PLASMA-MEMBRANE AND MITOCHONDRIAL TRANSPORT OF HEPATIC REDUCED GLUTATHIONE

The tripeptide glutathione (GSH) is a key nonprotein thiol that plays multiple critical functional and regulatory roles in cells. Hepatic tr ansport of GSH is a key process in the interorgan homeostasis of GSH. Hepatocellular GSH is available to other extrahepatic organs by its re lease into blood and bile through the sinusoidal and canalicular GSH c arriers, respectively. Their characterization at the molecular level h as been recently accomplished using the functional expression cloning strategy utilizing Xenopus laevis oocytes microinjected with the corre sponding cRNA from the sinusoidal (RsGshT) and canalicular (RcGshT) cl ones previously isolated and identified from cDNA libraries constructe d from hepatic size-fraction mRNAs expressing separately the sinusoida l and canalicular GSH transporters. These clones of 2.8 and 4.0 kb enc ode for proteins of 39.9 and 95.8 kD for RsGshT and RcGshT, respective ly, with 3 to 5 and 6 to 10 putative membrane-spanning domains. Their tissue distribution reveals that RsGshT is exclusively found in liver, contrasting with the distribution of RcGshT, which is found in nearly all tissues examined. Cellular GSH is also found in the mitochondrial matrix at a concentration similar to that in cytosol. However, mitoch ondria do not synthesize their own GSH, which originates from the oper ation of a transport carrier localized within the inner mitochondrial membrane. Its role is critical in maintaining a functionally competent organelle and in cell viability. Expression studies in Xenopus oocyte s have allowed the identification of the hepatic mitochondrial GSH car rier (RmGshT), which displays distinct functional features from both R sGshT and RcGshT, such as ATP stimulation and inhibitor specificity, s uggesting that RmGshT is encoded by a gene distinct from that of the p lasma membrane GSH carriers.