Abstract

The membrane permeabilities of acetaminophen glucuronide and sulfate produced through conjugative metabolism were examined in isolated rat hepatocytes. The glucuronide formed in hepatocytes was gradually released into the medium and its intracellular level decreased. Release of the sulfate formed in hepatocytes occurred more rapidly and its intracellular level remained almost constant. The permeability of acetaminophen was so rapid that it caused instantaneous equilibrium between hepatocytes and the medium. Its intracellular level thus decreased by conjugation reactions, but compensation for this decrease from the medium was soon made so that a constant intracellular level was resumed. The uptake of both preformed glucuronide and sulfate into hepatocytes indicated carrier-mediated transport. From these results, a pharmacokinetic model is proposed in which conjugative metabolism occurs in two consecutive steps: conjugative reactions of the parent compound taken up instantaneously into hepatocytes and membrane transport of conjugates into the medium. Changes in the amount of acetaminophen, its glucuronide, and its sulfate in the hepatocytes and medium as a function of time simulated according to the model closely agreed with those actually observed. Consequently, membrane permeability of the conjugates was concluded to be essential for conjugative metabolism.