Lactose Chemistry Digestion and Metabolism

Lactose is a galactoside composed of glucose and galactose. This compound is found in the milk of most mammals, although in marsupials and mono-tremes it appears in conjugated form. It is entirely lacking in the milk of the California sea lion and related pinnipedia of the Pacific Basin. The reason for the latter is that lactose is synthesized in the mammary gland from two monosaccharides, glucose and galactose, by the enzyme galactosyltransferase, which requires the presence of alpha-lactalbumin -not produced by the mammary glands of the sea lion. Thus the concentration of lactose in the milk of placental animals ranges from a low of zero to a high of 7 percent (Sunshine and Kretchmer 1964).

Lactose is digested in the small intestine by the lactase (a galactosidase), which is located in the brush border of the epithelial cells of intestinal villi. This enzyme is anchored to the membrane by a hydrophobic tail of amino acids. Detailed descriptions have recently been published of the intimate intracellular metabolism of lactase (Castillo et al. 1989; Quan et al. 1990).

In most mammals the activity of lactose-digesting lactase is high during the perinatal period; after weaning the activity declines to about 10 percent of its original value. But in certain human groups the enzyme activity remains elevated throughout the lifetime of most of their members (Kretchmer 1977). Examples of these groups include northern Europeans, people of Magyar-Finnish extraction, and two African tribes, the Fulani and the Tussi.

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