Phosphatidylinositol Synthesis in Castor Bean Endosperm

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Abstract
CDP-diglyceride:inositol transferase [EC 2.7.8] in endoplasmic reticulum fractions from castor bean (Ricinus communis) endosperm was partially characterized. The enzyme had a pH optimum of 8.5 and required Mn2+ for activity. Maximal activity was at 1.5 mM MnCl2. A Km of 0.30 mM was calculated for myo-inositol and 1.35 mM was estimated for CDP-dipalmitoylglyceride. Concentrations of CDP-dipalmitoylglyceride above 1.2 mM inhibited the enzyme. A deoxycholate concentration of 0.1% (wt/vol) stimulated the reaction slightly while Triton X-100 inhibited at all concentrations tested. Some incorporation of myo-inositol into phosphatidylinositol occurred in the absence of CDP-diglyceride.