Elsevier

Biochemical Pharmacology

Volume 64, Issue 4, 15 August 2002, Pages 553-564
Biochemical Pharmacology

Commentary
Toxic, halogenated cysteine S-conjugates and targeting of mitochondrial enzymes of energy metabolism

https://doi.org/10.1016/S0006-2952(02)01076-6Get rights and content

Abstract

Several haloalkenes are metabolized in part to nephrotoxic cysteine S-conjugates; for example, trichloroethylene and tetrafluoroethylene are converted to S-(1,2-dichlorovinyl)-l-cysteine (DCVC) and S-(1,1,2,2-tetrafluoroethyl)-l-cysteine (TFEC), respectively. Although DCVC-induced toxicity has been investigated since the 1950s, the toxicity of TFEC and other haloalkene-derived cysteine S-conjugates has been studied more recently. Some segments of the US population are exposed to haloalkenes either through drinking water or in the workplace. Therefore, it is important to define the toxicological consequences of such exposures. Most halogenated cysteine S-conjugates are metabolized by cysteine S-conjugate β-lyases to pyruvate, ammonia, and an α-chloroenethiolate (with DCVC) or an α-difluoroalkylthiolate (with TFEC) that may eliminate halide to give a thioacyl halide, which reacts with ϵ-amino groups of lysine residues in proteins. Nine mammalian pyridoxal 5′-phosphate (PLP)-containing enzymes catalyze cysteine S-conjugate β-lyase reactions, including mitochondrial aspartate aminotransferase (mitAspAT), and mitochondrial branched-chain amino acid aminotransferase (BCATm). Most of the cysteine S-conjugate β-lyases are syncatalytically inactivated. TFEC-induced toxicity is associated with covalent modification of several mitochondrial enzymes of energy metabolism. Interestingly, the α-ketoglutarate- and branched-chain α-keto acid dehydrogenase complexes (KGDHC and BCDHC), but not the pyruvate dehydrogenase complex (PDHC), are susceptible to inactivation. mitAspAT and BCATm may form metabolons with KGDHC and BCDHC, respectively, but no PLP enzyme is known to associate with PDHC. Consequently, we hypothesize that not only do these metabolons facilitate substrate channeling, but they also facilitate toxicant channeling, thereby promoting the inactivation of proximate mitochondrial enzymes and the induction of mitochondrial dysfunction.

Section snippets

Historical

In 1916, it was reported that cattle fed soybean meal extracted with trichloroethylene developed aplastic anemia [1]. The toxic compound present in trichloroethylene-extracted soybean meal was identified as the cysteine S-conjugate DCVC [2]. DCVC induces aplastic anemia only in cattle [3], but is nephrotoxic in all experimental animals tested (for reviews, see [4], [5], [6], [7], [8], [9], [10]). Cysteine S-conjugates are intermediates in the mercapturate pathway, which was discovered over 100

Cysteine S-conjugate β-lyases

Although the mercapturate pathway can lead to detoxification, it can also result in bioactivation (i.e. formation of a metabolite that is more toxic than the parent electrophile). Such bioactivation occurs with haloalkenes and dichloroacetylene and is generally brought about by the action of cysteine S-conjugate β-lyases on the corresponding halogenated cysteine S-conjugates.

Cysteine S-conjugate β-lyases contain PLP and catalyze the biotransformation of cysteine S-conjugates to aminoacrylate [CH

Selenocysteine Se-conjugate β-lyases

Selenocysteine Se-conjugates are β-lyase/transaminase substrates of highly purified rat kidney cytGTK [23]. The compounds are also substrates of multiple cysteine S-conjugate β-lyases in human and rat kidney cytosol [23], [24]. It was suggested that selenocysteine Se-conjugates might be useful as prodrugs to target pharmacologically active selenol compounds to human kidney [24]. A flavin-containing monooxygenase in rat liver microsomes converts Se-benzyl-l-selenocysteine and S-benzyl-l-cysteine

