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Metabolism and Enzymology

Phosphatidylethanolamine Synthesis by Castor Bean Endosperm ¹

A Base Exchange Reaction

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

A base exchange reaction for synthesis of phosphatidylethanolamine by the endoplasmic reticulum of castor bean (Ricinus comminus L. var Hale) endosperm has been examined. The calculated Michaelis-Menten constant of the enzyme for ethanolamine was 5 micromolar and the optimal pH was 7.8 in the presence of 2 millimolar CaCl₂. L-Serine, N-methylethanolamine and N,N-dimethylethanolamine all reduced ethanolamine incorporation, while D-serine and myo-inositol had little effect. These inhibitions of ethanolamine incorporation were found to be noncompetitive and ethanolamine also noncompetitively inhibited L-serine incorporation by exchange. The activity of the ethanolamine base exchange enzyme was affected by several detergents, with the best activity being obtained with the zwitterionic defjtergent 3-3-cholamidopropyl) dimethylammonio-2-hydroxyl-1-propanesulfonate.