Zero-trans and infinte-cis uptake of galactose in human Erythrocytes

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