<prompt> Which of the following is not metabolized by acetylation:HydralazinePhenytoinIsoniazidProcainamide  <response>: Ans. B: Phenytoin Phenytoin is extensively metabolised in the liver to 5-4-hydroxypheny1-5 phenyl-hydantoin, which is inactive. This para hydroxylation of phenytoin is carried out by cytochrome P450 2C9. This enzyme also hydroxylates tolbutamide and warfarin. This explains the interaction with these substances. The p-hydroxylated phenytoin is in turn conjugated to its glucuronide. Phenytoin hydroxylation is capacity-limited because of the saturable enzyme systems in the liver. At therapeutic doses, metabolism is nonlinear first-order kinetics, while at toxic doses the metabolism is linear zero-order kinetics. Phenytoin undergoes entero-hepatic recycling. 