Inhibition of glucose transport in human erythrocytes by ubiquinone Q0

Searches of the protein data bases revealed limited homologies between several regions of the human erythrocyte glucose transporter containing a relative abundance of hydrogen-bonding amino-acid side chains, and proteins of the NADH-ubiquinone oxidoreductase family. This raised the possibility the binding sites for glucose and ubiquinone may be similar in the respective proteins. Experimental studies demonstrated that ubiquinone Q0 does in fact inhibit both glucose entry and glucose exit in human erythrocytes with kinetics consistent with the existence of ubiquinone binding sites at both the exofacial and endofacial sides of the transporter. Glucose transport was also inhibited by the water-soluble tryptophan-inactivating agent, dimethyl(2-hydroxy-5-nitrobenzyl)sulphonium bromide, and this is consistent with the presence of tryptophan residues in two of the exofacial amino-acid sequences proposed as candidates for involvement in glucose binding sites.