The human cytochrome P4507B1: catalytic activity studies

Abstract

The cytochrome P4507B1 (P4507B1) in the human hippocampus is responsible for the production of 7α-hydroxylated derivatives of dehydroepiandrosterone (DHEA) and other 3β-hydroxylated neurosteroids. Minor quantities of the 7β-hydroxylated derivatives are also produced. Neuroprotective action of these 7-hydroxysteroids was reported. Recombinant human P4507B1 was prepared from yeast coexpressing the human hippocampal P450 cDNA and the human P450 reductase genes. Microsomal P4507B1 activity was tested in the presence of NADPH and 14C-labeled steroid substrates to deduce kinetic parameters and to study inhibitor responses. The K M values obtained for DHEA, pregnenolone, epiandrosterone, 5α-androstane-3β,17β-diol and estrone were 1.90 ± 0.06, 1.45 ± 0.03, 1.05 ± 0.12, 0.8 ± 0.04 and 1.20 ± 0.26 μM, respectively. Production of limited amounts of 7β-hydroxylated derivatives was also observed, but only with DHEA, 5α-androstane-3β,17β-diol and epiandrosterone. K M values determined for 7β-hydroxylation were identical to those for 7α-hydroxylation. The DHEA 7α-hydroxylation was inhibited by estrone and estradiol (mixed type inhibition) and by the [25–35] β-amyloid peptide (non-competitive inhibition). These results indicate that in human, the 7-hydroxylation catalysed by P4507B1 preferentially takes place on DHEA, 5α-androstane-3β,17β-diol and epiandrosterone with major and minor formation of 7α- and 7β-hydroxylated derivatives, respectively. Both estrogens and a β-amyloid component inhibit the P4507B1-mediated production of the 7-hydroxysteroid metabolites.