Cytochrome P-450 isozymes in metabolic activation of Δ⁹-tetrahydrocannabinol by rat liver microsomes

Δ⁹-Tetrahydrocannabinol (THC) was incubated with a reconstituted system consisting of dilauroylphosphatidylcholine, NADPH-cytochrome c reductase, cytochrome b₅, and cytochrome P-450 (P-450) isozyme UT-2, UT-4, or UT-5, which was purified from liver microsomes of adult male rats. It was biotransformed by UT-2 to 11-OH-Δ⁹-THC and 3'-OH-Δ⁹-THC, and by UT-4 to 8β-OH-Δ⁹-THC and 11-OH-Δ⁹-THC. UT-5, however, showed only a little activity for 11-OH-Δ⁹-THC formation. Activity of the isozyme UT-2 for 11-OH-Δ⁹-THC formation from Δ⁹-THC was calculated to be 4.07 nmol/min/nmol P-450, while those of UT-2 for the formations of 16α-OH-testosterone (16α-OH-T), 2α-OH-T, and androstenedione from testosterone were 14.7, 6.6, and 2.2 nmol/min/nmol P-450, respectively. Anti-P-450 UT-2-IgG fraction obtained from rabbit serum dose-dependently suppressed formations of 16α-OH-T, 2α-OH-T, and androstenedione from testosterone with liver microsomes of adult male rats. The antibody, in the amount that inhibited above 90% of 16α-OH-T and 2α-OH-T formations from testosterone, also reduced 80% of the microsomal formations of 11-OH-Δ⁹-THC and 3'-OH-Δ⁹-THC from Δ⁹-THC, as compared with control experiments using preimmune IgG fraction. These results indicate that UT-2, the major male-specific P-450 isozyme catalyzing testosterone oxidation, plays the major role in the formation of active metabolites of Δ⁹-THC, such as 11-OH-Δ⁹-THC and 3'-OH-Δ⁹-THC, in adult male rats.