Substrate Specificity of the H+-Sucrose Symporter on the Plasma Membrane of Sugar Beets (Beta vulgaris L.)

Previous results (TJ Buckhout, Planta [1989] 178: 393-399) indicated that the structural specificity of the H(+)-sucrose symporter on the plasma membrane from sugar beet leaves (Beta vulgaris L.) was specific for the sucrose molecule. To better understand the structural features of the sucrose molecule involved in its recognition by the symport carrier, the inhibitory activity of a variety of phenylhexopyranosides on sucrose uptake was tested. Three competitive inhibitors of sucrose uptake were found, phenyl-alpha-d-glucopyranoside, phenyl-alpha-d-thioglucopyranoside, and phenyl-alpha-d-4-deoxythioglucopyranoside (PDTGP; K(i) = 67, 180, and 327 micromolar, respectively). The K(m) for sucrose uptake was approximately 500 micromolar. Like sucrose, phenyl-alpha-d-thioglucopyranoside and to a lesser extent, PDTGP induced alkalization of the external medium, which indicated that these derivatives bound to and were transported by the sucrose symporter. Phenyl-alpha-d-3-deoxy-3-fluorothioglucopyranoside, phenyl-alpha-d-4-deoxy-4-fluorothioglucopyranoside, and phenyl-alpha-d-thioallopyranoside only weakly but competively inhibited sucrose uptake with K(i) values ranging from 600 to 800 micromolar, and phenyl-alpha-d-thiomannopyranoside, phenyl-beta-d-glucopyranoside, and phenylethyl-beta-d-thiogalactopyranoside did not inhibit sucrose uptake. Thus, the hydroxyl groups of the fructose portion of sucrose were not involved in a specific interaction with the carrier protein because phenyl and thiophenyl derivatives of glucose inhibited sucrose uptake and, in the case of phenyl-alpha-d-thioglucopyranoside and PDTGP, were transported.