The conditions for maximal activity of ethanolamine phosphokinase from rat
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چکیده
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 Ioo ooo X g for 6o min. The enzyme activity had a pH opt imum at 8.5 and an apparent I(m for ethanolamine of I , IO 4 M at an ATP and Mg 2+ concentration of 3.o 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 o. 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,Ndimethylethanolamine 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 I 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.
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