Cytochrome P450 3A4, the most abundant P450 form in human liver, exhibits a very broad substrate specificity and is of great importance for drug metabolism. The interindividual difference in the hepatic expression of CYP3A4 is considerable. In order to investigate possible genetic factor(s) causing this variation, the rate of 6beta-hydroxylation of testosterone in human liver microsomes prepared from 46 different human liver samples was determined and the 5'upstream region (+10 to -490 bp) was sequenced from genomic DNA isolated from 39 of these livers. We found a 31-fold variation of the testosterone hydroxylase activity between the samples. However, a very high sequence homology between the CYP3A4 5'-upstream regions sequenced from the 78 different alleles was found. In fact, only three variant nucleotide exchanges were identified, all causing a -290 A-->G mutation (CYP3A4-V) in a so called nifedipine specific element (NFSE). The importance of this element and the polymorphism was evaluated by gel shift analysis. Competition experiments revealed that the binding of nuclear proteins, although having lower affinity to the CYP3A4-V form of the element, was unspecific in nature. In accordance, no influence of this polymorphism was seen on the microsomal testosterone hydroxylase activity in vitro. It is concluded that the promoter region of CYP3A4 is highly conserved, the only polymorphism being in the NFSE, which however does not influence the enzyme expression in liver to a significant degree. This casts doubt of a previously described relationship between the CYP3A4-V allele and cancer in the prostate and leukaemia.
Copyright 1999 Academic Press.