y8-Galactosidase (8-D-galactoside galactohydrolase, EC is often known as lactase because it catalyzes the hydrolysis of lactose into its constituent sugars—viz., lactose + H2O ! galactose + glucose. All lactases are ^-galactosidases but some ^-galactosi-dases, including those from plant cells and mammalian organs other than the intestine, have little or no activity on lactose because their function is to hydrolyze other galactosyl moieties, including glycolipids, glyco-proteins, and mucopolysaccharides (1).

^-Galactosidase hydrolyzes O-glycosyl bonds and exhibits strict specificity for the glycone part of the substrate (Fig. 1). The only changes that allow activity (albeit reduced) are replacement of the hydroxylmethyl group on carbon 6 with a methyl group or a hydrogen atom. Consequently, the enzyme shows some activity on a-L-arabinosides and ^-D-fucosides. Other changes such as methylation of the hydroxyl on carbons 2, 3, 4, or 6; loss of the pyranose ring structure; or conversion to the a-anomeric form, lead to loss of activity (1). Replacement of the glycosidic oxygen with sulfur causes loss of activity but not loss of binding affinity (1). The enzyme shows wide tolerance for the structure of the aglycone which may be another sugar, an alkyl group or an aryl group. The nature of the aglycone, however, strongly influences the kinetic parameters (1).

Activity of intestinal lactase in mammals decreases after weaning, and this leads to some degree of lactose intolerance in many population groups. Consequently, the enzyme is used in clinical settings to aid lactose digestion and in commercial operations to produce low-lactose diary products.

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