Spontaneous and enzyme-induced dimer formation and its role in membrane permeability III. The mechanism of movement of glucose across the human erythrocyte membrane

1. 1. The rate of penetration of glucose, fructose and sorbose across the membrane of the human red blood cell has been followed by an optical method, and studied as a function of sugar concentration. 2. 2. The mutual effect of the sugars on penetration from a mixture of two sugars has also been studied. 3. 3. The data cannot be accounted for on the basis of previously suggested models which involve complex formation between single sugar molecules and some membrane component. 4. 4. But an adequate explanation arises from a model in which pairs of sugar molecules are bound by some membrane component (here termed a "dimeriser") and are induced to interact with one another to form a dimer. Such a dimer can penetrate the membrane far more readily than can the single sugar molecules. 5. 5. Thus in a similar fashion as for glycerol transfer the membrane carrier for glucose is a second glucose molecule. 6. 6. Fructose and sorbose similarly penetrate following induced dimer formation. 7. 7. Fructose and sorbose cannot form effective dimers with glucose but, when bound to the "dimeriser", can interact with one another to form a complex which can then readily cross the membrane.