Inhibitory effect of phenelzine on oxidative microsomal enzyme systems of rat liver

The inhibitory effects of phenelzine on the hepatic microsomal demethylation of aminopyrine, N,N-dimethylaniline, and p-nitroanisole on the hydroxylation of aniline and on the pharmacokinetics of antipyrine were investigated in the rat. Phenelzine produced a competitive and noncompetitive inhibition of the demethylation of p-nitroanisole and N,N-dimethylaniline, respectively, but was a mixed-type inhibitor of the aminopyrine N-demethylase and aniline hydroxylase. The inhibition constant, Ki, varied between 0.06 to 0.25 mM depending on the substrate used. Preincubation of phenelzine for 30 min with the microsomal homogenate prior to substrate addition doubled its inhibitory effect. Phenelzine induced a type II spectral change when combined with oxidized cytochrome P-450 with a Ks value of 0.4 mM. The administration of one dose of 50 mg∙kg−1 of phenelzine sulfate concomitantly with 50 mg∙kg−1 of antipyrine resulted in a significant decrease of the serum elimination of antipyrine. The serum half-life, apparent volume of distribution, and total body clearance of antipyrine were modified to 3.6 h, 294.1 mL∙kg−1, and 56.8 mL∙h−1∙kg−1, respectively, from 1.5 h, 666.7 mL∙kg−1, and 312.5 mL∙h−1∙kg−1 when antipyrine was administered alone. It is concluded that the inhibitory effect of phenelzine on the microsomal oxidative reactions of rat liver is related to its interaction with cytochrome P-450.