Kinetic studies with liver galactokinase.

1. Kinetic measurements of the forward reaction catalysed by ATP-galactose phosphotransferase were carried out with a purified preparation from pig liver. 2. The rate of reaction at pH7.8 is dependent on the concentration of MgATP(2-) rather than total ATP or magnesium chloride concentrations. 3. The effect of changes in pH on K(m) (galactose), K(m) (MgATP(2-)) and V(max.) was studied. 4. Of several possible nucleotide substrates only ATP and deoxyATP were effective. 5. The initial-velocity patterns both in the absence and presence of products were determined. 6. Galactose 1-phosphate is a non-competitive inhibitor when either galactose or MgATP(2-) was the variable substrate. 7. MgADP(-) was a non-competitive inhibitor with galactose and a competitive inhibitor with MgATP(2-) as variable substrate. 8. These results are consistent with an ordered reaction pathway in which galactose combines with an initial enzyme-MgATP(2-) complex.