Lactose, Composed of Glucose and Galactose, Is the Main Carbohydrate of Mammalian Milk
Lactose is the main carbohydrate of most mammals. It is composed of glucose and galactose. Blood glucose is the main precursor molecule for lactose, with propionate an important precursor for glucose in ruminants.
Lactose is formed under the direction of lactose synthetase, an enzyme composed of α- Iactalbumin (a milk protein) and galactosyl transferase. Lactose synthesis is held in abeyance until immediately before term because progesterone is inhibitory for the formation of α- Iactalbumin. Prolactinl on the other hand, is stimulatory for the formation of lactose synthetase. Animals must have the enzyme lactase present in the jejunum for lactose to be cleaved (to glucose and galactose) and used. Lactase is present in most mammalian young but is sometimes not present in adult animals, including humans. In the absence of lactase, lactose can have an osmotic effect in the gastrointestinal tract, which can lead to diarrhea.
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