Lactose.html



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Lactose
IUPAC name	(2S,3R,4R,5S,6R)-6- (hydroxymethyl)-5-((2S,3R,4S,5R,6R)- 3,4,5-trihydroxy-6-(hydroxymethyl)tetrahydro- 2H-pyran-2-yloxy)tetrahydro-2H-pyran-2,3,4-triol
Other names	Milk Sugar
Identifiers
CAS number	63-42-3
Properties
Molecular formula	C12H22O11
Molar mass	342.29648 g/mol
Appearance	white solid
Solubility in water	0.216 g/mL
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

Lactose (also referred to as milk sugar) is a sugar that is found most notably in milk. Lactose makes up around 2–8% of milk (by weight). The name comes from the Latin word for milk, plus the -ose ending used to name sugars. Its systematic name is β-D-galactopyranosyl-(1↔4)β-D-glucopyranose.

Chemistry

Lactose is a disaccharide that consists of β-D-galactose and β-D-glucose fragments bonded through a β1-4 glycosidic linkage.

Solubility

Lactose has a solubility of 1 in 4.63 measured %w/v. This translates to 0.216 g of lactose dissolving readily in 1 mL of water.

The solubility of lactose in water is 18.9049 g at 25°C, 25.1484 g at 40°C and 37.2149 g at 60°C per 100 g solution. Its solubility in ethanol is 0.0111 g at 40°C and 0.0270 g at 60°C per 100 g solution.¹

Digestion of Lactose (Catabolism)

Main article: Lactose intolerance

Infant mammals are fed on milk by their mothers, which is rich in the carbohydrate Lactose. To digest it, an enzyme called lactase (β-D-galactosidase) is secreted by the intestinal villi. This enzyme cleaves the lactose molecule into its two subunits; glucose and galactose for absorption.

Since lactose occurs mostly in milk, in most mammals the production of its digestive enzyme, lactase, gradually decreases with maturity, due to lack of constant consumption.

Many people who live in Europe, the Middle East, India, and parts of East Africa, maintain normal lactase production into adulthood. In many of these areas, milk from mammals such as cattle, goats, and sheep is used as a large source of food. Hence, it was in these regions that genes for lifelong lactase production first evolved. The genes of lactose tolerance have evolved independently in various ethnic groups. ²

Production of Lactose (Anabolism)

Lactose is produced (an anabolic pathway) in the mammary glands during lactation. Lactose is formed when galactose and glucose bind each other. The reaction first starts with two glucose molecules. One of them is converted to an active glucose molecule (UDP-glucose). The UDP-glucose is converted to UDP-galactose by the enzyme UDP-glucose 4-epimerase. The UDP-galactose is then added to the other glucose (by the enzyme galactosyltransferase) to form the disaccharide lactose.

References