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

Summary

The rate of exit ofd(+)-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 ofV _max andK _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, whereK _m is large and consequently underestimatesK _m . An improved modification of the method, suggested by Bolis, Luly and Wilbrandt, overcomes this difficulty and leads to more reliable values forV _max andK _m . These new results show thatE _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 inE _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.