Phospholipid Synthesis in the Squid Giant Axon: Enzymes of Phosphatidylinositol Metabolism

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Abstract
The properties of several enzymes of phospholipid metabolism in axoplasm extruded from squid giant axons were examined. The synthetic enzymes, CDP-diglyceride: inositol transferase (EC 2.7.8.11), ATP:diglyceride phosphotransferase, diglyceride kinase (EC 2.7.2.-), and phosphatidylinositol kinase (EC 2.7.1.67), were all present in axoplasm. Phospholipid exchange proteins, which catalyzed the transfer of phosphatidylinositol and phosphatidylcholine between membrane preparations and unilamellar lipid vesicles, were also found. Conditions under which the synthesis of CDP-diglyceride, phosphatidylserine, and phosphatidylinositol-4,5-diphosphate could be measured were not found. Subcellular fractionation by differential centrifugation showed that axoplasmic inositol transferase and phosphatidylinositol kinase activities were largely microsomal, while diglyceride kinase and exchange protein activities were primarily cytosolic.