Starting from the spectrophotometric method of Ballard optimal reaction conditions for measurements of galactokinase in piglet liver were systematically studied. These are (final conc. in the test): 100 mM triethanolamine-HCl buffer, 33 mM KCl, 16.5 mM NaF (inhibiting ATPase), 5 mM cysteine hydrochloride, 0.33 mM NADH2, 1 U pyruvate kinase and lactic dehydrogenase, 0.5 mM phosphoenolpyruvate, 1.5 mM galactose, 0.5 mM ATP and 1 mM MgCl2, final pH 7.5. 2. An optimal substrate concentration, a Mg: ATP-ratio of 2:1, pH-stability and addition of activators are important for the determination of galactokinase activity in the supernatant fraction of pig liver. 3. Using the optimized method galactokinase activity of pig liver in dependence on age, with particular reference to the perinatal period, was determined. 4. Galactokinase activity of liver of newborn piglets is 7 times that of adult pigs. In the suckling period the activity remains relatively constant at this high level and decreases remarkably immediately after weaning. 5. Galactokinase of liver of newborn piglets differs in kinetic properties (lower Km of ATP, higher maximal reaction velocity) from the enzyme of adult pigs, which is still insufficient to make sure the existence of two different forms of the enzyme.
