The temperature dependence of the uptake of glucose by exchange transport is investigated at two pH values (7.5 and 5). The Arrhenius's energy of activation, the heat of activation, the free energy of activation and the entropy of activation are calculated. The parameters are different at pH 7.5 and 5. For both pH levels the heat of activation and the entropy of activation change abruptly at 20°C. This finding leads us to assume that at this 1:emperature a change in the structure of the membrane takes place.

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1. A method was developed which allowed the measurement of sugar uptake into red cells after only 4 sec.2. 3. For these conditions, we found linear plots against time, which enabled us to determine the initial velocity for the uptake of label, i.e. the unidirectional influx.4. Influx depended on the widely varying cis-concentration of glucose in the medium in the manner of a simple saturation-relationship. The parameters J,,, and K m , determined from a reciprocal plot, were different for the three chosen [St,]-conditions. Some of these parameters were comparable to values of other papers, determined by measuring the efflux of glucose from red cells. 5.The results were compared with predictions derived from two models of a mobile carrier: the symmetrical model of Levine, Oxender and Stein [I] and the asymmetrical, but passive model of Geck [2]. The relationship between glucose uptake plotted against time and influx plotted against cis-concentration corresponded to both models. Some, but not all, ratios between the measured parameters matched the predictions of the symmetrical model, whereas all ratios were compatible with the asymmetrical model. 6. From both models, the translocation resistance comes out to be much greater for the free carrier than for the glucose-carrier complex: 10 to 30 fold a t 20 "C and circa 200 fold a t 0 "C. Hence, the mobility of the free carrier appears to be very much reduced in the cold.

Steroids inhibit the exchange transport of glucose in human erythrocytes. The extent of inhibition is roughly correlated to the affinity of the steroids to the membrane lipids. All C-21-steroids tested show a competitive inhibition while the C-19-steriods show different types of inhibition. 5Beta-androstane-3,17-dione acts as a competitive inhibitor. The inhibition by testosterone is of mixed type, while with androst-4-ene-3,17-dione and 5alpha-androstane-3,17-dione a non-competitive inhibition is observed. In this case two inhibitor molecules can be bound per transport molecule. The "non-competitive" inhibitors compete also to some extent with the glucose binding. This effect, however, is at high inhibitor concentrations masked by the more powerful non-competitive inhibition. Competitive and non-competitive inhibitors compete with each other. The structural requirements for the different types of inhibition are discussed.

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