Kinetics of acetyl CoA: arylamine N-acetyltransferase from rapid and slow acetylator human liver

Abstract

Michaelis-Menten kinetic constants for sulfamethazine (SMZ) and p-aminobenzoic acid (PABA) metabolism were determined in 2 very rapid, 8 intermediate, and 10 slow acetylator human livers. The mean apparent KM values for the monomorphic substrate PABA were 70 +/- 20, 180 +/- 50, and 310 +/- 30 microM for the slow, intermediate, and very rapid enzymes, respectively, whereas the polymorphic substrate SMZ exhibited little interphenotypic KM variation. Compared to the slow enzymes, the rapid acetylators exhibited mean apparent Vmax values 15- and 20-fold greater for PABA and SMZ, respectively. Furthermore, under in vitro conditions where enzyme saturation was achieved, PABA showed polymorphic acetylation characteristics, in contrast to events in vivo, where monomorphic acetylation patterns were observed. As the acetyl coenzyme A concentration in the reaction mixture was increased, the KM and Vmax for PABA increased, in accordance with "ping-pong" kinetic principles. As occurs with polymorphic substrates, a binary ping-pong mechanism appears to govern N-acetylation of monomorphic substrates in human liver, as evidenced by initial velocity patterns and limiting values for the KM and Vmax for PABA.