title: Inhibitions of sugar transport produced by ligands binding at opposite sides of the membrane. Evidence for simultaneous occupation of the carrier by maltose and cytochalasin B.
abstract: This study examines inhibitions of human erythrocyte D-glucose uptake at ice temperature produced by maltose and cytochalasin B. Maltose inhibits sugar uptake by binding at or close to the sugar influx site. Maltose is thus a competitive inhibitor of sugar uptake. Cytochalasin B inhibits sugar transport by binding at or close to the sugar efflux site and thus acts as a noncompetitive inhibitor of sugar uptake. When maltose is present in the uptake medium, Ki(app) for cytochalasin B inhibition of sugar uptake increases in a hyperbolic manner with increasing maltose. When cytochalasin B is present in the uptake medium, Ki(app) for maltose inhibition of sugar uptake increases in a hyperbolic manner with increasing cytochalasin B. High concentrations of cytochalasin B do not reverse the competitive inhibition of D-glucose uptake by maltose. These data demonstrate that maltose and cytochalasin B binding sites coexist within the glucose transporter. These results are inconsistent with the simple, alternating conformer carrier model in which maltose and cytochalasin B binding sites correspond to sugar influx and sugar efflux sites, respectively. The data are also incompatible with a modified alternating conformer carrier model in which the cytochalasin B binding site overlaps with but does not correspond to the sugar efflux site. We show that a glucose transport mechanism in which sugar influx and sugar efflux sites exist simultaneously is consistent with these observations.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Massachusetts Medical Center, Worcester 01605.
authors.forename: A
authors.initials: A
authors.lastname: Carruthers
authors.forename: A L
authors.initials: AL
authors.lastname: Helgerson
mesh_terms: D001665:Binding Sites; D001693:Biological Transport, Active / Q000187:drug effects; D001786:Blood Glucose / Q000378:metabolism*; D003571:Cytochalasin B / Q000494:pharmacology*; D004910:Erythrocyte Membrane / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008024:Ligands; D008320:Maltose / Q000494:pharmacology*; D008433:Mathematics; D008954:Models, Biological; D009004:Monosaccharide Transport Proteins / Q000037:antagonists & inhibitors / Q000097:blood*
chemical_list: D001786:Blood Glucose; D008024:Ligands; D009004:Monosaccharide Transport Proteins; D003571:Cytochalasin B; D008320:Maltose