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

膜脂通过其特定的代谢途径在信号传递中发挥重要作用。特别是，有许多报道称，新形成的二酰基甘油（DAG）通过受体活化与蛋白激酶C结合，从而增强其Ca^<2+>-敏感性。因此，DAG作为蛋白激酶C的激活剂在包括分泌系统在内的许多细胞系统中引起了广泛的关注。在本研究中，我们对DAG的酰基链组成和相关代谢途径进行了表征，试图澄清DAG在分泌细胞中的起源和消失。大鼠颌下腺和舌下唾液腺匀浆中DAG的酰基链主要由棕榈酸和硬脂酸等饱和脂肪酸组成（占总脂肪酸的79%以上）。油酸虽然占总油酸的13%，但多不饱和脂肪酸却很少。饱和脂肪酸富集的这种特征与三酰甘油的酰基链组成相似，而不是磷脂酰胆碱和磷脂酰乙醇胺，尽管1-硬脂酰-2-花生四烯酰基-DAG被认为是蛋白激酶c的有效激活剂。因此，DAG在信号传递中形成的特征被认为是在细胞活化过程中磷脂转化为DAG或去酰基-再酰化的结果。在脱酰-再酰化途径中，我们明确了微粒体磷脂酶A_2、1-酰基甘油磷酸酰基转移酶和1-酰基甘油酰胆碱酰基转移酶的底物特异性。此外，通过比较1-烯基甘油磷胆碱酰基转移酶的性质，确定了醚链磷脂的组成及其侧链组成。另一方面，在分泌颗粒中检测到与微粒体不同的膜结合磷脂酶A_2；提出了颗粒膜脂的需要量与膜融合之间的功能关系。这些结果被认为有助于澄清脂质代谢对细胞分泌刺激的反应。少

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