Testosterone metabolism in rat brain is differentially enhanced by phenytoin-inducible cytochrome P450 isoforms

Many cytochrome P450 (P450) isoforms are selectively inducible by xenobioti cs, e.g. pharmaceuticals like the anti-epileptic drug phenytoin, Some of th ese P450 enzymes are involved in the metabolism of gonadal hormones and are of great importance, especially in early brain development, In this study, the hydroxylation of testosterone by rat brain microsomes from control and phenytoin-induced animals was examined by use of high performance liquid c hromotography (HPLC) provided with a photodiode array detector (PDA), In co ntrol rats, testosterone is converted by cytochrome(s) P450 to 6 alpha-hydr oxytestosterone (OHT) as the main metabolite and 6 beta-OHT as well as andr ostenedione as minor metabolites, After phenytoin treatment, brain microsom es showed a strong increase of testosterone metabolism to 2 alpha-, 6 beta- , 16 alpha-, 16 beta-OHT and androstenedione, whereby 16 alpha-OHT was the main degradation product. These metabolites indicated the action of isoform s of the P450 subfamilies CYP2B, CYP2C and CYP3A. Inhibition experiments wi th antibodies against CYP2B1/2 and with the CYP2B specific inhibitor orphen adrine indicated the occurrence of members of this subfamily which are know n to catalyse the oxidation of testosterone to 16 alpha-OHT, 16 beta-OHT an d androstenedione. Western blots revealed the phenytoin-inducible expressio n of CYP2B1 and the constitutive expression of CYP3A, The latter is involve d in the GP-hydroxylation of testosterone which was found correspondingly i n control microsomes. Distinct CYP2C isoforms involved in the hydroxylation of testosterone in phenytoin-induced microsomes are not yet identified. Th e highly increased testosterone metabolism by phenytoin-dependent induction of specific cytochrome P450 isoforms in adult rat brain illustrates the po tential influence of exogenous substances on internal regulative and metabo lic pathways in the brain.