Cytochrome P450 2E1 is the primary enzyme responsible for low-dose carbon tetrachloride metabolism in human liver microsomes

Abstract

We examined which human CYP450 forms contribute to carbon tetrachloride (CCl 4) bioactivation using hepatic microsomes, heterologously expressed enzymes, inhibitory antibodies and selective chemical inhibitors. CCl 4 metabolism was determined by measuring chloroform formation under anaerobic conditions. Pooled human microsomes metabolized CCl 4 with a K m of 57 μM and a V max of 2.3 nmol CHCl 3/min/mg protein. Expressed CYP2E1 metabolized CCl 4 with a K m of 1.9 μM and a V max of 8.9 nmol CHCl 3/min/nmol CYP2E1. At 17 μM CCl 4, a monoclonal CYP2E1 antibody inhibited 64, 74 and 83% of the total CCl 4 metabolism in three separate human microsomal samples, indicating that at low CCl 4 concentrations, CYP2E1 was the primary enzyme responsible for CCl 4 metabolism. At 530 μM CCl 4, anti-CYP2E1 inhibited 36, 51 and 75% of the total CCl 4 metabolism, suggesting that other CYP450s may have a significant role in CCl 4 metabolism at this concentration. Tests with expressed CYP2B6 and inhibitory CYP2B6 antibodies suggested that this form did not contribute significantly to CCl 4 metabolism. Effects of the CYP450 inhibitors α-naphthoflavone (CYP1A), sulfaphenazole (CYP2C9) and clotrimazole (CYP3A) were examined in the liver microsome sample that was inhibited only 36% by anti-CYP2E1 at 530 μM CCl 4. Clotrimazole inhibited CCl 4 metabolism by 23% but the other chemical inhibitors were without significant effect. Overall, these data suggest that CYP2E1 is the major human enzyme responsible for CCl 4 bioactivation at lower, environmentally relevant levels. At higher CCl 4 levels, CYP3A and possibly other CYP450 forms may contribute to CCl 4 metabolism.