Possible function of astrocyte cytochrome P450 in control of xenobiotic phenytoin in the brain: In vitro studies on murine astrocyte primary cultures

[4-C-14]Phenytoin underwent a rapid cellular uptake by diffusion within 5 m in when applied in a concentration of 10 muM to mouse brain astrocyte cultu res. Subsequently, a slow linear increase of intracellular radioactivity in dicated metabolic trapping of the drug, with final concentrations reaching 144 pmol phenytoin/mg protein in the astrocytes, Phenytoin levels from 1 to 10 muM decreased cell viability by 15%. The action of cytochrome P450 pres ent in astrocytes in concentrations of 16-17 pmol P450/mg protein could exp lain these slight cytotoxic effects by generating intermediate metabolites of phenytoin, In contrast, concentrations of 50 muM strongly inhibited cell proliferation, A Cyp2c29 immunorelated P450 isoform was expressed in nearl y all astrocytes in culture. Intracellular [4-C-14]phenytoin was degraded t o its major metabolites dihydrodiol, p-HPPH, and m-HPPH through a P450-depe ndent reaction with a specific activity of 0.66 pmol/min x mg protein, or 0 .12 pmol/min x mg protein as measured in cell homogenates, These data under score the importance of astrocytes as brain cells active in the detoxificat ion of foreign substrates, but also in their toxification due to reactive m etabolites generated during these metabolic processes. After diffusionary i nflux of drugs and other xenobiotics, the astrocyte P450 monooxygenases per form an essential role in the mediation of toxicity most frequently encount ered in highly vulnerable neurons. (C) 2000 Academic Press.