Effects of hexoses and anions on the erythritol permeability of human red cells

1. The effect of hexoses and of the anions chloride, thiocyanate, and salicylate on the permeability of human red cells to [ 14 C]erythritol has been studied. 2. It was confirmed that erythritol competes with glucose, mannose, and galactose for the hexose transfer system of the red cell membrane. Approximately 25% of the erythritol influx was insensitive to the presence of hexoses or phloretin. Identical maximum degrees of inhibition were obtained with 0·3 M glucose and with phloretin (0·5 × 10 -3 M ). In the absence of competing inhibitors the erythritol permeability, P , was 1·2 × 10 -7 cm/sec at 38° C. At maximum inhibition P was 0·3 × 10 -7 cm/sec. 3. Erythritol is able to penetrate the membrane by two pathways, only one of which is sensitive to hexoses. Both hexose-sensitive and hexose-insensitive erythritol influx are well described by first-order diffusion kinetics. The affinity of erythritol for the hexose transfer system is very low, and the half saturation constants of hexoses can be determined from their ability to retard erythritol permeation. The following values were found for the half saturation of the transport system with hexoses at 38° C: glucose 6 m M , mannose 11 m M , and galactose 40 m M . 4. Thiocyanate and salicylate reduce the hexose-sensitive fraction of erythritol influx, but the hexose-insensitive erythritol permeability is not affected when chloride is replaced by the foreign anions. This applies to the whole temperature range between 0 and 38° C, where the ionic permeabilities of red cells have been shown to be profoundly changed by thiocyanate and salicylate.