Phosphatidylcholine Synthesis in Castor Bean Endosperm : Occurrence of an S-Adenosyl-l-Methionine:Ethanolamine N-Methyltransferase.

Methylethanolamine synthesis by S-adenosyl-l-methionine:ethanolamine N-methyltransferase from an extract of castor bean (Ricinus communis L. var Hale) endosperm was characterized. The apparent Michaelis-Menten constants of the enzyme for ethanolamine and S-adenosyl-l-methionine were estimated to be 6.7 and 1.4 mum, respectively, although the K(m) for ethanolamine is imprecise because of strong substrate inhibition. The pH optimum was 8.0, and a divalent cation was required for activity, with Mg(2+) giving the greatest stimulation at 5 mm. The enzyme was inhibited by calcium in the micromolar range and relatively high concentrations of ethanolamine (above about 7 mum). The activity was found in the 119,000g supernatant fraction and, therefore, appears to be cytoplasmic. The potential roles of S-adenosyl-l-methionine:ethanolamine N-methyltransferase in choline and phosphatidylcholine synthesis are discussed.