Phosphatidylglycerol Synthesis in Castor Bean Endosperm

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Abstract
The synthesis of phosphatidylglycerol in castor bean (Ricinus communis var. Hale) endosperm tissue was found to be located in both the endoplasmic reticulum and mitochondrial fractions separated on sucrose density gradients. The enzyme of both fractions attained maximum activity at 5 mm Mn2+, 0.075% Triton X-100, and pH 7.3. The addition of dithiothreitol produced little effect, but sulfhydryl inhibitors reduced activity in both systems. Cytidine diphosphate-diglyceride exhibited an apparent Michaelis constant for the endoplasmic reticulum enzyme of 2.8 μm and for the mitochondrial enzyme of 2.0 μm; the maximum reaction rate was achieved at about 20 μm. For the second substrate, glycerol-phosphate, the apparent Michaelis constant for both fractions was about 50 μm and maximum velocity was reached at 400 μm. The specific activity of the mitochondrial enzyme was generally twice that of the endoplasmic reticulum.