Role of human hepatic cytochrome P-450s in territrem A metabolism

The ability of human liver microsomal preparations (HLM1, 2, 3, and 5), microsomes from human lymphoblasts expressing different cytochrome P-450 (CYP450) isoforms, and CYP3A4 cDNA-transfected V79 Chinese hamster cells to metabolize territrem A (TRA) was studied. The only metabolite generated by any of these preparations was 6beta-hydroxymethyl-6beta-demethylterritrem A (MA(1)). MA(1) formation was observed with all four human liver microsomal samples. Of the eight microsomal preparations from human lymphoblasts expressing different cytochrome P-450 enzymes (1A2, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1, and 3A4) examined, only those expressing CYP2C9, CYP2D6, or CYP3A4 metabolized TRA, with that expressing CYP3A4 being the most active. No TRA metabolites were formed by control V79MZ cells, but MA(1) was formed by CYP3A4 cDNA-transfected V79 Chinese hamster cells. In order to investigate which CYP450 isoforms were involved in MA(1) formation in the human liver microsomal preparations, the effects of six isoform-specific chemical inhibitors (furafylline, sulfaphenazole, omeprazole, quinidine, ketaconazole, and diethyldithiocarbamate) and anti-3A4, anti-2C9, and anti-2D6 antibodies on TRA metabolism by HLM2 and HLM5 were examined. MA(1) formation was markedly inhibited by ketaconazole, with quinidine and sulfaphenazole having less of an effect. Anti-CYP3A4 antibody markedly inhibited MA(1) formation, while antibodies against CYP2C9 or CYP2D6 had little effect. The amount of MA(1) formed using different HLM preparations was related to the 6beta-testosterone hydroxylase activity and CYP3A4 protein content of the preparations. These results suggest that CYP3A4 is the major enzyme involved in TRA metabolism by human liver microsomes, with CYP2C9 and CYP2D6 playing a minor role.