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 methods, the membrane and the gel filtration methods, were quite similar. The uptake was highest for glucose and subsequently, in order, for fructose, sucrose and sorbitol. It was not stimulated by addition of ATP, although the vesicles could create a proton gradient. However, the uptakes were significantly inhibited by p-chloromercuribenzene sulphonate (PCMBS, SH-reagent and inhibitor of sugar transporter). Further, the PCMBS-sensitive uptakes of glucose and fructose saturated with their increased concentrations. Thus, these PCMBS-sensitive uptakes are mediated by the transporter of facilitated diffusion. The uptakes of glucose or fructose each had two Km values. Km values for glucose were 0.35 and 18 mM, and those for fructose were 1.6 and 25 raM. The uptake of 0.2 mM glucose was inhibited by 2 mM fructose and that of 2 mM fructose was inhibited by 2 mM glucose, but neither was inhibited by sucrose 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 transport system differed from the sucrose and sorbitol transport systems.