KINETICS ANALYSIS OF THE PLASMA-MEMBRANE SUCROSE H+ SYMPORTER FROM SUGAR-BEET (BETA-VULGARIS L) LEAVES

The kinetics behavior of the H+-sucrose (Suc) symporter was investigated in plasma membrane vesicles from sugar beet (Beta vulgaris L.) leaves by analyzing the effect of external and internal pH (pH(o) and pH(i), respectively) on Suc uptake. The apparent K-m for Suc uptake increased 18-fold as the pH(o) increased from 5.5 to 7.5. Over this same pH(o) range, the apparent V-max for Suc uptake remained constant. The effects of pH(i) in the presence or absence of internal Suc were exclusively restricted to changes in V-max. Thus, proton concentration on the inside of the membrane vesicles ([H+](i)) behaved as a noncompetitive inhibitor of Suc uptake. The K-m for the proton concentration on the outside of the membrane vesicles was estimated to be pH 6.3, which would indicate that at physiological apoplastic pH Suc transport might be sensitive to changes in pH(o). On the other hand, the [H+](i) for half-maximal inhibition of Suc uptake was approximately pH 5.4, making regulation of Suc transport through changes in [H+](i) unlikely. These results were interpreted in the framework of the kinetics models for co-transport systems developed by D. Sanders, U.-P. Hansen, D. Gradmann, and C. L. Slayman U Membr Biol [1984] 77: 123-152). Based on their analysis, the behavior of the Suc symporter with respect to the [H+](i) is interpreted as an ordered binding mechanism by which the binding of Suc on the apoplastic side of the membrane and its release on the symplastic side precedes that of H+ (i.e. a first-on, first-off model).