Glutathione transport across intestinal brush-border membranes: effects of ions, pH, delta psi, and inhibitors

We characterized glutathione transport in brush-border membrane vesicles (BBMV) that were prepared from rabbit small intestine in which gamma-glutamyl transpeptidases (gamma-glutamyltransferases, EC 2.3.2.2) had been inactivated by a specific affinity-labeling reagent (AT125). Intact GSH transport was strongly increased by the presence of Na+, K+, LI+, Ca2+ and Mn2+ and, of all these, the Ca2+ activation effect was prevalent. This cation effect was selective and catalytic but not energetic; Vmax obtained in the presence of both Na+ and Ca2+ was about 6-times higher than it was in their absence, while Km did not change. Moreover, these cations almost completely eliminated GSH binding on the membrane surface. Na+ activation cannot be explained as a stimulation effect on the Na+-H+ antiport system, since a GSH proton-driven transport was excluded. We determined a pH optimum (7.5), while low or high extravesicular pH values diminished the GSH uptake rate. The Ca2+ effect on GSH transport, when an electrical potential difference was imposed across BBMV, was different from that of monovalent cations. Indeed, experiments performed by valinomycin-induced K+ diffusion potential or by anion substitution showed that the GSH transport system was an electroneutral process in the presence of Na+ or K+, but that it was electrogenic in the presence of Ca2+ or in the absence of extravesicular cations. These results suggest that GSH is also cotransported with these cations, without its accumulation inside vesicles. Moreover, since GSH is negatively charged, the effect of pH changes and of cation activation on GSH transport is arguably mediated by changes in the ionization state of certain groups as the carrier site and of GSH itself, indicating the electrostatic nature of GSH binding sites on the transporter. The high Ca2+ activation effect is perhaps also partly due to fluidity changes in the lipoproteic microenvironment of the GSH transporter. Moreover, this transport system has high affinity with GSH, given the low Km value (17 microM) and the fact that it was only inhibited by GSH S-derivatives and by GSH monoethyl ester, which probably share the same transport system.