The enzymes shown in Figure 13.13 participate in mammary gland in the synthesis of lactose in milk. Lactose is formed from UDP-Gal plus glucose by lactose synthase, in the presence of the protein -lactalbumin. The endergonic synthesis of UDP-Gal in this tissue proceeds smoothly because of its high rate of conversion to lactose.

An enzyme called galactosyltransferase is closely related to lactose synthase. It catalyzes the reaction below.

UDP-Galactose + N-Acetylglucosamine -> UDP + N-Acetyllactosamine

The common form of the enzyme contains a single polypeptide subunit. The presence of an additional subunit, -lactalbumin, changes the specificity of the enzyme so that lactose is synthesized instead.

UDP-Galactose + Glucose -> UDP + Lactose

This modified enzyme is lactose synthase. Synthesis of -lactalbumin is activated hormonally in mothers shortly after giving birth. The -lactalbumin protein combines with preexisting galactosyltransferase, changes its specificity, and activates the large amount of lactose synthesis needed for milk production.