Purification and properties of a novel beta-galactosidase or exo-(1-->4)-beta-D-galactanase from the cotyledons of germinated Lupinus angustifolius L. seeds.

PMID 7764618


The main polysaccharide component of the thickened cell walls in the storage parenchyma of Lupinus angustifolius L. cotyledons is a linear (1-->4)-beta-linked D-galactan, which is mobilised after germination (L. A. Crawshaw and J.S.G Reid, 1984, Planta 160, 449-454). The isolation from the germinated cotyledons of a beta-D-galactosidase or exo-(1-->4)-beta-D-galactanase with a high specificity for the lupin galactan is described. The enzyme, purified using diethylaminoethyl-cellulose, carboxymethyl-cellulose and affinity chromatography on lactose-agarose, gave two bands (major 60 kDa, minor 45 kDa) on sodium dodecyl sulphate-gel electrophoresis, and two similar bands on isoelectric focusing (major, pI 7.0, minor pI 6.7, both apparently possessing enzyme activity). The minor component cross-reacted with an antiserum raised against, and affinity-purified on, the major band. Both components had a common N-terminal sequence. The minor component was probably a degradation product of the major one. The enzyme had limited beta-galactosidase action, catalysing the hydrolysis of p-nitrophenyl-beta-D-galactopyranoside and (1-->4)- and (1-->6)-beta-linked galactobioses. Lactose [beta-D-galactopyranosyl-(1-->4)-D-glucose] was hydrolysed only very slowly and methyl-beta-D-galactopyranoside not at all. Lupin galactan was hydrolysed rapidly and extensively to galactose, whereas other cell-wall polysaccharides (xyloglucan and arabinogalactan) with terminal non-reducing beta-D-galactopyranosyl residues were not substrates. A linear (1-->4)-beta-linked galactopentaose was hydrolysed efficiently to the tetraose plus galactose, but further sequential removals of galactose to give the tetraose and lower homologues occurred more slowly. Galactose, gamma-galactonolactone and Cu+2 were inhibitory. No endo-beta-D-galactanase activity was detected in lupin cotyledonary extracts, whereas exo-galactanase activity varied pari passu with galactan mobilisation. Exo-galactanase protein was detected, by Western immunoblotting of cotyledon extracts, just before the activity could be assayed and then increased and decreased in step with the enzyme activity. The exo-galactanase is clearly a key enzyme in galactan mobilisation and may be the sole activity involved in depolymerising the dominant (1-->4)-beta-galactan component of the cell wall.

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