Lactase-phlorizin hydrolase, which hydrolyzes
lactose, the major
carbohydrate in milk, plays a critical role in the nutrition of the mammalian neonate.
Lactose intolerance in adult humans is common, usually due to low levels of small intestinal
lactase. Low
lactase levels result from either intestinal injury or (in the majority of the world's adult population) alterations in the genetic expression of
lactase. Although the mechanism of decreased
lactase levels has been the subject of intensive investigation, no consensus has yet emerged. Recent studies have begun to define the cellular and molecular biology of this
enzyme. In animals and humans, a glycosylated precursor is proteolytically cleaved to yield the mature
enzyme on the microvillus membrane of the enterocyte, bound to the
lipid bilayer only by a hydrophobic anchor sequence. The
enzyme hydrolyzes
lactose,
phlorizin, and glycosylceramides. A decline in
lactase specific activity occurs at the time of weaning in most mammalian species; in most humans who have low
lactase activity as adults, the decline occurs at approximately 3-5 years of age. In a few human groups, the elevated juvenile level of
lactase specific activity persists throughout adulthood. These developmental patterns of
lactase expression are most likely regulated at the level of gene transcription.