Ethanolamine phosphokinase: Activity and properties during liver development

Abstract

The conditions for maximal activity of ethanolamine phosphokinase from rat liver were determined. All of the activity was located in the supernatant fraction after centrifugation at 100 000 x g for 60 min. The enzyme activity had a pH optimum at 8.5 and an apparent Km for ethanolamine of 1[middle dot]10-4 M at an ATP and Mg2+ concentration of 3.0 mM. The enzyme uses the Mg-ATP complex as substrate and is inhibited by free ATP. Choline inhibits ethanolamine phosphokinase. Maximal inhibition is obtained at choline concentrations of 0.4 mM. Treatment of the supernatant with Sephadex G-25 or by dialysis causes an increase in the maximal amount of inhibition obtained with choline. The inhibition by choline is non-competitive with ethanolamine and competitive with ATP. Ethanolamine phosphokinase is inhibited by N,N-dimethylethanolamine and N-methylethanolamine but not by betaine, phosphoryl choline, CDP-choline or phosphorylethanolamine.The activity of ethanolamine phosphokinase is low in -5-day fetal liver. The activity increases from -5 days to -2 days and drops at 1 day after birth. An endogenous inhibitor of ethanolamine phosphokinase is present in all preparations but at different levels. The drop in activity in 1-day-old animals is due to the presence of higher amounts of inhibition by the endogenous inhibitor within the preparation. Evidence indicates that the endogenous inhibitor is choline.