Biosynthesis of microsomal phospholipids and mitochondrial polyglycerophosphatides in rapidly sedimenting endoplasmic reticulum.

Rapidly sedimenting endoplasmic reticulum (RSER), which is known to be a complex between endoplasmic reticulum and mitochondria, was isolated from rat liver and purified through a sucrose density gradient by centrifugation according to a well established procedure previously published by G. C. Shore and J. R. Tata. This complex was characterized by microsomal (NADPH-cytochrome c reductase) and mitochondrial (succinate-cytochrome c reductase and NADP-isocitrate dehydrogenase) marker enzymes and was examined for the ability to synthesize microsomal lipids and mitochondrial polyglycerophosphatides. Results of these experiments showed that the RSER is capable of synthesizing key microsomal lipids, i.e., phosphatidic acid, phosphatidylcholine, and neutral lipids, as well as mitochondrial phosphatidylglycerosphate and phosphatidic acid, phosphatidylcholine, and neutral lipids, as well as mitochondrial phosphatidylglycerophosphate and phosphatidylglycerol. Furthermore, the level of synthesis of these lipids paralleled the level of activity of microsomal and mitochondrial marker enzymes found in the RSER preparation. Details of these experimental findings and some implications are discussed in view of the possible functional role of RSER.