Lipid analysis of a novel type of cell secretion in the exocrine pancreas: the pancreasomes.

A novel type of cell secretion termed 'microvesicular secretion' has been described recently in the exocrine pancreas. According to this process, microvesicles are released by acinocytes in the pancreas acinar lumen. These microvesicles, identified as 'pancreasomes', were characterized by the presence of a major glycoprotein component originating in the exocrine acinar cell. In the present work, phospholipids of pancreasomes have been identified. Five classes of phospholipid were found: phosphatidylcholines, phosphatidylethanolamines, lysophosphatidylcholines, sphingomyelins and another minor class of ninhydrin-positive phospholipid (phosphatidylserines or lysophosphatidylethanolamines). The ratios of neutral lipids to phospholipids were particularly high (3:1), as estimated by GLC of their fatty acid content. Analysis of fatty acid composition of pancreasomes lipids revealed a very high proportion of two saturated fatty acids, palmitic (40%) and stearic (24%), whereas two main unsaturated fatty acids, oleic (17%) and linoleic (8%), were found in smaller proportions. Differential scanning calorimeter studies on washed pancreasomes indicated that there was no lipid phase transition in their membrane, despite the absence of cholesterol. Our observations show that pancreasomes have an unusual lipid composition and confirm our previous conclusion based on protein analysis that the release of pancreasomes occurs according to an hitherto undescribed type of secretion, in which a glycoprotein is released associated with specific domains of the luminal plasma membrane.