Abstract
The metabolism of testosterone to 6β-hydroxytestosterone (6β-OH-T) is a commonly used assay to evaluate human CYP3A enzyme activities. However, previous reports have indicated that CYP3A7 also produces 2α-hydroxytestosterone (2α-OH-T) and that a 2α-OH-T/6β-OH-T ratio may be a unique endogenous biomarker of the activity of the enzyme. Until now, the full metabolite and kinetic profile for testosterone hydroxylation by CYP3A7 has not been fully examined. To this end, we performed a complete kinetic analysis of the 6β-OH-T, 2α-OH-T, and 2β-hydroxytestosterone metabolites for recombinant Supersome CYP3A4, CYP3A5, and CYP3A7 enzymes and monitored metabolism in fetal and adult human liver microsomes for comparison. In general, a decrease in the velocity of the reaction was observed between CYP3A4 and the two other enzymes, with CYP3A7 showing the lowest metabolic capacity. Interestingly, we found that the 2α-OH-T/6β-OH-T ratio varied with substrate concentration when testosterone was incubated with CYP3A7, suggesting that this ratio would likely not function well as a biomarker for CYP3A7 activity. In silico docking studies revealed at least two different binding modes for testosterone between CYP3A4 and CYP3A7. In CYP3A4, the most energetically favorable docking mode places testosterone in a position with the methyl groups directed toward the heme iron, which is more favorable for oxidation at C6β, whereas for CYP3A7 the testosterone methyl groups are positioned away from the heme, which is more favorable for an oxidation event at C2α In conclusion, our data indicate an alternative binding mode for testosterone in CYP3A7 that favors the 2α-hydroxylation, suggesting significant structural differences in its active site compared with CYP3A4/5.