The synthetic reaction to make N-acetyl--lactosamine is catalyzed by galactosyltransferase.

UDP-Galactose + N-Acetylglucosamine -> UDP + N-AcetylLactosamine

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

UDP-Galactose + Glucose -> UDP + Lactose

This modified enzyme, termed lactose synthase, is found in animals only in mammary gland, where it synthesizes the major sugar of milk. The polypeptide that modifies the specificity of the enzyme is the mammary gland protein -lactalbumin. Synthesis of -lactalbumin is activated hormonally in mothers shortly after giving birth. The protein combines with preexisting galactosyltransferase, changes its specificity, and activates the large amount of lactose synthesis needed for milk production.