FRUCTAN CHEMICAL-STRUCTURE AND SENSITIVITY TO AN EXOHYDROLASE

A fructan exohydrolase selective for (2 --> 1)-linked terminal fructosyl linkages, isolated from barley (Hordeum vulgare L. cv. Morex) stems and leaf sheaths, was used to elucidate the chemical structures of several oligomeric fructans extracted from liliaceous and graminaceous species. Products released by enzymic and mild acid hydrolysis were separated by reversed-phase high-performance liquid chromatography. Gas-liquid chromatography-mass spectrometry of partially methylated alditol acetates permitted unequivocal deduction of many linkage sequences, first of the hydrolysis products and then of the original oligomers. We found that bifurcose, a tetrasaccharide formed by addition of a fructosyl unit to O-6 of the central fructose residue of 1-kestose, was a central molecule in the generation of the branched, oligomeric fructans of wheat (Triticum aestivum L. cv. Fidel). These arise by the extension of both (2 --> 1)- and (2 --> 6)-linked chains from the bifurcose branch-point residue. Some of the (2 --> 6)-linked units that slowly accumulate in oligomers may arise in vivo from selective hydrolysis, by fructan exohydrolases, of (2 --> 1)-linked terminal units at branch point residues rather than by the action of (2 --> 6)-specific synthases. Limited hydrolysis by specific exohydrolases in vitro coupled with separation of the oligomeric products constitutes an effective approach to the sequence analysis of complex oligosaccharides.