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-β-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 β- d-Glc p-(1→4)-β- d-Man p-(1→, III β- d-Glc p-(1→4)-[α- d-Gal p-(1→6)]-β- d-Man p-(1→, and IV β- d-Glc p-(1→4)-[β- d-Gal p-(1→2)-α- d-Gal p-(1→6)]-β- d-Man p-(1→4)-β- d-Glc p-(1→4)-β- d-Man p-(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 β-(1→4)-linked d-glucopyranosyl and d-mannopyranosyl residues, with approximately one third of the latter carrying side-chains at O-6 of single α- d-Gal p-(1→ residues (50% of the branches) or the disaccharide β- d-Gal p-(1→2)-α- d-Gal p-(1→ (50% of the branches), the substituted residues being separated by three or five unsubstituted monosaccharide units.