title: Sulfhydryl substituents of the human erythrocyte hexose transport mechanism.
abstract: 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.
mesh_terms: D001692:Biological Transport / Q000187:drug effects; D001786:Blood Glucose / Q000378:metabolism; D002463:Cell Membrane Permeability; D002784:Cholesterol / Q000097:blood; D003571:Cytochalasin B / Q000494:pharmacology; D004910:Erythrocyte Membrane / Q000378:metabolism*; D005033:Ethylmaleimide / Q000494:pharmacology; D005947:Glucose / Q000494:pharmacology; D006601:Hexoses / Q000097:blood*; D006801:Humans; D008301:Maleimides / Q000494:pharmacology; D011725:Pyridines / Q000494:pharmacology; D013438:Sulfhydryl Compounds / Q000097:blood*; D013439:Sulfhydryl Reagents / Q000494:pharmacology
chemical_list: D001786:Blood Glucose; D006601:Hexoses; D008301:Maleimides; D011725:Pyridines; D013438:Sulfhydryl Compounds; D013439:Sulfhydryl Reagents; C044107:glutathione maleimide; D003571:Cytochalasin B; D002784:Cholesterol; D005947:Glucose; D005033:Ethylmaleimide