SYNTHESIS OF INULIN OLIGOMERS IN TISSUE-SLICES, PROTOPLASTS AND INTACT VACUOLES OF JERUSALEM-ARTICHOKE

Tissue slices and protoplasts of Jerusalem artichoke (Helianthus tuberosus L. cv. Bianco) incorporate radioactivity from glucose into sucrose, 1-kestose, nystose (inulin tetrasaccharide) and other fructan oligomers, and intact isolated vacuoles absorb sucrose and incorporate radioactivity from sucrose into 1-kestose. Gas-liquid chromatography and radiogas proportional counting demonstrated that, in all cases, the glucosyl and internal fructosyl units of 1-kestose contained substantial amounts of radioactivity whereas the terminal fructosyl unit was essentially devoid of label. We propose that incorporation of radioactivity into 1-ketose is primarily from transfer of the unlabeled terminal fructosyl unit of nascent 1-kestose to labeled sucrose by fructan:fructan fructosyl transferase and not by sucrose:sucrose fructosyl transferase. Such reactions do not result in a net gain in fructan, and hence, more definitive estimations of SST activity and the net synthesis of fructans in vivo are needed. Our success preserving fructan metabolism in isolated vacuoles, however, provides a system to examine more closely the role of the tonoplast in vectorial transport of sucrose into the vacuole, glucose and fructose out of the vacuole, and the association of these transport processes with net increases in synthesis of 1-kestose and fructans.