Kinetic studies of latent microsomal UDP-glucuronyltransferases. Kinetics of glucuronidation in intact and perturbant-treated membranes.

Double-reciprocal plots (with UDP-glucuronate as varied substrate) of the rate of glucuronidation of p-nitrophenol by the latent UDP-glucuronyltransferases of intact guinea pig and rat liver microsomal membranes (prepared with 154 mM KCl and 0.25 M sucrose) were continuously curved concave-downwards. Good fits to the kinetic data were obtained by using two different calculation methods which assume that two forms (high K and low K) of the transferase catalyse the reaction simultaneously. No evidence of cooperativity in binding of UDP-glucuronate to the enzyme was found. When latency of the enzymes of these preparations was destroyed by disrupting the membranes with Triton X-100 or lysophosphatidylcholine, double-reciprocal plots were linear. With guinea pig membranes, lysophosphatidylcholine generated an activated single-enzyme form obeying the simple Michaelis-Menten rate law; K for the activated species was close to that (K1) for the native low K form and its value of V was greater than the combined maximum velocities (V1 + V2) of the two forms in intact membranes. With rat membranes, both perturbants produced a single activated form also with V greater than (V1 + V2) and with K2 greater than K greater than K1. These results are discussed and are consistent with the view of transferase latency which envisages that there are two populations (buried and exposed) of enzyme molecules in intact microsomal membranes. The effects of membrane perturbants on the kinetic parameters of the two native transferase forms were assessed by accounting for the possibility that the reactivity of the buried transferase is controlled by the rate of transport of UDP-glucuronate across the membrane matrix. The data are compatible with a model which supposes that UDP-glucuronate gains access to the buried population by a process with the kinetic characteristics of a facilitated transport system.