REEXAMINATION OF HEXOSE EXCHANGES USING RAT ERYTHROCYTES - EVIDENCE INCONSISTENT WITH A ONE-SITE SEQUENTIAL EXCHANGE MODEL, BUT CONSISTENT WITH A 2-SITE SIMULTANEOUS EXCHANGE MODEL

(1). The kinetic parameters of zero-trans net uptake and infinite-trans uptake of 3-O-methyl-D-glucoside, 2-deoxy-D-glucose and D-mannose into rat red cells at 24 degrees C were measured after taking account of the linear diffusion components of flux. (2). Zero-trans exits of 3-O-methyl-D-glucoside and D-mannose from rat cells were also measured. (3). After correction for linear flux via non-specific routes, the V-max of zero-trans uptake of 3-O-methyl-D-glucoside was significantly higher, (1.25 +/- 0.06 mu mol(10 min)(-1)(ml cell water)(-1)) than the corresponding parameters of mannose or 2-deoxy-D-glucose, (0.33 +/- 0.01 and 0.39 +/- 0.01 mu mol(10-min)(-1)(ml cell water)(-1), respectively; P < 0.001). (4). After correction for linear flux via non-specific uptake routes, the V-max of zero-trans exit of 3-O-methyl-D-glucoside is significantly higher (1.70 +/- 0.1 mu mol(10 min)(-1)(ml cell water)(-1)) than the corresponding value for mannose exit flux, (1.10 +/- 0.1 mu mol(10 min)(-1)(ml cell water)(-1); P < 0.001). (5). The acceleration ratio, i.e., the ratio of infinite-trans influx V-max/zero-trans influx V-max of mannose by mannose (9.12 +/- 0.03) is significantly higher than that of 3-O-methyl-D-glucose by 3-O-methyl-D-glucose (2.77 +/- 0.14)(P < 0.001). (6). The one-site simple carrier model of glucose transport in which sugar exchange is viewed as a sequential process, predicts that the acceleration ratio of the more rapidly moving sugar 3-O-methyl-D-glucose by 3-O-methyl-D-glucose should be greater than that of the slower sugar, mannose by mannose. Hence, the observed findings are inconsistent with the one-site model, but confirm the earlier disputed studies of Miller, D.M. (1968; Biophys. J. 8, 1329-1338). (7). A two-site model, in which sugar exchange is considered as a simultaneous process, predicts that the acceleration ratio of mannose influx by mannose should be higher than for 3-O-methyl-D-glucose by 3-O-methyl-D-glucose. The data are, therefore, consistent with a two-site model.