Zero-trans and infinite-cis uptake of galactose in human erythrocytes.

1. The zero-trans and infinite-cis uptake of galactose into human erythrocytes was measured as a function of galactose concentration at 20 degrees C. 2. A special procedure, the "cis-trans test" has been developed to determine the directionality of an asymmetric transport carrier. 3. Using the "cis-trans test" and results obtained by phloretin inhibition, could show the existance of two sites mediating galactose uptake. The kinetic parameters of the high affinity site are K1 equals 11 mM; V1 equals 16 mmol - cell unit-1-min-1 and of the low affinity site: K2 equals 286 mM; V2 equals 21 mmol-cell unit-2-min-1. 4. The infinite-cis Km, using an intergrated rate equation treatment was 21 mM and that found by a direct preloading procedure was 25 mM. The existence of a high affnity site at the inner side of the membrane was thus confirmed.