CHOLINE KINASE AND ETHANOLAMINE KINASE-ACTIVITY IN THE CYTOSOL OF NERVE-ENDINGS FROM RAT FOREBRAIN

Both choline kinase and ethanolamine kinase are present in the cytosol of nerve endings prepared from rat brain as are the products of their action, phosphocholine (84 nmol/g fresh wt of brain) and phosphoethanolamine (190 nmol/g fresh wt of brain). In contrast with the enzymes from the cytosol of whole brain, both are as equally active at pH 7.5 as 9.0. Determination of kinase activity in membrane-containing tissue samples at pH 9 gives low values because of the activity of alkaline phosphatase. Choline kinase, but not ethanolamine kinase, requires Mg2+ in excess of that required for the formation of the MgATP complex and is inhibited by an excess of free ATP. The Km for choline is 2.6 mM and for ethanolamine is 2.2 mM. The differing requirements for ATP and Mc2+ and the inhibition of choline kinase, but not ethanolamine kinase, by hemicholinium-3 suggest either the presence of 2 separate enzymes or 2 different active sites on the same enzyme.