| Lactase | |||||||
|---|---|---|---|---|---|---|---|
| Identifiers | |||||||
| Symbol | LCT | ||||||
| Alt. symbols | LAC; LPH; LPH1 | ||||||
| NCBI gene | 3938 | ||||||
| HGNC | 6530 | ||||||
| OMIM | 603202 | ||||||
| RefSeq | NM_002299 | ||||||
| UniProt | P09848 | ||||||
| Other data | |||||||
| EC number | 3.2.1.108 | ||||||
| Locus | Chr. 2q21 | ||||||
| |||||||
Lactase (EC3.2.1.108) is anenzyme produced by many organisms and is essential to the complete digestion of wholemilk. It breaks down the sugarlactose into its component parts,galactose andglucose. Lactase is found in thebrush border of thesmall intestine of humans and other mammals. People deficient in lactase or lacking functional lactase may experience the symptoms oflactose intolerance after consumingmilk products.[1] Microbialβ-galactosidase (often loosely referred to as lactase) can be purchased as afood supplement and is added to milk to produce "lactose-free" milk products.
Lactase is an enzyme that some people are unable to produce in their small intestine.[2] Technology to produce lactose-free milk, ice cream, and yogurt was developed by theUSDA Agricultural Research Service in 1985.[3] This technology is used to add lactase to milk, thereby hydrolyzing the lactose naturally found in milk, leaving it slightly sweet but digestible by everyone.[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.
Lactase supplements can be used to treatlactose intolerance.[6]
Lactase produced commercially can be extracted both fromyeasts such asKluyveromyces fragilis andKluyveromyces lactis and from molds, such asAspergillus niger andAspergillus oryzae.[7] Its primary commercial use in supplements is to break down lactose in milk to make it suitable for people with lactose intolerance.[8][9] TheU.S. Food and Drug Administration has not independently evaluated these products.[10]
Lactase (or a similar form ofβ-galactosidase) is also used to screen forblue white colonies in themultiple cloning sites of variousplasmid vectors inEscherichia coli or other bacteria.[11][12]
The temperature optimum for human lactase is about 37 °C[13] and thepH optimum is 6.[14]
Inmetabolism, the β-glycosidic bond inD-lactose is hydrolyzed to formD-galactose andD-glucose, which can be absorbed through the intestinal walls and into the bloodstream. The overall reaction that lactase catalyzes is as follows:
The catalytic mechanism ofD-lactose hydrolysis retains the substrate anomeric configuration in the products.[15] While the details of the mechanism are uncertain, the stereochemical retention is achieved off a double displacement reaction. Studies ofE. 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.[16] The removal of theD-glucose leaving group may be facilitated by Mg-dependent acid catalysis.[16] The enzyme is liberated from the α-galactosyl moiety upon equatorial nucleophilic attack by water, which producesD-galactose.[15]
Substrate modification studies have demonstrated that the 3′-OH and 2′-OH moieties on the galactopyranose ring are essential for enzymatic recognition and hydrolysis.[17] The 3′-hydroxy group is involved in initial binding to the substrate while the 2′- group is not necessary for recognition but needed in subsequent steps. This is demonstrated by the fact that a 2-deoxy analog is an effective competitive inhibitor (Ki = 10mM).[17] Elimination of specific hydroxyl groups on the glucopyranose moiety does not eliminate catalysis.[17]
Lactase also catalyzes the conversion ofphlorizin tophloretin and glucose.
Lactase (Lactaid commercially) is used as a medication for lactose intolerance. Since it is an enzyme, its function can be inhibited by the acidity of the stomach. However, it is packaged in an acid-proof tablet, allowing the enzyme to pass through the stomach intact and remain in the small intestine. 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. Since the enzyme is not absorbed, it is excreted during the next bowel movement.[citation needed]
Preprolactase, the primary translation product, has a single polypeptide primary structure consisting of 1927 amino acids.[18] It can be divided into five domains: (i) a 19-amino-acid cleavedsignal sequence; (ii) a large prosequence domain that is not present in mature lactase; (iii) the mature lactase segment; (iv) a membrane-spanning hydrophobic anchor; and (v) a short hydrophilic carboxyl terminus.[18] The signal sequence is cleaved in theendoplasmic reticulum, and the resulting 215-kDa pro-LPH is sent to theGolgi 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.[20]
Mature human lactase consists of a single 160-kDa polypeptide chain that localizes to the brush border membrane of intestinal epithelial cells. It is oriented with the N-terminus outside the cell and the C-terminus in the cytosol.[18] LPH contains two catalytic glutamic acid sites. In the human enzyme, the lactase activity has been connected to Glu-1749, while Glu-1273 is the site of phlorizin hydrolase function.[21]
In humans, lactase is encoded by a single genetic locus on chromosome 2.[22] It is expressed exclusively by mammalian small intestine enterocytes and in very low levels in the colon during fetal development.[22] Humans are born with high levels of lactase expression. In most of the world's population, lactase transcription is down-regulated after weaning, resulting in diminished lactase expression in the small intestine,[22] which causes the common symptoms of adult-type hypolactasia, or lactose intolerance.[23] TheLCT gene provides the instructions for making lactase. Lactose intolerance in infants (congenital lactase deficiency) is caused by mutations in theLCT gene. 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.[25] This mutation has allowed almost half of the world's population to metabolize lactose without symptoms. Studies have linked the occurrence of lactase persistence to two differentsingle-nucleotide polymorphisms about 14 and 22 kilobases upstream of the 5'-end of the LPH gene.[26] Both mutations, C→T at position -13910 and G→ A at position -22018, have been independently linked to lactase persistence.[27]
The lactase promoter is 150 base pairs long and is located upstream of the site of transcription initiation.[27] The sequence is highly conserved in mammals, suggesting that critical cis-transcriptional regulators are located nearby.[27]Cdx-2,HNF-1α, andGATA have been identified as transcription factors.[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.[28] Developmentally regulated DNA-binding proteins may down-regulate transcription or destabilize mRNA transcripts, causing decreased LPH expression after weaning.[28]
