Purification and characterisation of a galactoglucomannan from kiwifruit (Actinidia deliciosa)

A galactoglucomannan (GGM) has been purified from the primary cell walls of ripe kiwifruit. A combination of barium hydroxide precipitation, anion exchange- and gel-permeation chromatography gave a chemically homogeneous polymer with a 1:2:2 galactose-glucose-mannose ratio and a molecular weight range of 16-42 kDa. Complete hydrolysis of the polymer with endo-1,4-beta -mannanase (EC 3.2.1.78) from Aspergillus niger gave a mixture of oligosaccharides, three of which (II, III, IV) accounted for more than 80%, of the GGM. Structural characterisation of these oligosaccharides and the original polysaccharide was achieved by linkage analysis, 1D and 2D NMR spectrometry and enzymatic hydrolysis. Oligosaccharide II beta -D-Glcp-(1 -->4)-beta -D-Manp-(1 -->, III beta -D-Glcp-(1 -->4)[alpha -oD-Galp-(1 -->6)]-beta -D-Manp-(1 -->, and IV beta -D-Glcp-(1 -->4)-[beta -D-Galp-(1 -->2)-alpha -D-Galp-(1 -->6)]-beta -D-Manp-(1 -->4)-beta -D-Glcp-(1 -->4)-beta -D-Manp-(l -, appeared in the molar ratio of 2.1:1. A trace amount of mannobiose (I) was detected, indicating that some of the mannosyl residues were contiguous. It is concluded that the predominant structural feature of kiwifruit GGM is a backbone of alternating beta-(1 -->4)-linked D-glucopyranosyl and D-mannopyranosyl residues, with approximately one third of the latter carrying side-chains at O-6 of single alpha -D-Galp-(1 --> residues (50% of the branches) or the disaccharide beta -D-Galp-(1 -->2)-alpha -D-Galp-(1 --> (50% of the branches), the substituted residues being separated by three or five unsubstituted monosaccharide units. (C) 2001 Elsevier Science Ltd. All rights reserved.