Deamination of amphetamines by cytochromes P450: Studies on substrate specificity and regioselectivity with microsomes and purified CYP2C subfamily isozymes

Abstract

Deamination or oxidative cleavage of the carbon-nitrogen bond in various phenylisopropylamines was examined in liver microsomes from rabbits and rats, and in reconstituted systems containing CYP2C subfamily isozymes. Kinetic studies of phenylacetone formation from six amphetamine (AP) derivatives, catalyzed by rabbit liver microsomes, indicated that AP had the highest apparent affinity (lowest Km) and increasing the size of the substituent on the nitrogen atom decreased the affinity. The values of maximal velocity increased with increasing size of the substituent. Experiments with purified CYP2C3 from rabbit liver gave similar results: this enzyme showed the highest activity for phenylacetone formation from benzphetamine (BZP) and showed lower activities with compounds having smaller nitrogen substituents. Based on these results, we conclude that among a series of AP derivatives, the parent phenylisopropylamine has the highest affinity for rabbit liver deaminase, where as BZP has the highest turnover. However, the intrinsic clearance (Vmax/Km) values for the individual reactions tended to be comparable. The rates of BZP and deprenyl N-demethylation by rat CYP2C11 and 2C13 were far greater than those of the reactions at other N-α-positions. This result indicated that rat CYP2C enzymes have a more rigid regioselectivity than rabbit CYP2C3 for the deamination/N-dealkylation of phenylisopropylamines.