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Lactase is an enzyme produced by many organisms. It is located in the brush border of the small intestine of humans and other mammals. Lactase is essential to the complete digestion of whole milk; it breaks down lactose, a sugar which gives milk its sweetness. Lacking lactase, a person consuming dairy products may experience the symptoms of lactose intolerance. Lactase can be purchased as a food supplement, and is added to milk to produce "lactose-free" milk products. Lactase (also known as lactase-phlorizin hydrolase, or LPH), a part of the β-galactosidase family of enzymes, is a glycoside hydrolase involved in the hydrolysis of the disaccharide lactose into constituent galactose and glucose monomers. Lactase is present predominantly along the brush border membrane of the differentiated enterocytes lining the villi of the small intestine.〔 In humans, lactase is encoded by the LCT gene. == Mechanism == The optimum temperature for human lactase is about 37°C for its activity and has an optimum pH of 6. 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. The overall reaction that lactase catalyzes is C12H22O11 + H2O → C6H12O6 + C6H12O6 + heat. The catalytic mechanism of D-lactose hydrolysis retains the substrate anomeric configuration in the products. While the details of the mechanism are uncertain, the stereochemical retention is achieved through a double displacement reaction. 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. The removal of the D-glucose leaving group may be facilitated by Mg-dependent acid catalysis.〔 The enzyme is liberated from the ''α''-galactosyl moiety upon equatorial nucleophilic attack by water, which produces D-galactose.〔 Substrate modification studies have demonstrated that the 3’-OH and 2’-OH moieties on the galactopyranose ring are essential for enzymatic recognition and hydrolysis. 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).〔 Elimination of specific hydroxyl groups on the glucopyranose moiety does not completely eliminate catalysis.〔 Lactase also catalyzes the conversion of phlorizin to phloretin and glucose. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Lactase」の詳細全文を読む スポンサード リンク
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