Kinetic studies on the effect of uridine diphosphate galactose and manganous ions on the reaction between lactose synthetase A protein from human milk and p-hydroxymercuribenzoate.

The inhibition of lactose synthetase A protein by p-hydroxymercuribenzoate at pH7.5 and 25 degrees C, which involves the reaction of one molecule of inhibitor with each molecule of enzyme, was decreased in rate by UDP-galactose, especially in the presence of Mn(2+). Pseudo-first-order rate constants for the reaction between 0.1mm-p-hydroxymercuribenzoate and free enzyme, the enzyme-UDP-galactose complex and the enzyme-Mn(2+)-UDP-galactose complex were 4.4x10(-2), 1.9x10(-2) and 0.3x10(-2)min(-1) respectively. The results also indicated that dissociation constants for UDP-galactose in the enzyme-UDP-galactose and enzyme-Mn(2+)-UDP-galactose complexes were 313 and 16mum respectively, the latter value being similar to the K(m) for UDP-galactose in the lactose synthetase reaction. The protective effect of UDP-galactose and the role of Mn(2+) ions in lactose synthetase are discussed.