INULIN SYNTHESIS BY A COMBINATION OF PURIFIED FRUCTOSYLTRANSFERASES FROM TUBERS OF HELIANTHUS-TUBEROSUS

Sucrose-sucrose 1-fructosyltransferase (1-SST) was purified 100-fold f rom tubers of Helianthus tuberosus L. The purified enzyme was essentia lly devoid of invertase activity and could be separated by isoelectric focusing into five isoforms which all were composed of two subunits ( 59 and 26 kDa). Fructan-fructan 1-fructosyltransferase (1-FFT) was pur ified from the same source [M. Luscher et al. (1993) New Phytologist 1 23, 437-442). When incubated individually with sucrose, 1-FFT was inac tive while 1-SST formed isokestose (trimer) and, upon prolonged incuba tion, some nystose (tetramer). When a combination of the two enzymes w as incubated with sucrose, a series of oligofructosides with a degree of polymerization of up to 20 was formed. Amino acid sequences of tryp tic peptide fragments from both 1-SST and 1-FFT indicate that these en zymes are highly homologous with plant invertases.