The sulfoxidation of the hexachlorobutadiene metabolite N-acetyl-S-(1,2,3,4,4-pentachlorobutadienyl)-L-cysteine is catalyzed by human cytochrome P450 3A enzymes.

The sulfoxidation of the mercapturic acid N-acetyl-S-(1,2,3,4,4-pentachlorobuta-1,3-dienyl)-L-cysteine (N-Ac-PCBC), a urinary metabolite of the renal toxin hexachlorobutadiene (HCBD), was studied in human liver microsomes and with purified cDNA expressed human liver cytochrome P450 (P450) enzymes. N-Acetyl-S-(1,2,3,4,4-pentachlorobuta-1,3-dienyl)-L-cysteine sulfoxide (N-Ac-PCBC SO) is a major urinary metabolite of HCBD in male rats; only liver microsomes from male rats catalyze the sulfoxidation of N-Ac-PCBC. Our results presented here show that human liver microsomes from both male and female donors are capable of oxidizing N-Ac-PCBC to the corresponding sulfoxide diastereomers. The correlation of N-Ac-PCBC sulfoxidation with the rates of oxidation of P450 enzyme specific substrates suggests that only P450 3A enzymes oxidize N-Ac-PCBC. Moreover, only gestodene and troleandomycin, two selective inhibitors for P450 of the 3A family, significantly reduced the rates of N-Ac-PCBC sulfoxidation. No reduction in sulfoxidation rates was observed with inhibitors for other P450 enzymes, i.e., diethyldithiocarbamate, 4-methylpyrazole, 7,8-benzoflavone, or sulfaphenazole. Incubations of N-Ac-PCBC with purified and reconstituted recombinant P450s 1A2, 2E1, 3A4, and 3A5 resulted in sulfoxide formation only with P450s 3A4 and 3A5. In summary, these results indicate that P450 from the 3A family may sulfoxidize N-Ac-PCBC. Since these P450 enzymes account for a major fraction of the P450 in human liver and are also present in human kidney, the sulfoxidation reaction may also be expected to occur in humans exposed to HCBD.