Bioactivation by cysteine S-conjugate β-lyases

Trichloroethylene is metabolized in part to DCVC, and, as noted above, this cysteine S-conjugate is nephrotoxic. The reactive metabolites generated from toxic cysteine S-conjugates by the action of β-lyases are cytotoxic to renal epithelial cells, and their cytotoxicity is associated with covalent modification of macromolecules, depletion of non-protein thiols (such as glutathione), and initiation of lipid peroxidation [27], but other mechanisms may contribute (see below). In the kidney, the S2

Mitochondria as targets for toxic cysteine S-conjugates

Much evidence suggests that mitochondria are especially vulnerable to the toxic effects of cysteine S-conjugates [78], [79], [80], [81], [82], [83], [84], [85], [86], [87], [88], [89], [90], [91], [92]: (a) Stonard and Parker [78] showed that DCVC progressively inhibited pyruvate/malate- and α-ketoglutarate-stimulated respiration in rat liver mitochondria. Moreover, because of the progressive nature of the inhibition, Stonard and Parker suggested that a DCVC metabolite is responsible. (b) The

Mammalian PLP-dependent cysteine S-conjugate β-lyases

In 1965, Colucci and Buyske identified a thiol metabolite of benzothiazolyl 2-sulfonamide in rabbits, rats, and dogs [95]; this was likely the first description in the literature of C-S lyase activity associated with the metabolism of a xenobiotic in mammalian tissues. Later, a mammalian cysteine S-conjugate β-lyase was described that cleaved cysteine S-conjugates of drugs [96]. A cysteine S-conjugate β-lyase from an enteric bacterium (Eubacterium limosum) has been characterized [97]. This

Summary of known mammalian cysteine S-conjugate β-lyases

A summary is provided in Table 1. Because different authors have used different assay conditions and different β-lyase substrates, it is not possible to compare directly the specific activities of these enzymes. For example, all of the cysteine S-conjugate β-lyases listed in Table 1 accept TFEC and DCVC as substrates, but only some of the enzymes accept BTC. Comparisons are also complicated because of different susceptibilities to syncatalytic inactivation. For example, cytGTK is not

Mechanism of the syncatalytic inactivation of PLP enzymes by cysteine S-conjugates

It has long been known that cytAspAT is syncatalytically inactivated by β-lyase substrates, such as β-chloro-l-alanine and l-serine-O-sulfate. Originally, inactivation was attributed to attack by the aminoacrylate intermediate on a crucial amino acid residue [138]. However, evidence from Metzler’s group [139], [140] showed that inactivation of pig heart cytAspAT and bacterial glutamate decarboxylase by l-serine-O-sulfate is accompanied by formation of a PLP-pyruvate aldol product. We showed

TFEC and toxicant channeling in mitochondrial enzyme complexes

As noted above, the E2k and E3 subunits of kidney KGDHC are thioacylated, but not the E2p and E3 subunits of PDHC, after administration of TFEC to rats [91]. KGDHC activity, but not PDHC activity, is also decreased in the kidneys of TFEC-treated rats. We have noted that KGDHC is strongly inhibited in PC12 cells exposed to 1 mM TFEC, but PDHC is not directly inhibited [141]. It has been suggested that KGDHC, but not PDHC, may be in close proximity to a cysteine S-conjugate β-lyase [94]. On the

Conclusion

Nine mammalian cysteine S-conjugate β-lyases are currently known (Table 1), and no doubt the list will grow in the future. The toxicity of cysteine S-conjugates may be attributable to at least two mechanisms: syncatalytic inactivation of β-lyases may lead to loss of crucial PLP-enzyme activity. For example, incubation of PC12 cells with 1 mM TFEC leads to a time-dependent loss of mitAspAT activity [141], which could lead to disruption of the malate:aspartate shuttle and compromised energy

Acknowledgements

Work from the authors’ laboratories cited in the text was supported in part by NIH Grants ES08421 and AG14930 (A.J.L.C.), R29 GM5196 and ES07033 (S.A.B.), and ES03127 (M.W.A.).

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