Origin and Disappearance of Diacylglycerol, a Protein Kinase C Activator, in Salivary Glands

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Membrane lipids play an important role in signal transmission via their specific metabolic pathways. Especially, there are many reports that newly formed diacylglycerol (DAG) through receptor activation binds to protein kinase C resulting in enhancement of its Ca^<2+>-sensitivity. Therefore, DAG attracts many attentions as an activator of protein kinase C in many cellular systems including secretory one. In this study, we have characterized the acyl chain composition of DAG and the related metabolic pathway to approach the clarification of origin and disappearance of DAG in secretory cells.The acyl chain of DAG from both homogenate of rat submandibular and sublingual salivary glands mainly consisted of saturated fatty acids (more than 79% of the total) such as palmitic and stearic acids. Although oleic acid existed 13% of the total, polyunsaturated ones were very little in DAG. This profile of saturated fatty acid enrichment was similar to the acyl chain composition of triacylglycerol … More rather than those of phosphatidylcholine and phosphatidylethanolamine although 1-stearoyl-2-arachidonoyl-DAG was known as a potent activator of protein kinase C. Therefore, the feature of DAG formed in signal transmission was thought to result from conversion from phospholipid to DAG or deacylation-reacylation during cell activation. Concerning the deacylation-reacylation pathway, we clarified the substrate specificities of microsomal phospholipase A_2, 1-acylglycerophosphate acyltransferase and 1-acylglycerophosphocholine acyltransferase. Furthermore, composition of ether-linked phospholipids and their side chain composition were established comparing properties of 1-alkenylglycerophosphocholine acyltransferase. On the other hand, membrane-bound phospholipase A_2 having different substrate specificity from the microsomal one was detected in secretory granules ; functional relationship between the requirement of granular membrane lipids and membrane fusion was also suggested.These results are thought to contribute the clarification of lipid metabolism responding to secretory stimulus in the cells. Less