Effects of temperature on the transport of galactose in human erythrocytes.

The transport of galactose in human erythrocytes has been resolved recently into a mechanism which involves two asymmetric carriers operating in antiparallel fashion. The effects of temperature on this mediated transport system in the range of 0--25 degrees C show the following features. The Michaelis constants for zero-trans influx and efflux and for equilibrium-exchange efflux, do not vary with temperature. Arrhenius plots of the maximal velocities show breaks between 3 and 15 degrees C with activation energies two- to threefold larger below the break than above it. The relative contribution of the two types of carriers to the total transport rate is not affected by temperature. The kinetic properties of the prevalent type of carriers are analysed in terms of the simple carrier model as formulated by Lieb & Stein (1974). This analysis shows that the relative concentration of the unloaded carrier at the inner interface of the membrane increases upon cooling. The free energy of translocation of the unloaded carrier and the change in entropy involved in this step are significantly larger in the low temperature range (0--5 degrees C) than in the higher range (15--25 degrees C). The results are discussed briefly in terms of possible lipid-protein interaction and the physicochemical nature of the erythrocyte membrane.