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

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. 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 plus or minus 96 mmol/l isotonic cell water and Km equals 240 plus or minus 57, the V/K ratio being 1.01 plus or minus 0.04 min-1. 3. The equibirum-exchange procedure yielded a maximal velocity of 432 plus or minus 44 mmol/cell unit per min and K equals 138 plus or minus 57, the V/K ratio being3.19 plus or minus 0.52 min-1. 4. The infinite-trans uptake revealed a high affinity site at the outer face of the membrane having a maximal velocity of 239 plus or minus 11 mmol/cell unit per min, and K equals 21 plus or minus 2 mM. 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.