The metabolism of fructose polymers in plants. Transfructosylation in tubers of Helianthus tuberosus L.

1. A transfructosylase was separated from Jerusalem artichoke-tuber extracts. The partially purified enzyme was practically free from fructosans and its specific activity was increased more than sevenfold. 2. The enzyme was highly specific for terminal beta-(2-->1')-linked fructofuranosyl residues. 3. The most active donor found was 1(F)-fructosylsucrose, which had about five times the reactivity of the next higher homologues (degree of polymerization 4-6); (1-fructosyl)(2)fructose was intermediate in reactivity; sucrose was inactive. 4. Sucrose, and polymers of high degree of polymerization (>20), were the most efficient acceptors for transferred fructosyl residues of the compounds tested. 5. A variety of transferase activities appeared to be catalysed by the enzyme, namely, dismutation (;self-transfer') giving rise to a series of polymers ranging in degree of polymerization both above and below that of the original substrate, transfer to polymer giving rise to limited quantities of higher polymers, and transfer to sucrose; in the last case 1(F)-fructosylsucrose/sucrose transfer (resulting in exchange labelling of sucrosyl groups) was used to investigate the mechanism of the reaction more fully. 6. Since fructose transfer can be demonstrated in the living plant tissue, the inter-relationships of the various activities and their significance in fructosan metabolism are discussed.