Oxidation mechanism of 7-hydroxy-Delta(8)-tetrahydrocannabinol and 8-hydroxy-Delta(9)-tetrahydrocannabinol to the corresponding ketones by CYP3A11

A cDNA isolated from a C57BL/6 mouse liver cDNA library had the identical n ucleotide sequence in coding region with the mouse CYP3A11, and the NH2-ter minal sequence was also identical to that of cytochrome P450 (P450) MDX-B, a microsomal alcohol oxygenase. The COS-7 cells transfected with the CYP3A1 1 expression vector formed 7-oxo-Delta (8)-tetrahydrocannabinol (7-oxo-Delt a (8)-THC) from 7 alpha- and 7 beta -hydroxy-Delta (8)-THC. An immunologica lly related protein with P450 MDX-B was expressed in the COS-7 cell microso mes. The cell microsomes expressed CYP3A11; COS-3A11 catalyzed the oxidatio n of 7-hydroxy-Delta (8)-THC and 8-hydroxy-Delta (9)-THC to 7-oxo-Delta (8) -THC and 8-oxo-Delta (9)-THC, respectively, in a reconstituted system. O-18 derived from atmospheric oxygen was incorporated into about 30% of the cor responding ketones formed from 7 alpha -hydroxy-Delta (8)-THC and 8 beta -h ydroxy-Delta (9)-THC by mouse hepatic microsomes, P450 MDX-B, and COS-3A11, although incorporation of the stable isotope into the oxidized metabolites from 7 beta -hydroxy-Delta (8)-THC and 8 alpha -hydroxy-Delta (9)-THC was negligible. O-18, however, was not incorporated into 7-oxo-Delta (8)-THC fo rmed from 7 alpha -hydroxy-Delta (8)-THC by using cumene hydroperoxide inst ead of NADPH under O-18(2). When O-18-labeled 7 alpha -hydroxy-Delta (8)-TH C and 8 beta -hydroxy-Delta (9)-THC were incubated with above enzymes under air, about 30% of the ketones formed released O-18 from a hydroxy group at the 7 and 8 positions in the course of the oxidation. These results sugges t that 7 alpha -hydroxy-Delta (8)-THC and 8 beta -hydroxy-Delta (9)-THC may be oxidized to the corresponding ketones by CYP3A11 via a gem-diol pathway . 7 beta -Hydroxy-Delta (8)-THC and 8 alpha -hydroxy-Delta (9)-THC may be a lso converted to the ketones through a stereoselective dehydration of an en zyme-bound gem-diol rather than through a direct hydrogen extraction as a p eroxy form of the enzyme.