ATP-dependent transport of reduced glutathione in yeast secretory vesicles

Turnover of cellular reduced glutathione (GSH) is accomplished predominantly by export into the extracellular space; however, the plasma membrane transport mechanisms that mediate GSH efflux are not well characterized. The present study examined GSH transport using secretory vesicles isolated from the sec6-4 mutant strain of Saccharomyces cerevisiae. In contrast with studies in mammalian membrane vesicles, GSH transport in yeast secretory vesicles was mediated largely by an ATP-dependent, low-affinity pathway (K-m 19+/-5 mM). ATP-dependent [H-3]GSH transport was cis-inhibited by substrates of the yeast YCF1 transporter, including sulphobromophthalein, glutathione S-conjugates and the alkaloid verapamil, and was competitively inhibited by S-(2,4-dinitrophenyl)glutathione (DNP-SG). Similarly, GSH competitively inhibited ATP-dependent [H-3]DNP-SG transport, with a K-i of 18 +/- 2 mM, but had no effect on ATP-dependent [H-3]taurocholate transport. ATP-dependent GSH transport was not affected by either membrane potential or pH-gradient uncouplers, but was inhibited by 4,4'-di-isothiocyanato-stilbene-2,2'-disulphonate, probenecid and sulphinpyrazone, which are inhibitors of mrp1 and mrp2, mammalian homologues of the yeast YCFI transporter. Western blot analysis of the secretory vesicle membrane fraction confirmed the presence of Ycf1p. These results provide the first direct evidence for low-affinity, ATP-dependent transport of GSH, and demonstrate that this ATP-dependent pathway displays kinetic characteristics similar to those of the yeast YCFI transporter.