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Metabolism and Enzymology

Phosphatidylcholine Synthesis in Castor Bean Endosperm ¹

Occurrence of an S-Adenosyl-L-Methionine:Ethanolamine N-Methyltransferase

Department of Botany, Louisiana State University, Baton Rouge, Louisiana 70803-1705

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 µm, 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 Mg²⁺ 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 µM). 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.

¹ Supported by National Science Foundation grant Nos. DCB-8703739 and DCB-9003817.

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