Purification and Properties of Sucrose:Sucrose Fructosyltransferases from Barley Leaves and Onion Seeds

Fructan biosynthesis in primary leaves of barley (Hordeum vulgare L. cv. Gerbel) was induced by continuous illumination of excised leaves. No fructans were synthesized during the first 8 h of induction, although enzymes were present which were able to synthesize 1-kestose from sucrose. In this first period no fructan:fructan fructosyltransferase (FFT) activity was detectable in an in vitro assay. Fructan accumulation occurred simultaneously with the appearance of FFT activity. Using FPLC ion exchange chromatography we were not able to separate a sucrose:sucrose fructosyltransferase (SST) activity from the hydrolytic activity caused by acid invertase. Km's of 10 mM and 300 mM were found for the hydrolytic and fructosyltransferase activity, respectively. Both activities were inhibited to the same extent by addition of pyridoxal or pyridoxin. These results provide additional evidence that both activities in barley could be the result of only one enzyme. A distinct enzyme with only SST activity was isolated from onion seeds. The enzyme had a molecular weight of 68,000, a pH optimum of 5.5 and a Km of 250 mM.