Abstract
We have studied the expression of the hepatic GSH transport system in Xenopus laevis oocytes. Injection of rat liver poly(A)+ RNA resulted in the functional expression of the GSH transport system determined as GSH efflux from GSH loaded oocytes. Expression required 3-5 days to process the liver mRNA. Methionine, cystathionine, and sulfobromophthalein (BSP)-GSH inhibited the efflux of GSH from liver mRNA-injected oocytes according to their known cis or transactions on hepatocytes, namely BSP-GSH from inside and methionine and cystathionine from outside. The expressed hepatic GSH transport system also mediated the uptake of intact GSH into the oocyte, consistent with the bidirectional operation of this facilitative transporter. The uptake of GSH into mRNA-injected oocytes was inhibited by BSP-GSH in chloride-free conditions. Finally, two different mRNA size fractions encoded for hepatic GSH transport activity (uptake or efflux): a 2.0-2.5-kilobase size class, which expressed GSH transport (uptake or efflux) completely inhibited by BSP-GSH (compatible with sinusoidal GSH transport), and a 3.5-4.0-kilobase size class, which expressed GSH transport (uptake or efflux) not inhibited by BSP-GSH. These results demonstrate that hepatic GSH transport can be expressed in Xenopus oocytes and mRNA of two distinct sizes encode for GSH transporters.
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