Plasma membrane and mitochondrial transport of hepatic reduced glutathione.

Abstract

The tripeptide glutathione (GSH) is a key nonprotein thiol that plays multiple critical functional and regulatory roles in cells. Hepatic transport of GSH is a key process in the interorgan homeostasis of GSH. Hepatocellular GSH is available to other extrahepatic organs by its release into blood and bile through the sinusoidal and canalicular GSH carriers, respectively. Their characterization at the molecular level has been recently accomplished using the functional expression cloning strategy utilizing Xenopus laevis oocytes microinjected with the corresponding cRNA from the sinusoidal (RsGshT) and canalicular (RcGshT) clones previously isolated and identified from cDNA libraries constructed from hepatic size-fraction mRNAs expressing separately the sinusoidal and canalicular GSH transporters. These clones of 2.8 and 4.0 kb encode for proteins of 39.9 and 95.8 kD for RsGshT and RcGshT, respectively, 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, mitochondria do not synthesize their own GSH, which originates from the operation 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 oocytes have allowed the identification of the hepatic mitochondrial GSH carrier (RmGshT), which displays distinct functional features from both RsGshT and RcGshT, such as ATP stimulation and inhibitor specificity, suggesting that RmGshT is encoded by a gene distinct from that of the plasma membrane GSH carriers.