Lactase is an enzyme present in the gut that is responsible for breaking down complex lactose sugars into simpler sugars such as glucose and galactose which can then be utilized for energy and bodily functions.
Lactase is located in the brush border membrane of small-intestinal cells called enterocytes. The enzyme is located in the apex of the brush border membrane where it is attached by its C-terminal end, with the rest of the molecule projecting into the intestinal lumen.
Lactase is at its most active during infancy and is vital for the infant's survival when milk is the main source of nutrition. The enzyme then becomes less active after the weaning phase and in adults this decline in activity is referred to as "lactase non-persistence." Conversely, the lactase persistent state describes when a high level of lactase activity is maintained beyond the weaning phase and throughout life.
Lactase persistent individuals can usually hydrolyze large amounts of lactose and can consume milk and dairy without complications. Those with lactase non-persistence, however, may show symptoms of lactose intolerance.
Genetics of lactase production and decline
Usually, as the weaning period ends in humans, lactase production declines and among communities that do not consume dairy products (vegans for example), the decline can be by as much as around 90% by time a child reaches about 4 years of age.
Lactase is encoded by a single gene (LCT) of approximately 50 kb located on chromosome 2. The gene has 17 exons and codes for an mRNA molecule which has 6274 bases with 1927 amino acid residues.
Lactase has also been shown to remain persistent in people who have specific chromosomal mutations. For example, those with mutations on chromosome 2 do not have a reduction in lactase production. With time, these individuals continue to break down lactose and have no difficulty in consuming milk and dairy products. This seems to be an evolutionary change that has occurred as humans have adapted to milk and dairy consumption, especially in northern Europe and east African populations.
Lactase persistence in people reaching adulthood is a dominant genetic trait and lactose intolerance a recessive trait. The gene associated with lactose intolerance is a noncoding variation in the MCM6 gene.