Biochemistry of Lactose and Related Carbohydrates

SUMMARY The biosynthesis of lactose represents a unique biochemical system where one protein, α-lactalbumin, acts as a protein modifier of an enzyme, a galactosyltransferase, whose principal function is to transfer galactose to the carbohydrate side chain of glycoproteins. The mammary gland is unique in that it has the ability to synthesize α-lactalbumin and this synthesis is presumably under hormonal control. Kinetic studies have shown that the substrates are added in the order, UDP-galactose and glucose and the products are released in the order, lactose and UDP. α-Lactalbumin adds after the substrates and is released prior to the products. The kinetic mechanism postulated can describe the known properties of the lactose synthetase system and also explains how the galactosyltransferase may be involved in both lactose and glycoprotein biosynthesis as depicted in the model presented in Fig. 3. It is also apparent that the Golgi apparatus plays an important part in the synthesis and secretion of lactose and glycoproteins. The presence of a wide variety of carbohydrate containing compounds in milk must be in part a reflection of the complexity of the milk secretory process. Further work is necessary to determine the origin of these compounds which may be synthesized in the milk, like many of the oligosaccharides, or be the products of degradation of cells or cell organelles.