Regulatory subunit of lactose synthase, changes the substrate specificity of galactosyltransferase in the mammary gland making glucose a good acceptor substrate for this enzyme. This enables LS to synthesize lactose, the major carbohydrate component of milk. In other tissues, galactosyltransferase transfers galactose onto the N-acetylglucosamine of the oligosaccharide chains in glycoproteins. Lactose synthase (LS) is a heterodimer of a catalytic component, beta1,4-galactosyltransferase (beta4Gal-T1) and a regulatory component, alpha-lactalbumin (LA). Mammary gland specific. Secreted in milk. Belongs to the glycosyl hydrolase 22 family.