation of glutathione conjugates, which are processed via the mercapturic biosynthetic pathway, to acety-lated cysteine -conjugates. The unacetylated premercapturic acids are substrates for cysteine 5-conjugate P-lyase, whereas the acetylated cysteine -conjugates, the mercapturic acids, do not function as substrates for the enzymes. 

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