Subcellular sites of synthesis of phosphatidylglycerol and phosphatidylinositol in type II pneumonocytes

The subcellular sites of synthesis of phosphatidylinositol and phosphatidylglycerol in type II pneumonocytes isolated from lungs of adult rats were investigated. Microsomes contained approximately 34% of the total cellular activity of CDP-diacylglycerol: inositol 3-phosphatidyltransferase (EC 2.7.8.11) but less than 10% of the total activity of CDP-diacylglycerol: glycerol 3-phosphate phosphatidyltransferase (EC 2.7.8.5) and the latter activity could be attributed to mitochondrial contamination of the microsomal fraction. A crude mitochondrial fraction contained approximately 90% of the total cellular activity of CDP-diacylglycerol: glycerol 3-phosphate phosphatidyltransferase and also contained more than 50% of the total CDP-diacylglycerol: inositol 3-phosphatidyltransferase activity. When the crude mitochondrial fraction was subfractionated by use of discontinuous density gradient centrifugation, the distribution of CDP-diacylglycerol: inositol 3-phosphatidyltransferase activity was similar to that of NADPH: cytochromec reductase activity but dissimilar to the distribution of CDP-diacylglycerol: glycerol 3-phosphate phosphatidyltransferase activity which resembled that of succinate dehydrogenase. Electron microscopy of subcellular fractions revealed that fractions rich in CDP-diacylglycerol: inositol 3-phosphatidyltransferase activity contained numerous smooth membrane vesicles, whereas fractions rich in CDP-diacylglycerol: glycerol 3-phosphate phosphatidyltransferase activity contained mainly mitochondria. We conclude that the subcellular sites of synthesis of phosphatidylinositol and phosphatidylglycerol in type II pneumonocytes are most likely the endoplasmic reticulum and mitochondria, respectively. The results of this investigation were presented, in part, at the 73rd annual meeting of the American Society of Biological Chemists