EFFECTS OF THE ANTICONVULSANT, VALPROATE, ON THE EXPRESSION OF COMPONENTS OF THE CYTOCHROME-P-450-MEDIATED MONOOXYGENASE SYSTEM AND GLUTATHIONE S-TRANSFERASES

It has been shown previously that the anticonvulsant agent, sodium val proate, induces certain cytochrome P-450 monooxygenase activities and decreases glutathione S-transferase activity. We have used Western blo tting, RNase protection assays and Northern blot hybridization to dete rmine the effects of valproate on the abundance of individual componen ts of the cytochrome P-450 monooxygenase and of glutathione S-transfer ase subunits. Due to the short half-life of the drug in rats we have u sed an in vitro experimental system comprised of rat hepatocytes co-cu ltured with rat primitive biliary epithelial cells. Valproate was show n to be a potent inducer of two members of the cytochrome P-450 (CYP)2 B subfamily, CYP2B1 and 2B2. The induction of the proteins was mediate d at the level of the mRNAs, with the mRNA for CYP2B1 being more highl y induced than that for CYP2B2. The drug also induced, but to a much l esser extent, two important components of the cytochrome-P-450-mediate d monooxygenase system, NADPH-dependent cytochrome P-450 reductase and cytochrome b(5), and their corresponding mRNAs. Thus, the effects of valproate on cytochromes P-450 and other components of the cytochrome- P-450-mediated monooxygenase system mimic those of another, structural ly diverse, antiepileptic drug, phenobarbital. However, in contrast to phenobarbital, which induces glutathione S-transferase subunits 1, 2, 3, 4 and 7, valproate selectively decreases the abundance of subunits 3 and/or 4. It has been shown previously that CYP2B1 is involved in t he production of metabolites of valproate implicated in hepatotoxicity . The induction of this protein by valproate would thus contribute sub stantially to the hepatotoxic effects associated with the drug.