Phosphatidylethanolamine Synthesis by Castor Bean Endosperm A Base Exchange Reaction

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 CaCl 2 . 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.