Enzymatic transformation of mercapturic acids derived from halogenated alkenes to reactive and mutagenic intermediates.
The metabolism of the mercapturic acids S-pentachlorobutadienyl-N-acetylcysteine (N-Ac-PCBC), S-trichlorovinyl-N-acetylcysteine (N-Ac-TCVC) and S-dichlorovinyl-N-acetylcysteine (N-Ac-DCVC) by subcellular fractions from male rat liver and kidney homogenates was studied. As a model compound, N-Ac-PCBC, 14C labelled, was synthesised. It was intensively metabolised by cytosolic but not by microsomal enzymes from rat liver and kidney. The major metabolite identified by GC/MS was pentachlorobutadienylcysteine, the amount produced being highest in kidney cytosol. Metabolic conversion of 14C-N-Ac-PCBC by kidney and liver cytosol resulted in covalent binding of radioactivity to protein, binding was strongly inhibited by the beta-lyase inhibitor aminooxyacetic acid (AOAA). N-Ac-TCVC and N-Ac-DCVC were also transformed by cytosolic enzymes to the corresponding cysteine conjugates (trichlorovinylcysteine and dichlorovinylcysteine). The three mercapturic acids tested were strong mutagens in the Ames-test after addition of rat kidney cytosol. In the absence of cytosol, N-Ac-TCVC and N-Ac-DCVC were weakly but definitely mutagenic, whereas N-Ac-PCBC was not. In contrast to N-Ac-PCBC, the "direct" mutagens N-Ac-TCVC and N-Ac-DCVC were both transformed to pyruvate by bacterial (S. typhimurium TA100) homogenate 100,000 g supernatants. It is concluded that mercapturic acids are deacetylated to the corresponding cysteine conjugates by cytosolic (N-Ac-PCBC, N-Ac-TCVC and N-Ac-DCVC) and bacterial enzymes (N-Ac-TCVC and N-Ac-DCVC) and further cleaved to reactive and mutagenic intermediates by mammalian and/or bacterial beta-lyase. The observed activation mechanisms for the mercapturic acids, whose formation from hexachlorobutadiene, tetrachloroethylene and trichloroethylene has been proven, might contribute to the nephrotoxicity and nephrocarcinogenicity of the parent alkenes.
Vamvakas
S
S
Dekant
W
W
Berthold
K
K
Schmidt
S
S
Wild
D
D
Henschler
D
D
D000107:Acetylation; D000111:Acetylcysteine / Q000031:analogs & derivatives* / Q000378:metabolism; D000475:Alkenes / Q000378:metabolism*; D000818:Animals; D001426:Bacterial Proteins / Q000378:metabolism; D001711:Biotransformation; D002070:Butadienes; D013437:Carbon-Sulfur Lyases; D003600:Cytosol / Q000378:metabolism; D007668:Kidney / Q000378:metabolism; D008099:Liver / Q000378:metabolism; D008190:Lyases / Q000378:metabolism; D008297:Male; D009152:Mutagenicity Tests; D051381:Rats; D012486:Salmonella typhimurium / Q000187:drug effects; D013347:Subcellular Fractions / Q000378:metabolism
D000475:Alkenes; D001426:Bacterial Proteins; D002070:Butadienes; C049705:S-1,2-dichlorovinyl-N-acetylcysteine; C053445:S-trichlorovinyl-N-acetylcysteine; C049739:N-acetyl-S-pentachloro-1,3-butadienylcysteine; D008190:Lyases; D013437:Carbon-Sulfur Lyases; C019969:S-alkylcysteine lyase; D000111:Acetylcysteine