Functional re-evaluation of the putative glutathione transporters, RcGshT and RsGshT.

Transport systems that mediate glutathione (GSH) efflux from hepatocytes into blood plasma and bile have been characterized extensively in sinusoidal and canalicular membrane vesicles, and recent reports describe two candidate GSH transport proteins: the rat sinusoidal GSH transporter (RsGshT) and rat canalicular GSH transporter (RcGshT). However, studies in our laboratory have been unable to confirm the function of these gene products. Xenopus laevis oocytes injected with either rat liver mRNA, the cRNA for RcGshT or the cRNA for RsGshT did not transport GSH at a higher rate than water-injected oocytes, when measured either as 3H-GSH uptake or efflux, at low or high GSH concentrations, or in the presence or absence of acivicin to inhibit gamma-glutamyltransferase activity. In contrast, transport of 3H-taurocholate was markedly accelerated in oocytes injected with rat liver mRNA or the cRNA for the Na(+)-taurocholate cotransporting polypeptide (Ntcp), confirming the integrity of the mRNA and the viability of the oocytes. Northern blot analysis failed to detect an RcGshT transcript in rat liver total RNA or rat liver mRNA. Of significance, the RcGshT and RsGshT cDNA sequences are similar to those found in the Escherichia coli K-12 genome, indicating possible cloning artifacts. Further studies are needed to resolve this discrepancy, and to isolate and characterize hepatic GSH transport proteins.