The effect of temperature on the competitive inhibition of sorbose transfer in human erythrocytes by glucose

The detection of the enthalpy of binding of D‐glucose of human red blood cell membranes by microcalorimetry

“…Estimates of the number of specific D-glucose binding sites per cell range from 2 to 5 X 10' [22-241; if we take a figure of 6 X lo-l3 g of non-haemoglobin protein per ghost [l l] and our saturation enthalpy of about 50 mJ (g NHP)-r the enthalpy of binding lies in the range 91 to 36 kJ mol-'. The lower figure is very close to that found from the kinetic method of inhibition of sorbose entry into erythrocytes by glucose [6] at 15°C. Unfortunately the level of precision of the data is not high and the possibility that several distinct binding sites exist cannot be ruled out.…”

“…This estimate of the half-saturation constant is in line with the values of K, between 1 and 10 mM reported by various workers using several different techniques. The sign of the enthalpy change agrees with that predicted using the kinetic method of inhibition of sorbose entry by glucose [6] and the equilibrium exchange experiments of Levine and Stein [9] and Jung et al…”

Structural requirements for binding to the sugar-transport system of the human erythrocyte

“…Human erythrocyte suspension. This was prepared by a method similar to that of Levine et al (1971). Out-dated transfusion blood, which had been stored at 4°C, was washed with 3 x 6 vol.…”

“…Inhibition constants. Inhibition constants were measured by the method of Levine et al (1971) at 25°C. The sugar to be tested (1.5 ml, usually 33.3 mm) in 25mM-sodium phosphate buffer, pH7.4, containing 1 % NaCl, was added to 2.5ml of cell suspension previously adjusted to 50 % packed cell volume.…”

“…It is generally accepted that L-sorbose enters the erythrocyte by the system that transports D-glucose, although it has a very low affinity for the carrier (LeFevre & Davies, 1951;Sen & Widdas, 1962). It has been shown by Levine et al (1971) that if the entry of L-sorbose into the erythrocyte is measured in the presence and absence of a second sugar that binds strongly to the carrier, an inhibition constant K, can be measured which represents, on the symmetrical carrier model where k+l/k-1 = k_3/k+3, the true dissociation constant of the carrier-sugar complex. In the asymmetric carrier model, where this is not true, Scheme 1.…”