Inhibition of hexose transport by adenosine derivatives in human erythrocytes

The human erythrocyte membrane carriers for hexoses and nucleosides have several structural features in common. In order to assess functional similarities, the effects of adenosine derivatives on hexose transport and cytochalasin B binding sites were studied. Adenosine inhibited zero-trans uptake of 3-O-methylglucose half-maximally at 5 mM, while more hydrophobic adenosine deaminase-resistant derivatives were ten- to 20-fold more potent transport inhibitors. However, degradation of adenosine accounted for very little of this difference in potency. Hexose transport was rapidly inhibited by N6-(L-2-phenylisopropyl)adenosine at 5 degrees C in a dose-dependent fashion (EC50 = 240 microM), to lower the transport Vmax without affecting the Km. A direct interaction with the carrier protein was further indicated by the finding that N6-(L-2-phenylisopropyl)adenosine competitively inhibited 3H]cytochalasin B binding to erythrocytes (Ki = 143 microM) and decreased 3H]cytochalasin B photolabeling of hexose carriers in erythrocyte ghosts. The cross-reactivity of adenosine and several of its derivatives with the hexose carrier suggests further homologies between the carriers for hexoses and nucleosides, possibly related to their ability to transport hydrophilic molecules through the lipid core of the plasma membrane.