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.
D001665:Binding Sites; D001667:Binding, Competitive; D006801:Humans; D006902:Hydroxymercuribenzoates / Q000494:pharmacology*; D007700:Kinetics; D007788:Lactose Synthase / Q000037:antagonists & inhibitors*; D008345:Manganese / Q000494:pharmacology*; D008895:Milk, Human / Q000201:enzymology; D014531:Uridine Diphosphate Galactose / Q000494:pharmacology*; D014539:Uridine Diphosphate Sugars / Q000494:pharmacology*
D006902:Hydroxymercuribenzoates; D014539:Uridine Diphosphate Sugars; D014531:Uridine Diphosphate Galactose; D008345:Manganese; D007788:Lactose Synthase