Abstract

Several approaches have been developed to estimate in vivo the intrinsic clearances of enzymes that catalyze the formation of chemically reactive metabolites that do not escape the organs in which they are formed. Two basic models are considered. Model 1 is a general model in which the chemically reactive metabolite is inactivated by a combination of a pseudo-first order reaction, such as a reaction with large pools of protein, and a second order reaction with a depletable endogenous substance, such as glutathione or an enzyme. Model 2 is a special case, in which at low doses of the parent compound the reactive metabolite preferentially reacts with a depletable endogenous substance, such as glutathione. In developing both models we have assumed that the rate of formation of reactive metabolite follows first order kinetics and that the concentration of reactive metabolite in liver reaches a steady state almost instantaneously. In developing Model 2 we also have assumed that the depletion of hepatic reduced glutathione is due solely to the formation of glutathione conjugates. The uses of the approaches based on Model 2 were illustrated by studying the effects of a marginally toxic dose of acetaminophen on the depletion and subsequent repletion of hepatic glutathione in hamsters. From the calculated rate of synthesis of glutathione in liver, the fraction of the dose of acetaminophen converted to the glutathione conjugate in liver, and the clearance for the formation of glutathione conjugate in vivo was estimated and was found to be similar to that obtained with hepatic 9,000 g supernatant preparations. Other uses of the models are described.