Bidirectional transport of glutathione in hepatocyte membranes: Does glutathione efflux stimulate organic anion uptake?

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Abstract

We determined the trans effects of extracellular reduced glutathione (GSH) on the rate of efflux of endogenous labeled GSH from freshly isolated rat hepatocytes. The presence of GSH (10 mM) in the medium significantly stimulated the fractional rate of efflux of [35S]GSH from 5.2 to 12.6%/15 min (p < 0.01). This effect was concentration-dependent, had sigmoid type of kinetics (D50 of 0.32 mM), and was reversible upon removal of external GSH. trans-Stimulation (counter-transport) was also observed with 5 mM oxidized glutathione (GSSG) and ophthalmic acid (fractional [35S]GSH efflux: 13.4% ± 4.1 and 8.8% ± 2.3 in 15 min, respectively, compared with control: 4.7 ± 2.5/15 min). Bromosulphthalein-glutathione (BSP-GSH, 5 mM) in Krebs buffer inhibited the fractional [35S]GSH efflux (1.1%/15 min), whereas in Cl−--free buffer, GSH efflux was stimulated (14.2%/15 min) compared with control. trans-Stimulation was independent of chloride. BSP-GSH cis-inhibited and trans-stimulated the initial rate of GSH transport in basolateral-enriched membrane vesicles (bLPM) but not in canalicular-enriched membrane vesicles (cLPM). γ-Glutamyl compounds also cis-inhibited and trans-stimulated GSH transport in bLPM vesicles. GSH-depleted hepatocytes incubated with 10 mM [35S]GSH accumulated more GSH than repleted cells, but the initial rate of uptake of radioactivity was faster in repleted cells. In contrast, repleted hepatocytes incubated with tracer or 50 μM [35S]GSH did not take up GSH. Thus, the sinusoidal membrane GSH transporter exhibits low affinity kinetics with sigmoid features for both GSH uptake and trans-stimulation of efflux, explaining the lack of uptake of GSH at low physiologic extracellular concentrations. Therefore, our findings support and explain the widely held view that GSH transport is unidirectional under physiologic conditions. However, the efflux of GSH may also occur in exchange for the uptake of organic anions and γ-glutamyl compounds. (J Biol Chem 1992;267:22256–22264.)

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