2005 年 28 巻 4 号 p. 646-651
The NADH-dependent activity by hepatic microsomes of Japanese monkeys for 7-oxo-Δ8-tetrahydrocannabinol (7-oxo-Δ8-THC) formation from 7β-hydroxy-Δ8-THC exhibited about 70% of the NADPH-dependent activity (100%) at the substrate concentration of 72.7 μM, although NADPH was an obligatory cofactor for maximal activity. Both NADH- and NADPH-dependent activities were significantly inhibited by the typical P450 inhibitors, such as SKF525-A and metyrapone. Both activities were almost completely inhibited by the NADPH-P450 reductase inhibitor diphenyliodonium chloride. The ratio of NADH- and NADPH-dependent activities varied significantly according to the substrate concentration. Interestingly, the NADH-dependent activity was higher than that of NADPH at low substrate concentrations of 13—50 μM. The ratio was also affected by the cofactor concentration. In the reconstituted system of CYP3A8 purified from hepatic microsomes of Japanese monkeys as a major enzyme responsible for the NADPH-dependent oxidation, NADH as well as NADPH could sustain the oxidation of 7β-hydroxy-Δ8-THC to the corresponding ketone. The NADH-dependent oxidation of 7β-hydroxy-Δ8-THC by monkey livers is mainly catalyzed by CYP3A8 as well as the NADPH-dependent oxidation. These results indicate that NADH as a cofactor may be also useful for the oxidation of 7β-hydroxy-Δ8-THC, and that the cofactor requirement for the reaction is varied by the concentrations of substrate and/or cofactor.
