α-Lactalbumin and the Lactose Synthetase Reaction

An improved procedure for the isolation of the A protein of lactose synthetase is presented. The Km for UDP-galactose is not influenced by the concentration of glucose or α-lactalbumin but these compounds alter the maximum velocity of the reaction. There is a reciprocal relationship between the concentration of glucose and α-lactalbumin in the lactose synthetase assay and α-lactalbumin lowers the apparent Km of glucose. It is suggested that the physiological function of α-lactalbumin is to lower the Km of glucose so that it may be used maximally for the synthesis of lactose. Lactose may be synthesized at maximum rates by the A protein in the absence of α-lactalbumin but in the presence of high concentrations of glucose. Different preparations of the A protein have variable activities.