Biphasic kinetics of imipramine N-oxidation in rat brain microsomes

Imipramine (IMI) N-oxidase activity in brain microsomes from rats of both sexes was determined by high performance liquid chromatography, and compared with the results in rat liver microsomes. Brain and liver microsomal IMI N-oxidation was sensitive to thermal inactivation and had an optimal pH at around 9.0. IMI N-oxidase activity (15.54 pmol/min/mg protein) in brain microsomes was about one-hundredth that of liver microsomes (2.08 nmol/min/mg protein) at a substrate concentration of 5 mM. IMI N-oxidase activities in both brain and liver microsomes displayed biphasic kinetics that associated a low Km-low Vmax element with a high Km-high Vmax component. Furthermore, a significant sex difference was observed in Vmax values (male>female) in both phases, but Km values were similar between male and female rats, resulting in a significant sex difference (male>female) in intrinsic clearance values (Vmax/Km) of the low-Km and the high-Km phases.