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Abstract
1. Diazepam metabolism and its association with mephenytoin hydroxylase were studied in vitro using human and rat livers.
2. Enzyme kinetic parameters were obtained for the formation of p-hydroxydiazepam (p-hydroxy-DZP), N-desmethyldiazepam (NDZ), and temazepam (TMZ) from diazepam (DZP) in rat liver fractions. The Km values for formation in rat of p-hydroxy-DZP, NDZ and TMZ were 14·3 (SEM) μM, 44·4 and 63·8, respectively; clearance values calculated from Vmax/Km were 5.7, 3.2 and 4.9 ml/g per min, respectively.
3. Mephenytoin (MP) competitively inhibited, in rat liver, the formation of NDZ, but not the formation of p-hydroxy-DZP or TMZ; in human liver neither NDZ nor TMZ formation was inhibited by MP.
4. In seven different human livers the formation of p-hydroxy-DZP represented a minor pathway compared to the formation of NDZ and TMZ.