Flavin-containing monooxygenase-dependent stereoselective S-oxygenation and cytotoxicity of cysteine S-conjugates and mercapturates

The metabolism of cysteine S-conjugates of both cis- and trans-1,3-dichloropropene in the presence of rat kidney microsomes and purified flavin-containing monooxygenase from hog liver was investigated in vitro. Preliminary studies with isolated rat kidney cells demonstrated that cysteine S-conjugates were quite toxic to the cells in a process which was consistent with a role of the flavin-containing monooxygenase in the bioactivation of the nephrotoxins. Putative S-oxide metabolites of cysteine S-conjugates were chemically synthesized, and diastereomers were separated and identified by spectroscopic means. The metabolic products of cysteine S-conjugates were identified by comparing the chemical properties of the metabolites with authentic synthetic cysteine S-conjugate S-oxides. Surprisingly, S-conjugate S-oxygenase activity was not observed with rat kidney microsomes but was present when cysteine S-conjugates were incubated with the highly purified flavin-containing monooxygenase from hog liver. The kinetic parameters indicated that considerable S-oxygenase stereoselectivity and structural selectivity was observed: cis cysteine S-conjugates were preferred substrates and N-acetylation of cysteine S-conjugates decreased substrate activity. S-Oxygenation was considerably diastereoselective and diastereoselectivity was much greater for cysteine S-conjugates with higher Vmax values. Cysteine S-conjugate S-oxides were not indefinitely stable, and under certain conditions, the S-oxides underwent a [2,3]-sigmatropic rearrangement to acrolein. Formation of acrolein or other electrophilic products from S-(chloropropenyl)cysteine conjugate S-oxides may contribute to the renal effects observed for S-(chloropropenyl)cysteine conjugates.