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

Abstract

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.

Abundant Rodent Furan-Derived Urinary Metabolites Are Associated with Tobacco Smoke Exposure in Humans

“…This evidence supports the hypothesis that sulfoxide 3 detected in urine is formed enzymatically and not as a result of air oxidation. Cytochrome P450 and flavin-containing monooxygenase (FMO) are two enzymes capable of oxidizing mercapturic acids to their sulfoxides, with CYP3A enzymes being the most active. − …”

Biotransformation, Excretion, and Nephrotoxicity of the Hexachlorobutadiene Metabolite (E)-N-Acetyl-S-(1,2,3,4,4-pentachlorobutadienyl)-l-cysteine Sulfoxide

“…Previous studies revealed a discrepancy in the formation and/or in the handling of N-AcPCBC-SO in rats: In incubations of N-AcPCBC with liver microsomes two diastereomers, (R)-and (S)-N-AcPCBC-SO, were formed in a 1:1 molar ratio by rat P450 3A 1/2 and human P450 3A 4/5, but after administration of HCBD to male rats only one diastereomer was found to be excreted with urine (8,9,11). To elucidate the molecular mechanism for this discrepancy, a 1:1 mixture of (R)-and (S)-N-AcPCBC-SO was given to male and female Wistar rats.…”

“…A structurally related sulfoxide derived from S-(1,2-dichlorovinyl)-L-cysteine was also shown to exhibit β-lyase-independent toxicity (10). A discrepancy, however, having not been clarified yet, was observed in both experiments (8,9,11): In incubations of N-AcPCBC with rat liver microsomes two diastereomers (R)-and (S)-N-AcPCBC-SO were formed in a nearly 1:1 molar ratio by P450 3A 1/2, but after administration of HCBD to male rats only one diastereomer was found to be excreted with urine.…”

Sex-related differences in renal toxicodynamics in rodents

“…Covalent binding of this reactive metabolite to macromolecules results in toxicity [133,134]. A sex-specific metabolite, N-acetyl-S-(1,2,3,4,4-pentachlorobutadienyl)-L-cysteine (N-ac-PCBC) sulfoxide, was identified in the urine of male rats treated with HCBD [43] with an apparent role of the male-specific CYP3A2 as catalyst of sulfoxidation [135]; the sulfoxidation of N-ac-PCBC is also catalyzed by human CYP3A enzymes [136].…”