[4-(14)C]Phenytoin underwent a rapid cellular uptake by diffusion within 5 min when applied in a concentration of 10 microM to mouse brain astrocyte cultures. Subsequently, a slow linear increase of intracellular radioactivity indicated metabolic trapping of the drug, with final concentrations reaching 144 pmol phenytoin/mg protein in the astrocytes. Phenytoin levels from 1 to 10 microM decreased cell viability by 15%. The action of cytochrome P450 present in astrocytes in concentrations of 16-17 pmol P450/mg protein could explain these slight cytotoxic effects by generating intermediate metabolites of phenytoin. In contrast, concentrations of 50 microM strongly inhibited cell proliferation. A Cyp2c29 immunorelated P450 isoform was expressed in nearly all astrocytes in culture. Intracellular [4-(14)C]phenytoin was degraded to its major metabolites dihydrodiol, p-HPPH, and m-HPPH through a P450-dependent 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 underscore the importance of astrocytes as brain cells active in the detoxification of foreign substrates, but also in their toxification due to reactive metabolites generated during these metabolic processes. After diffusionary influx of drugs and other xenobiotics, the astrocyte P450 monooxygenases perform an essential role in the mediation of toxicity most frequently encountered in highly vulnerable neurons.
Copyright 2000 Academic Press.