any of a group of enzymes found in the small intestine, liver, and kidney of mammals that catalyze the breakdown of lactose (milk sugar) into the simple sugars glucose and galactose. Lactase is particularly abundant during infancy. The enzyme is thought to be produced by the mucous membrane cells that line the intestinal walls; granules localize in the brush border (a chemical barrier through which food must pass to be absorbed) that coats the intestinal villi.