The temperature dependence of the facilitated transport of d(+)-glucose across the human red cell membrane

The rate of exit of d(+)-glucose from human red cells was measured as a function of the extracellular glucose concentration over the temperature range 12 to 47°C. The results were analyzed at each temperature, according to the kinetic model of Widdas and of Rosenberg and Wilbrandt, in terms of the apparent maximum exit rate (V_max) and the apparent dissociation constant (K_m) of the carrier-glucose complex. When the values of V_max and K_m were obtained by the same graphical method as that used by Sen and Widdas, the results were very similar to theirs insofar as the effect of temperature is concerned. In particular, the apparent standard enthalpy of dissociation (ΔH_m) of the carrier-glucose complex does not vary with temperature, whereas the apparent activation energy (E_max) for the translocation of the carrier increases strongly with decreasing temperature. It is shown that the explanation of these findings given by Dawson and Widdas is internally inconsistent. Furthermore, the graphical method as used by these authors is unreliable at higher temperatures, where K_m is large and consequently underestimates K_m. An improved modification of the method, suggested by Bolis, Luly and Wilbrandt, overcomes this difficulty and leads to more reliable values for V_max and K_m. These new results show that E_max decreases, and ΔH_m increases, as the temperature is raised. This behavior is shown to be consistent with the modified kinetic model for sugar transport proposed by Wilbrandt, in which the translocation rate of the loaded carrier is assumed to be different from that of the empty carrier. The changes in E_max and ΔH_m with temperature are the result of the difference in true activation energies for the translocation of the loaded and empty carrier.