[4] Without lactase, lactose-intolerant people pass the lactose undigested to the colon[5] where bacteria break it down, creating carbon dioxide which leads to bloating and flatulence.

[10] Lactase (or a similar form of β-galactosidase) is also used to screen for blue white colonies in the multiple cloning sites of various plasmid vectors in Escherichia coli or other bacteria.

[14] In metabolism, the β-glycosidic bond in D-lactose is hydrolyzed to form D-galactose and D-glucose, which can be absorbed through the intestinal walls and into the bloodstream.

Studies of E. coli lactase have proposed that hydrolysis is initiated when a glutamate nucleophile on the enzyme attacks from the axial side of the galactosyl carbon in the β-glycosidic bond.

In the small intestine it can act on ingested lactose molecules, allowing the body to absorb the digested sugar which would otherwise cause cramping and diarrhea.

[18] The signal sequence is cleaved in the endoplasmic reticulum, and the resulting 215-kDa pro-LPH is sent to the Golgi apparatus, where it is heavily glycosylated and proteolytically processed to its mature form.

[19] The prodomain has been shown to act as an intramolecular chaperone in the ER, preventing trypsin cleavage and allowing LPH to adopt the necessary 3-D structure to be transported to the Golgi apparatus.

Mature human lactase consists of a single 160-kDa polypeptide chain that localizes to the brush border membrane of intestinal epithelial cells.

Mutations are believed to interfere with the function of lactase, causing affected infants to have a severely impaired ability to digest lactose in breast milk or formula.

[24] Some population segments exhibit lactase persistence resulting from a mutation that is postulated to have occurred 5,000–10,000 years ago, coinciding with the rise of cattle domestication.

[27] Studies of hypolactasia onset have demonstrated that despite polymorphisms, little difference exists in lactase expression in infants, showing that the mutations become increasingly relevant during development.