Lactase; Origin, gene expression, localization, and function

Lactase-phlorizin hydrolase is a small intestinal enzyme responsible for the hydrolysis of the carbohydrate lactose in mammalian milk. During the neonatal period the enzyme is crucial for the nutrition of humans and most other mammals. Subsequently, the specific activity of lactase decreases to low adult levels. In most adult humans and other mammals, large amounts of lactose are no longer tolerated, and lactose ingestion may lead to gastrointestinal symptoms. People of Caucasian extraction and a few isolated other groups form a clear exception with regard to this pattern; high levels of lactase activity are maintained, enabling these people to consume diary products during adult life. This review will describe the role of lactase in the digestion of lactose in mammalian milk. The function and origin of the enzyme will be outlined, and the review will examine relevant issues regarding the consumption of lactose and the clinical syndrome of lactose intolerance. Furthermore, insight provided by molecular and cell biology into gene structure, promoter function, gene transcription, localization of expression in the small intestine, biosynthesis of lactase protein, and enzyme specificities will be outlined. Current thinking with respect to the regulation of expression of lactase during development, and the differences in expression between species and different human populations will also be discussed.