Identification of the human and rat P450 enzymes responsible for the sulfoxidation of S-methyl N,N-diethylthiolcarbamate (DETC-ME). The terminal step in the bioactivation of disulfiram.

The present study investigated the role of rat and human cytochrome P450 enzymes in the sulfoxidation of S-methyl N,N-diethylthiolcarbamate (DETC-Me) to S-methyl N,N-diathylthiolcarbamate sulfoxide (DETC-Me sulfoxide), the putative active metabolite of disulfiram. DETC-Me sulfoxidation by microsomes from male and female rats treated with various cytochrome P450-enzyme inducers suggested that multiple enzymes can catalyze this reaction, and these include, CYP1A1/2, CYP2B1/2, and CYP3A1/2. All cDNA-expressed human cytochrome P450 enzymes examined catalyzed the sulfoxidation of DETC-Me. The turnover rates (min-1) of DETC-Me sulfoxidation by the cDNA-expressed cytochrome P450 enzymes ranked as follows: CYP3A4 > CYP2A6 = CYP2C9 > CYP1A2 > CYP2B6 = CYP2E1 > CYP1A1 > CYP2D6. Interestingly, CYP3A4 ranked first or last, depending on whether or not additional NADPH-cytochrome P450 reductase was coexpressed in the lymphoblastoid cells. This complicated estimates of the contribution of CYP3A4 to DETC-Me sulfoxidation by human liver microsomes. The sample-to-sample variation in DETC-Me sulfoxidation by bank of human liver microsomes (N=13) correlated highly with coumarin 7-hydroxylation (r=0.88) and testosterone 6beta-hydroxylation (r=0.90), suggesting that CYP2A6 and CYP3A4/5 contribute to the sulfoxidation of DETC-Me by human liver microsomes. Although, chlorzoxazone 6-hydroxylation (a marker for CYP2E1) correlated poorly with DETC-Me sulfoxidation, the correlation improved from r=0.07 to r=0.44 when DETC-Me sulfoxidation was studied in the presence of the CYP2A6 inhibitor, coumarin. Similarly, when DETC-Me sulfoxidation was studied in the presence of diethyldithiocarbamate (DDTC), the inhibited DETC-Me sulfoxidase activity correlated better (r=0.50) with chlorzoxazone 6-hydroxylase, compared with DETC-Me sulfoxidase activity in the absence of DDTC (r=0.09). Polyclonal antibodies against CYP2E1 caused a modest inhibition (30%) of DETC-Me sulfoxidation by human liver microsomes. Anti-CYP3A1 antibodies completely inhibited DETC-Me sulfoxidation by cDNA-expressed CYP3A4. Under similar conditions, DETC-Me sulfoxidation by human liver microsomes was only partially inhibited by anti-CYP3A1 antibodies. Although studies with the rat and cDNA-expressed cytochrome P450 enzymes suggested that CYP1A2 contributed to DETC-Me sulfoxidation, this reaction was not inhibited by either furafylline ( a mechanism-based inhibitor of CYP1A2) or antibodies against CYP1A1/2. A significant role for CYP2C9 was excluded by the inability of sulfaphenazole to inhibit the sulfoxidation of DETC-Me by human liver microsomes. Collectively, these data suggest that multiple cytochrome P450 enzymes can catalyze the sulfoxidation of DETC-Me. In human liver microsomes the CYP2A6, CYP2E1, and CYP3A4/5 all contribute significantly to the sulfoxidation of DETC-Me. It is interesting to note that DDTC, the reduced metabolite of disulfiram, is known to inhibit these same enzymes. The ability of DDTC to block the formation of DETC-Me sulfoxide may explain why the dose of disulfiram required to produce a disulfiram-ethanol reaction in alcoholics is so variable and often inadequate.