Glucose-induced thermal stabilization of the native conformation of GLUT 1

The glucose transporter, GLUT 1, was purified from erythrocyte membranes and incorporated into vesicles of erythrocyte lipids. These protein-containing vesicles were studied with differential scanning calorimetry. It was found that the protein underwent an irreversible denaturation at 68.5 +/- 0.2 degreesC (at a scan rate of 0.25 degreesC/min) which was shifted to 72.6 +/- 0.2 degreesC in the presence of 500 mM D-glucose, while 500 mM L-glucose or 10 microM cytochalasin B did not produce a significant shift. The calorimetric enthalpy was found to be 150 kcal/mol, independent of the presence of D-glucose. On a weight basis this value is lower than that for soluble proteins, but it is comparable to values obtained with other integral membrane proteins. The van't Hoff enthalpy is similar to the calorimetric enthalpy, within the experimental error, indicating that the transition is not likely to be cooperative. The activation energy is estimated from both the scan rate dependence of the transition temperature and from the shape of the DSC curve. The presence of 500 mM D-glucose slightly decreases the activation energy. It is concluded that the shift to a higher denaturation transition temperature in the presence of D-glucose is not a result of increased kinetic stability of GLUT 1.