The metabolism of phenytoin by isolated hepatocytes and hepatic microsomes from male rats

Previous in vivo and in vitro studies have indicated that the K(m) for phenytoin hydroxylation is about 30 μM. Yet, the drug shows dose-dependent kinetics suggesting a K(m) of about 5 μM. The present studies indicate the discrepancy is not due to active transport of the drug in the hepatocyte or a decrease in the K(m) due to the low pO₂ of the portal vein leading to uncompetitive inhibition. Studies in both hepatocytes and microsomes indicate the presence of a high affinity hydroxylase with a K(m) of 2 to 5 μM. These data suggest that this enzyme is the one primarily involved in the metabolism of phenytoin.