NEUROSTEROID METABOLISM - 7-ALPHA-HYDROXYLATION OF DEHYDROEPIANDROSTERONE AND PREGNENOLONE BY RAT-BRAIN MICROSOMES

Two 'neurosteroids', dehydroepiandrosterone (DHEA) and pregnenolone (PREG), are converted by rat brain microsomes into polar metabolites, identified as the respective 7alpha-hydroxylated (7alpha-OH) derivatives by the 'twin ion' technique of g.l.c.-m.s. with deuterated substrates. The enzymic reaction requires NADPH and is stimulated 2-4-fold by EDTA. Under optimal conditions (pH 7.4,0.5 mm-NADPH, 1 mM-EDTA), the K(m) values for DHEA and PREG are 13.8 and 4.4 muM respectively, and the V(max.) values are 322 and 38-8 pmol/min per mg of microsomal protein respectively. Trace amounts of putative 7beta-OH derivatives of DHEA and PREG are detected. Oestradiol, at a pharmacological concentration of 5 mum, inhibits DHEA and PREG 7alpha-hydroxylation. Formation of 7alpha-hydroxylated metabolites is low in prepubertal rats and increases 5-fold in adults. Derivatives of PREG and DHEA, such as PREG sulphate, DHEA sulphate, progesterone and 3alpha-hydroxy-5alpha-pregnan-20-one, are known to be neuroactive. Therefore the quantitatively important metabolism to 7alpha-OH compounds may contribute to the control of neurosteroid activity in brain.