Lactose synthase complex

Lactose, the major milk sugar, is also responsible for the osmotic regulation of lactation, thus causing the movement of water into milk. This carbohydrate is synthesized in the secretory vesicles of the mammary glands by the lactose synthase complex. As lactose cannot diffuse out of the vesicles, it draws water into the vesicles by osmosis. Thus, the volume of milk produced is directly dependent on the amount of lactose synthesized. 

Lactose Synthases.—2-Diazoacetamido-2-deoxy-D-glucose accepted theD-galacto-syl residue transferred in a reaction catalysed by lactose synthase. Physicochemical studies suggested that the amino-sugar derivative makes contact with the A protein, rather than with a-lactalbumin, in the lactose synthase complex. 

Recently, the molten globule state of a-lactalbumin has been shown to possess antitumor activity when complexed with a fatty acid [36,37], and hence the protein may possess secondary biological activity in addition to the primary activity of native a-lactalbumin, i.e., substrate specificity modifier activity in a lactose synthase system [38,39]. The molten globule of a-lactalbumin thus provides an example of the folding intermediate of a protein exhibiting a secondary biological activity. 

UDP-A-acetyl-5-thio-D-galactosamine (UDP-5.SGalNAc) was active as a donor substrate of lactose synthase, the complex of galactosyltransferase (EC 2.4.1.38) and lactalbumin. By tliis method the disaccharide /9-5SGalNAc/9(1 4)GlcNAc was prepared. UDP-5.S GalNAc was synthesized from an A -acetyIgalactosaininc... 

D-Galactosyltransferases.—The D-galactosyltransferase associated with the lactose synthase system in cows milk exists as a monomer (mol. wt. 4.6 x 10 ), although its aggregation is promoted by 2-acetamido-2-deoxy-D-glucose. Complexation of the enzyme with a-lactalbumin was investigated. 

There is a single binding site for UDP-D-galactose on a lactose synthase -Mn + ion complex derived from bovine lactose synthase. Interactions of the complex with bovine a-lactalbumin were also studied. 

This enzyme [EC 2.4.1.22] is a protein complex of two proteins (designated A and B) and catalyzes the reaction of UDP-galactose with D-glucose to generate UDP and lactose. In the absence of the ce-lactalbumin (protein B), the enzyme catalyzes the transfer of galactose from UDP-galactose to A-acetylglucosamine (Le., the activity of A-acetyllactosamine synthase, EC 2.4.1.90). 

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