Metabolism of primaquine by liver homogenate fractions - Evidence for monoamine oxidase and cytochrome P450 involvement in the oxidative deamination of primaquine to carboxyprimaquine

The role of monoamine oxidase (MAO) and cytochrome P450 (P450) in the oxida tive deamination of primaquine by rat liver fractions was studied. Rat live r fractions including liver homogenate, mitochondria, microsomes and 100,00 0 g supernatant fractions were prepared from a pool of rat livers and chara cterised using benzylamine as a probe for MAO activity and N,N-dimethylbenz amide as a probe for P450 N-dealkylation activity. Incubation of all fracti ons with primaquine yielded carboxyprimaquine as the only metabolite detect able by HPLC. The mitochondrial fraction, which contained MAO activity but not P450 activity, presented the highest V-max/K-M value for the formation of carboxyprimaquine (8.5 x 10(-6) dm(3)mg(-1)h(-l)). A substantially lower V-max/K-M value (1.3 x 10(-6) dm(3)mg(-1)h(-1)) was obtained in the micros omal fraction, which contained P450 but not MAO activity. The liver homogen ate fraction presented a similar value (1.8 x 10(-6) dm(3)mg(-1)h(-1)), tho ugh it contained both enzyme systems. Incubations of all the fractions that presented MAO activity, in presence o f the MAO inhibitor pargiline, resulted in a marked inhibition of primaquin e oxidation. P450 inhibitor SKF 525-A effectively inhibited primaquine meta bolism in the microsomal fraction but inhibition in the liver homogenate wa s less effective. The results are consistent with an important role for MAO in primaquine biotransformation, though clearly metabolism by P450 has a c ontribution role.