Differentiation of erythrocyte-(GLUT1), liver-(GLUT2), and adipocyte-type (GLUT4) glucose transporters by binding of the inhibitory ligands cytochalasin B, forskolin, dipyridamole, and isobutylmethylxanthine

Abstract

The binding affinities of the glucose transporter isoforms GLUT1, GLUT2, and GLUT4 for the inhibitory ligands cytochalasin B, forskolin, dipyridamole, and isobutylmethylxanthine (IBMX) were compared in membranes from human erythrocytes and rat brain containing the erythrocyte-type glucose transporter (GLUT1), in membranes from rat liver containing the liver-type glucose transporter (GLUT2), and in membranes from adipocytes and heart containing predominantly the adipose/muscle-type glucose transporter (GLUT4). The binding affinities of cytochalasin B for GLUT1 and GLUT4 were virtually identical (KD) in membranes from erythrocytes, 190 nM; in brain, 130 nM; in adipocytes, 160 nM; and in heart, 170 nM). In contrast, no specific glucose-inhibitable binding of cytochalasin B was detected in liver membranes. The binding affinity for forskolin of GLUT1 was significantly lower than that of GLUT4 (KD in erythrocytes, 2360 nM; Kl in brain, 4360 nM; and KD in adipocytes, 200 nM; and in heart, 210 nM); specific glucose-inhibitable binding to GLUT2 was not detectable. Like forskolin, the glucose transport inhibitors dipyridamole (Kl in adipocyte membranes, 1.2 microM; in erythrocytes, greater than 40 microM) and IMBX (Kl in adipocyte membranes, 60 microM; and in erythrocytes, greater than 500 microM) bound with higher affinity to GLUT4 than to GLUT1. These data demonstrate striking differences of GLUT1, GLUT2, and GLUT4 with respect to their binding affinity for the inhibitory ligands cytochalasin B, forskolin, dipyridamole, and IBMX. It is suggested that the complex differences result from interaction of more than one heterogeneous binding site at the glucose transporters with the inhibitory ligand.