lactase The enzyme that metabolizes lactose. It is present in the stomach of neonatal mammals, but usually ceases to be secreted after weaning, hence milk is indigestible to most mammalian species after this time. In humans, it is a polymorphism: lactase is secreted throughout life in most people of north European and north Indian origin, but its frequency declines towards the tropics and most tropical, East Asian, Australian Aboriginal, Pacific, and Native American peoples cannot digest milk.

lactase The enzyme that hydrolyses lactose to glucose and galactose in the brush border of intestinal mucosal cells. See also disaccharide intolerance.

lactase (β-galactosidase) The enzyme that breaks down the milk sugar, lactose, to glucose and galactose by catalysing the hydrolysis of galactose residues.

lactase (lak-tayz) n. an enzyme, secreted by the glands of the small intestine, that converts lactose (milk sugar) into glucose and galactose during digestion.