Phospholipid Metabolism in Plant Mitochondria

Abstract

CDP choline:1,2-diacylglycerol cholinephosphotransferase (EC 2.7.8.2) and CDP ethanolamine: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 [ER]. 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 ER were found. The Km of the mitochondrial enzyme for CDP-choline was about 2 U less than that for the enzyme of the ER (8.0 and 10.0 mM respectively). The mitochrondrial enzyme activity was maximal above 10 mM Mg2+, whereas maximum ER activity was achieved by 4 mM. The ER enzyme was more stable at 37.degree. C than was that of the mitochondria. The mitochondrial cholinephosphotransferase represented .apprx. 1-2% of the total activity isolated from castor bean endosperm.