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Sulfhydryl substituents of the hexose transport mechanism of human erythrocyte membranes were studied with membrane-impermeant and -permeant maleimide derivatives. Three sulfhydryl classes have been identified on the basis of their reactivity toward the reagents and their effects on the transport mechanism. Type I sulfhydryl is located at the outer (exofacial) surface of the membrane and bound covalently on treatment of intact cells with the membrane-impermeant glutathione-maleimide. This sulfhydryl is required for the transport, and it is protected from alkylation, i.e., its reactivity toward maleimides is decreased by the presence of D-glucose or cytochalasin B. Type II sulfhydryl is also required for the transport, but it differs from type I in that D-glucose (but not cytochalasin B) increases the reactivity toward maleimides. Further, it is located at the endofacial surface of the membrane, since reaction with glutathione-maleimide occurs only in leaky ghosts and not in intact cells. Alkylation by glutathione-maleimide of type I and type II sulfhydryls increases the half-saturation for the binding of D-glucose to erythrocyte membranes. In contrast, inactivation of type III sulfhydryls by N-ethylmaleimide or dipyridyl disulfide decreases the half-saturation concentration for the binding of D-glucose and other transported hexoses to the membranes; nontransported sugars are not affected similarly. Type III sulfhydryl is not inactivated by the polar reagent glutathione-maleimide and is probably located in a nonpolar domain of the transport mechanism. Inactivation of either type I or II sulfhydryls decreases or eliminates the flux asymmetry of the hexose transport mechanism.

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