The alpha-D-glucosyl C-2 hydroxyl is required for binding to the H(+)-sucrose transporter in phloem

The specificity of uptake of sucrose into isolated phloem tissue from Apium graveolens has been investigated using a number of analogues of sucrose. The presence of a single saturable transport system for sucrose was confirmed using the double isotope method of Inui and Christensen (J. Gen. Physiol. 50 (1966) 203-224). 4-Hydroxyphenyl beta-D-fructofuranoside showed no inhibition of sucrose uptake, whereas 4-hydroxyphenyl alpha-D-glucopyranoside showed competitive inhibition with a Ki of 6.7 mM. 4-Methoxyphenyl alpha-D-glucopyranoside also inhibited sucrose uptake competitively (Ki = 6.0 mM). This compound was also synthesised radioactively labelled with 14C and its uptake into phloem tissue was conclusively demonstrated to occur by active transport on the same carrier as sucrose. Contrastingly, 4-methoxyphenyl alpha-D-2-deoxyglucopyranoside displayed non-competitive inhibition of sucrose influx (Ki = 2.5 mM) and uptake of the 14C-labelled compound was insensitive to FCCP, PCMBS and sucrose. We conclude that the hydroxyl group at the C-2 position on the glucopyranosyl moiety is essential for binding to the sucrose carrier in this tissue.