Zero-trans and equilibrium-exchange efflux and infinite-trans uptake of galactose by human erythrocytes

1. 1. The zero-trans and equilibrium exchange efflux and the infinite-trans uptake of galactose in human erythrocytes were measured as a function of galactose concentration at 20%. 2. 2. The zero-trans procedure with cells loaded with 285 mM galactose revealed a low affinity site for galactose transport at the inner face of the membrane having a maximal velocity of 255±96 mmol/1 isotonic cell water and K_m = 240±57 the V/K ratio being 1.01±0.04 min^-1. 3. 3. The equilibrium-exchange procedure yielded a maximal velocity of 432±44 mmol/cell unit per min and K = 138±57, the V/K ratio being 3.19±0.52 min^-1. 4. 4. The infinte-trans uptake revealed a high affinity site at the outer face of the membrane having a maximal velocity of 239±11 mmol/cell unit per min, and K = 21±2 mM. 5. 5. These results combined with previous findings (Ginsburg, H. and Stein, W. D. (1975) Biochim. Biophys. Acta 000,000-000) force us to reject the following models for sugar transport in human erythrocytes: a single asymmetric carrier; two symmetric carriers in parallel, the original form of the internal transfer model.