Purification of a beta-galactosidase from cotyledons of Hymenaea courbaril L. (Leguminosae). Enzyme properties and biological function.

Beta-galactosidases are enzymes that can be found in most living beings and in the plant kingdom its activity and genes have been detected in several tissues such as ripening fruits, developing leaves and flowers and storage tissues such as cotyledons. In plants, their activities are usually associated with the secondary metabolism or with oligosaccharide or polysaccharide degradation. Polysaccharide specific beta-galactosidases include beta-galactanases, which attack pectic polymers and beta-galactosidases that attack xyloglucans (XG). In the present work we purified an XG-specific beta-galactosidase (named hcbetagal) from cotyledons of developing seedlings of Hymenaea courbaril, a legume tree from the Neotropical region of the world. The enzyme has a molecular weight of 52-62 kDa and was shown to attack specifically xyloglucan oligosaccharides (XGOs) but not the polymer. It has a pH optimum between 3 and 4 and at this pH range the enzyme increases activity linearly up to 50 degrees C. Kinetic studies showed that hcbetagal is inhibited competitively by free galactose (K(i) = 3.7). The biochemical properties of hcbetagal as a whole suggest that it is involved in storage xyloglucan mobilisation during seedling development. Its high specificity towards XGOs, the low pH optimum and the fact that it is inhibited by its product (galactose) suggest that hcbetagal might be one of the biochemical control points in xyloglucan catabolism in vivo. A possible relationship with functional stability of the wall during cell death as cotyledons undergo senescence is discussed.