D-glucose-sensitive and -insensitive cytochalasin B binding proteins from microvillous plasma membranes of human placenta. Identification of the d-glucose transporter

Abstract

Cytochalasin B was found to bind to at least two distinct sites in human placental microvillous plasma membrane vesicles, one of which is likely to be intimately associated with the glucose transporter. These sites were distinguished by the specificity of agents able to displace bound cytochalasin B. [ 3H]Cytochalasin B was displaceable at one site by d-glucose but not by dihydrocytochalasin B; it was displaceable from the other by dihydrocytochalasin B but not by d-glucose. Some binding which could not be displaced by d- glucose + dihydrocytochalasin B was displaced by excess unlabeled cytochalasin B and may represent a third cytochalasin B binding site. Cytochalasin B can be photoincorporated into specific binding proteins by ultraviolet irradiation. d-Glucose specifically prevented such photoaffinity labeling of a microvillous protein component(s) of M r = 60 000 ± 2 000 as determined by urea-sodium dodecyl sulfate acrylamide gel electrophoresis. This d-glucose-sensitive cytochalasin B binding site of the placenta is likely to be either the glucose transporter or be intimately associated with it. The molecular weight of the placental glucose transporter agrees well with the most widely accepted molecular weight for the human erythrocyte glucose transporter. Dihydrocytochalasin B prevented the photoincorporation of [ 3H]cytochalasin B into a polypeptide(s) of M r = 53 000 ± 2 000. This component is probably not associated with placental glucose transport. This report presents the first identification of a sodium-independent glucose transporter from a normal human tissue other than the erythrocyte. It also presents the first molecular weight identification of a human glucose-insensitive high-affinity cytochalasin B binding protein.