Abstract

The human liver enzyme microsomal alcohol oxygenase was able to oxidize both 7α- and 7β-hydroxy-Δ⁸-tetrahydrocannabinol (7α- and 7β-hydroxy-Δ⁸-THC) to 7-oxo-Δ⁸-THC. The oxidative activity was determined by using a panel of 12 individual cDNA-expressed human cytochrome P450s (CYPs) (1A1, 1A2, 2A6, 2B6, 2C8, 2C9-Arg, 2C9-Cys, 2C19, 2D6-Met, 2D6-Val, 2E1 and 3A4). Among the CYP isoforms examined, CYP3A4 showed the highest activity for both of substrates. The metabolism of 7α- and 7β-hydroxy-Δ⁸-THC to 7-oxo-Δ⁸-THC was also detected for CYPs 1A1 (4.8% of CYP3A4), 1A2 (4.7%), 2A6 (2.3%), 2C8 (16.6%), and 2C9-Cys (5.4%), and CYPs 1A1 (0.4%), 2C8 (1.3%), 2C9-Arg (4.3%), and 2C9-Cys (0.9%), respectively. The 7α- and 7β-hydroxy-Δ⁸-THC microsomal alcohol oxygenase activities in human liver were significantly inhibited by addition of 100 μM troleandomycin, 1 μM ketoconazole, and anti-CYP3A antibody, although these activities were not inhibited by 1 μM 7,8-benzoflavone and 50 μM sulfaphenazole. When the substrates were incubated with the CYP3A4-expressed microsomes under oxygen-18 gas phase, atmospheric oxygen was incorporated into 35% of 7-oxo-Δ⁸-THC formed from 7α-OH-Δ⁸-THC, but only 12% of 7-oxo-Δ⁸-THC formed from 7β-OH-Δ⁸-THC. These results indicate that CYP3A4 is a major isoform responsible for the oxidation of 7α- and 7β-hydroxy-Δ⁸-THC to 7-oxo-Δ⁸-THC in liver microsomes of humans, although the oxidation mechanisms for 7α- and 7β-hydroxy-Δ⁸-THC might be different.