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-tran s uptake of 3-O-methyl-D-glucoside, 2-deoxy-D-glucose and D-mannose in to 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-ma x of zero-trans uptake of 3-O-methyl-D-glucoside was significantly hig her, (1.25 +/- 0.06 mu mol(10 min)(-1)(ml cell water)(-1)) than the co rresponding 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 u ptake routes, the V-max of zero-trans exit of 3-O-methyl-D-glucoside i s 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 accelera tion 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 hi gher 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 th at of the slower sugar, mannose by mannose. Hence, the observed findin gs are inconsistent with the one-site model, but confirm the earlier d isputed studies of Miller, D.M. (1968; Biophys. J. 8, 1329-1338). (7). A two-site model, in which sugar exchange is considered as a simultan eous process, predicts that the acceleration ratio of mannose influx b y 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.