Kinetic tests of models for sugar transport in human erythrocytes and a comparison of fresh and cold-stored cells

We studied the time course of the entry of galactose into human erythrocytes from an external concentration of 500 mM, and analyzed the data by an integrated rate equation treatment. We found evidence for only a single, high-affinity site for sugar at the inner face of the membrane. We studied the effect of pre-loading cells with galactose at various concentrations on the entrance of galactose into the cell from 128 mM, and compared the result we found with a previous report of a similar experiment from 500 mM external sugar. We found no evidence of other than a high affinity for sugar at the inner face of the membrane. The data reject a model in which sugar transport occurs on two asymmetric, oppositely directed carriers. We studied exchange of glucose into and out of the cells as a function of sugar concentration, taking care to minimize metabolism of sugar. We found no evidence for other than a single component for glucose exchange. Our data reject the 'allosteric pore' model for sugar transport. The explanation of the high-affinity site for sugar at the inner membrane face thus remains enigmatic. We find a very significant difference in the kinetics of glucose exchange when we compare freshly drawn and long cold-stored blood. The K_m for exchange was almost twice as large for cold-stored as for fresh blood.