Abstract

The capacities to inhibit coumarin 7-hydroxylase activity of human cytochrome P450 2A6 (CYP2A6) by organosulfur compounds were evaluated. Five dialkyl sulfides and five dialkyl disulfides, with alkyl chains from methyl to amyl, were examined. In addition to these chemicals, diallyl sulfide, diallyl disulfide, allyl methyl sulfide, allyln-propyl sulfide, allyl phenyl sulfide, diphenyl sulfide, diphenyl disulfide, difurfuryl disulfide, phenyl cyclopropyl sulfide, 2,2′-dipyridyl disulfide, 4,4′-dipyridyl sulfide, and 4,4′-dipyridyl disulfide were also examined for their capacity to inhibit CYP2A6. The membrane fraction of genetically engineeredEscherichia coli cells expressing CYP2A6 together with NADPH-cytochrome P450 reductase was used as an enzyme source. Dialkyl disulfides inhibited CYP2A6 more strongly than did dialkyl sulfides. Among dialkyl disulfides examined, di-n-propyl disulfide, contained in onion oil, was the most potent competitive inhibitor of CYP2A6, with a K_i value of 1.73 μM. Diallyl disulfide, present in garlic oil, inhibited CYP2A6 activity in a competitive/noncompetitive mixed manner, with theK_i value of 2.13 μM. Among all of the organosulfur compounds tested, 4,4′-dipyridyl disulfide was the most potent inhibitor of CYP2A6, with a K_i value of 60 nM, followed by 4,4′-dipyridyl sulfide, with aK_i value of 72 nM. These chemicals inhibited CYP2A6 in a competitive manner. The preincubation time did not affect the inhibitory effects of di-n-propyl disulfide, diallyl disulfide, 4,4′-dipyridyl disulfide, and 4,4′-dipyridyl sulfide on CYP2A6, indicating that these chemicals were not mechanism-based inhibitors of CYP2A6. 4,4′-Dipyridyl disulfide also inhibited midazolam 1′-hydroxylase activity of CYP3A4. We discovered 4,4′-dipyridyl disulfide to be a potent and relatively selective inhibitor of CYP2A6.