LACTASE - ORIGIN, GENE-EXPRESSION, LOCALIZATION, AND FUNCTION

Lactase-phlorizin hydrolase is a small intestinal enzyme responsible f or the hydrolysis of the carbohydrate lactose in mammalian milk. Durin g the neonatal period the enzyme is crucial for the nutrition of human s and most other mammals. Subsequently, the specific activity of lacta se decreases to low adult levels. In most adult humans and other mamma ls, large amounts of lactose are no longer tolerated, and lactose inge stion may lead to gastrointestinal symptoms. People of Caucasian extra ction and a few isolated other groups form a clear exception with rega rd to this pattern; high levels of lactase activity are maintained, en abling these people to consume dairy products during adult life. This review will describe the role of lactase in the digestion of lactose i n mammalian milk. The function and origin of the enzyme will be outlin ed, and the review will examine relevant issues regarding the consumpt ion of lactose and the clinical syndrome of lactose intolerance. Furth ermore, insight provided by molecular and cell biology into gene struc ture, promoter function, gene transcription, localization of expressio n in the small intestine, biosynthesis of lactase protein, and enzyme specificities will be outlined. Current thinking with respect to the r egulation of expression of lactase during development, and the differe nces in expression between species and different human populations wil l also be discussed.