Lactose is a sugar that is found most notably in milk. Lactose makes up around 2–8% of milk (by weight), although the amount varies among species and individuals. It is extracted from sweet or sour whey. The name comes from lacte, the Latin word for milk, plus the -ose ending used to name sugars.
Lactase (LCT), 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.
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Lactose is a disaccharide that consists of β-D-galactose and β-D-glucose molecules bonded through a β1-4 glycosidic linkage. Lactose makes up around 2-8% of the solids in milk. The name comes from the Latin word for milk, plus the -ose ending used to name sugars. Its empirical formula is C12H22O11 and its molecular weight is 342.3 g/mol.
Digestion of lactose: Infant mammals are fed on milk by their mothers. To digest it an enzyme called lactase (β1-4 disaccharidase) is secreted by the intestinal villi, and this enzyme cleaves the molecule into its two subunits for absorption.
Since lactose occurs mostly in milk, in most species the production of lactase gradually ceases with maturity, and they are then unable to metabolise lactose. This loss of lactase on maturation is also the default pattern in most adult humans. However, many people with ancestry in Europe, the Middle East, India, and the Maasai of East Africa, have a version of the gene for lactase that is not disabled after infancy, and in many of these cultures other mammals such as cattle, goats, and sheep are milked for food.
This fact may cast doubt on some arguments by proponents of the Paleolithic diet, who argue that human metabolic needs have not changed since the last ice age. The process of retaining infant characteristics into adulthood is one of the simplest routes of adaptation, and is known as neoteny.
Lactose intolerance is the inability to metabolize lactose, because of a lack of the required enzyme lactase in the digestive system. It is estimated that 75% of adults worldwide show some decrease in lactase activity during adulthood. The frequency of decreased lactase activity ranges from as little as 5% in northern Europe, up to 71% for Sicily, to more than 90% in some African and Asian countries.
Lactase (LCT), a member of the β-galactosidase family of enzyme, is involved in the hydrolysis of the disaccharide lactose into constituent galactose and glucose monomers. In humans, lactase is present predominantly along the brush border membrane of the differentiated enterocytes lining the villi of the small intestine.
Lactase is essential for digestive hydrolysis of lactose in milk. Deficiency of the enzyme causes lactose intolerance; most humans become lactose intolerant as adults.
Characteristics: Lactase has an optimum temperature of about 48°C (118.4°F) for its activity and an optimum pH of 6.5. In humans, the gene is localised on the second chromosome (2q21). Bacterial and Archaea lactase lack a membrane binding domain and free float around the cell, these also tend to be more general β-galactosidase that will cleave more than just lactose.
Gene: Lactase persistence, the genetic trait in which intestinal lactase activity persists at childhood levels into adulthood, varies in frequency in different human populations, being most frequent in northern Europeans and certain African and Arabian nomadic tribes, who have a history of drinking fresh milk. Lactase persistence is not due to mutations within the lactase gene, LCT, but to mutations outside the gene in control regions that regulate its expression. They are two intronic variations (C/T and G/A) in the MCM6 gene, MCM6, located approximately 14 kb (-13910) and 22 kb (-22018) upstream of LCT, respectively. C/T(-13910) variant has been shown that can function in vitro as a cis element capable of enhancing differential transcriptional activation of LCT promoter.
Industrial use: Lactase produced commercially can be extracted both from yeasts such as Kluyveromyces fragilis and Kluyveromyces lactis and from fungi, such as Aspergillus niger and Aspergillus oryzae. Its primary commercial use is to break down lactose in milk to make it suitable for people with lactose intolerance. Lactase is also used in the manufacture of ice cream. Because glucose and galactose are sweeter than lactose, lactase produces a more pleasant taste. Lactose also crystallises at the low temperatures of ice cream; however, its constituent products stay liquid and contribute to a smoother texture. Lactase is used in the conversion of whey into syrup.
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