CHARACTERIZATION OF HEXOSE TRANSPORTER FOR FACILITATED DIFFUSION OF THE TONOPLAST VESICLES FROM PEAR FRUIT

Tonoplast vesicles were prepared from the flesh tissue of mature pear fruit, Sugar uptakes into the vesicles determined by two different met hods, the membrane and the gel filtration methods, were quite similar. The uptake was highest for glucose and subsequently, in order, for fr uctose, sucrose and sorbitol. It was not stimulated by addition of ATP , although the vesicles could create a proton gradient. However, the u ptakes were significantly inhibited by p-chloromercuribenzene sulphona te (PCMBS, SH-reagent and inhibitor of sugar transporter). Further, th e PCMBS-sensitive uptakes of glucose and fructose saturated with their increased concentrations. Thus, these PCMBS-sensitive uptakes are med iated by the transporter of facilitated diffusion, The uptakes of gluc ose or fructose each had two K-m values. K-m values for glucose were 0 .35 and 18 mM, and those for fructose were 1.6 and 25 mM, The uptake o f 0.2 mM glucose was inhibited by 2 mM fructose and that of 2 mM fruct ose was inhibited by 2 mM glucose, but neither was inhibited by sucros e or sorbitol. O-methyl-glucose (OMG) also inhibited both the glucose and fructose uptakes. Therefore, the same transporter may mediate both glucose and fructose uptakes at lower concentrations; this hexose tra nsport system differed from the sucrose and sorbitol transport systems .