The role of alpha-lactalbumin and the A protein in lactose synthetase: a unique mechanism for the control of a biological reaction.

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Abstract
The studies were designed to define the roles of the A protein and [alpha]-LA [lactalbumin] in lactose synthetase. The separated A protein was a UDP-galactose : N-acetyl-glucosamine galactosyltransferase which catalyzes the following reaction: UDP-galactose + N-acetyl-D-glucosamine[forward arrow]N-acetyllactosamine + UDP. Under normal assay conditions, [alpha]-LA inhibits this reaction and allows synthesis of lactose in the presence of glucose. Thus, the [alpha]-LA modifies the substrate (acceptor) specificity of a galactosyltransferase from NAG [N-acetyl-D-glucosamine] to glucose. This appears to be a role hitherto not ascribed to a protein and [alpha]-LA may be termed a "specifier" protein. The mechanism by which it effects a change in substrate specificity appears to be complex, but it is possible that it represents a new type of molecular control of a biological reaction.