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Phosphatidylglycerol Synthesis in Castor Bean Endosperm

Kinetics, Requirements, and Intracellular Localization ¹

^a Thimann Laboratories, University of California, Santa Cruz, California 95060

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.

¹ This work was supported by the Atomic Energy Commission Contract AT-104/3.

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