Choline Kinase and Ethanolamine Kinase are Separate, Soluble Enzymes in Rat Liver

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Abstract
Choline kinase [EC 2.7.1.32] and ethanolamine kinase [EC 2.7.7.15] were located in the cytosol from rat liver and were copurified more than 500 fold by affinity chromatography. Kinetic properties of the 2 activities were determined. Choline kinase had a Km for choline of 0.033 mM and ethanolamine was a competitive inhibitor (Ki = 6.2 mM). Ethanolamine kinase had a Km for ethanolamine of 7.7 mM and choline was a mixed type of inhibitor with a Ki of 0.037 mM. Both enzyme activities responded in a similiar fashion to the adenylate energy charge. Betaine and choline phosphate partially inhibited both kinases with a 93% inhibition of the ethanolamine kinase by 5 mM choline phosphate. CTP and ethanolaminephosphate partially inhibited the ethanolamine kinase, but not the choline kinase. Other metabolites tested had negligible effects on both kinases. The affinity column purified enzyme was analyzed by disc gel electrophoresis which resolved the 2 activities. Although many of the properties of the 2 activities were similar, choline kinase and ethanolamine kinase must be separate enzymes. Analysis of rat liver cytosol by disc gel electrophoresis indicated 4 isoenzymes for choline kinase and ethanolamine kinase.