Phospholipid Metabolism in Plant Mitochondria: II. SUBMITOCHONDRIAL SITES OF SYNTHESIS OF PHOSPHATIDYLCHOLINE AND PHOSPHATIDYLETHANOLAMINE.

CDPcholine:1,2-diacylglycerol cholinephosphotransferase (EC 2.7.8.2) and CDPethanolamine:1,2-diacylglycerol ethanolaminephosphotransferase (EC 2.7.8.1) were detected in mitochondrial fractions from castor bean (Ricinus communis) endosperm. These activities were not due to contamination of the fractions with endoplasmic reticulum. The enzymes were localized on both the inner and outer mitochondrial membranes.Only minor kinetic differences between the phosphatidylcholine-synthesizing activities of intact mitochondria and of the endoplasmic reticulum were found. The K(m) of the mitochondrial enzyme for CDP-choline was about 2 units less than that for the enzyme of the endoplasmic reticulum (8.0 and 10.0 micromolar, respectively). The mitochondrial enzyme activity was maximal above 10 millimolar Mg(2+), whereas maximum endoplasmic reticulum activity was achieved by 4 millimolar. The endoplasmic reticulum enzyme was more stable at 37 C than was that of the mitochondria. The mitochondrial cholinephosphotransferase represented about 1 to 2% of the total activity isolated from castor bean endosperm.