Purification and kinetic properties of galactokinase from human placenta

Galactokinase from human placenta was purified about 350-fold using DEAE-Sephadex-A-50 chromatography followed by Sephadex-G-200 and CM-Sephadex-C-50 filtration. The final steps of purification involved electrofocusing and ammonium sulfate precipitation. In analytical disc electrophoresis the purified enzyme moved as a single protein band. Crude enzyme is extremely unstable. Purification of the enzyme increases the stability slightly. The activation energy of the enzyme was found to be 9500 Cal. Galactokinase has an absolute requirement for a divalent ion. The enzyme is maximally active in the presence of Mg²⁺, 5 mm. However, Mg²⁺ could be partially replaced by Ca²⁺, Co²⁺, and Mn²⁺. The enzyme is almost completely inhibited by p-chloromercuribenzoate and partially inhibited by N-ethylmaleimide. The K_m of placental galactokinase for galactose and ATP is 250 μm and 100 μm, respectively. Galactose-1-P appears to inhibit the enzyme in a competitive fashion with ATP with a K_i of 2.0 mm. The isoelectric point of galactokinase is 5.7 and the molecular weight is 58,000. The kinetic properties of placental galactokinase have been compared to the red cell galactokinase. The K_m of the placental enzyme for galactose is about twice as high as that of the red cell enzyme whereas the affinity of the placental enzyme for ATP is twice as high as that of the red cell enzyme. Co-chromatography of placental and adult erythrocyte extract resulted in the separation of two peaks of galactokinase activity, one peak with the kinetic characteristics of the purified placental enzyme and the other peak with the kinetic characteristics of the red cell enzyme.