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-D-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-(1-->, 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 0-6 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.