Mechanism of glutathione S-conjugate transport in canalicular and basolateral rat liver plasma membranes

Abstract

Transport of S-(2,4-dinitrophenyl)glutathione (DNP-SG), a model compound of glutathione S-conjugates, was studied in purified canalicular and basolateral (i.e. sinusoidal and lateral) rat liver plasma membrane (LPM) vesicles. Incubation of canalicular LPM vesicles with DNP-SG in the presence of ATP resulted in an uptake of DNP-SG into the vesicles. In contrast, in basolateral LPM vesicles the initial rate of ATP-dependent DNP-SG uptake was 3-4-fold lower than that of the canalicular LPM. This suggests that the ATP-dependent glutathione S-conjugates transport is localized in the canalicular membrane of the hepatocytes. The rate of DNP-SG uptake into canalicular LPM vesicles exhibited saturation kinetics with apparent Km values of 4 microM for DNP-SG and 260 microM for ATP. The DNP-SG uptake was inhibited by various glutathione S-conjugates and GSSG, not GSH. The bulkiness of S-substituent alkyl groups in the conjugates and cysteinylglycine of glutathione are essential for the inhibition. In contrast to the ATP-dependent process, the uptake of DNP-SG to basolateral LPM in the presence of KCl was 4-fold higher than that of canalicular LPM. The rate of DNP-SG uptake was enhanced by a valinomycin-induced K+ diffusion potential, and was inhibited by GSH. These results suggest that electrogenic transport of glutathione S-conjugates is localized in the basolateral membrane of the hepatocytes. These studies provide evidence that the ATP-dependent canalicular transport system functions in biliary secretion of glutathione S-conjugates.