Lactose Synthesis by a Golgi Apparatus Fraction from Rat Mammary Gland

LACTOSE synthetase (UDP galactose : D-glucose 1-galactosyl transferase, EC 2.4.1.22) catalyses the final step in the synthesis of lactose of milk and is composed of two subunits, both of which are required for activity1. The A subunit is membrane bound and, in the absence of the B subunit, transfers galactose from UDP galactose to N-acetylglucosamine with the formation of N-acetyl-lactosamine2–4. The B subunit is identical to the milk protein α-lactalbumin, and modifies the acceptor specificity of the A subunit to include glucose2,3. Galactosyl transferase activities have been specifically localized in Golgi apparatus from rat5 and bovine6 livers. Because the localization of lactose synthetase activity has been the subject of much speculation8, it was of interest to extend these studies to mammary tissue. We show here the A protein of lactose synthetase to be concentrated in a Golgi apparatus fraction from rat mammary gland.