Limited contribution of CYP3A5 to the hepatic 6beta-hydroxylation of testosterone.

The polymorphic expression of CYP3A5 in human livers is well established, but its significance for the entire hepatic CYP3A activity is disputed. We investigated the contribution of CYP3A5 to the CYP3A activity assessed as 6beta-hydroxylation of testosterone using baculovirus-expressed CYP3A4 and CYP3A5 and microsomes isolated from 47 Caucasian human livers. Under comparable conditions, the specific activities of baculovirus-expressed CYP3A4 and CYP3A5 were nearly identical. Among human livers tested, the Vmax of 6beta-testosterone hydroxylation varied 28-fold. Of the enzymes that are capable of catalyzing 6beta-hydroxylation of testosterone (CYP3A and CYP1A1), only CYP3A4 mRNA and protein expression correlated significantly with the Vmax values (r=0.51, p<0.001 and r=0.66, p<0.001, respectively). Neither consideration of the CYP3A5 polymorphism nor combining CYP3A4 mRNA expression with the expression of other CYP3A mRNA species increased the correlation. The five livers heterozygous for the CYP3A51 allele had a mean 6beta-testosterone hydroxylation Vmax value of 2,976 pmol/mg/min, compared with 3,798 pmol/mg/min in the homozygous CYP3A53 livers. Together, these data suggest that the specific activities of CYP3A4 and CYP3A5 towards testosterone are comparable. However, the contribution of CYP3A5 to 6beta-hydroxylation of testosterone in Caucasian livers is limited, due to the much lower expression levels of CYP3A5.