Abstract
The purpose of this study was to evaluate a contribution of polymorphic cytochrome P450 (CYP) 3A5 to the oxidation of diltiazem, midazolam and testosterone by liver microsomes from Japanese subjects. Twenty-seven liver samples were classified into three groups according to the CYP3A5 genotypes; CYP3A5*1/*1 (n=3), *1/*3 (n=12) and *3/*3 (n=12). The results of genotyping and immunochemical quantitation of CYP3A5 protein showed a good accordance between the CYP3A5 genotype and CYP3A5 content but not CYP3A4 content in liver microsomes. The expression levels of hepatic CYP3A5 protein ranged from 20 to 60% of the sum of CYP3A4 and CYP3A5 contents in subjects with at least one wild type allele (*1). The CYP3A5 contents correlated well with liver microsomal activities of diltiazem N-demethylation, midazolam 1′- and 4-hydroxylations and testosterone 6β-hydroxylation among subjects carrying at least one *1 allele. In addition, the correlation coefficients of CYP3A5 contents with the rates of diltiazem N-demethylation, midazolam 1′-hydroxylation and testosterone 6β- hydroxylation were higher than those of CYP3A4, although the value of CYP3A5 with the midazolam 4-hydroxylation rate was similar to that of CYP3A4. Kinetic analyses revealed a biphasic diltiazem N-demethylation in liver microsomes from subjects carrying the *1 allele. The apparent Vmax/Km values for recombinant CYP3A5 indicated the greater contributions to diltiazem N-demethylation and midazolam 1′-hydroxylation as compared with CYP3A4. These results suggest that polymorphic CYP3A5 contributes markedly to the drug oxidations, particularly diltiazem N-demethylation, midazolam 1′- hydroxylation and testosterone 6β-hydroxylation by liver microsomes from Japanese subjects.
