Kiwifruit beta-galactosidase: isolation and activity against specific fruit cell-wall polysaccharides
1993
Ross, G.S. | Redgwell, R.J. | MacRae, E.A.
A beta-galactosidase (EC 3.2.1.23) capable of degrading a number of fruit cell-wall polysaccharides in vitro, was isolated from ripening kiwifruit (Actinidia deliciosa [A. Chev.) C.F. Liang et A.R. Ferguson cv. Hayward). The enzyme has a molecular weight of approximately 60 kDa by gel permeation and consists of several basic isoforms. Several polypeptides were enriched during purification, with 33-, 46- and 67-kDa bands being predominant after sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The optimum activity of the enzyme against p-nitrophenyl-beta-D-galactopyranoside was at pH 3.2, but against a galactan purified from kifruit cell walls, it was at pH 4.9. The enzyme was specific for galactosyl residues in the beta-configuration, releasing galactose from a variety of kiwifruit cell-wall polysaccharide fractions including cell wall material, Na2CO3-soluble pectin, high-molecular-weight galactan, xyloglucan, and galactoglucomannan. A galactosylated glucuronomannan found throughout the kiwifruit plant was also a substrate for the enzyme. The results indicate that the enzyme attacks the non-reducing end of galactose side chains, cleaving single galactose residues which may be attached to the 2, 3, 4, or 6 position of the aglycone. Activity of the enzyme in-vitro was too low to account for the total loss of galactose from the cell walls during ripening. If the beta-galactosidase of this study is solely responsible for the removal of galactose from the cell wall during ripening then its in-vivo activity must be much greater than that observed in-vitro.
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