Hydrolytic activities of human pancreatic phospholipase A 2 and endotoxin-stimulated endogenous phospholipase A 2 toward membrane phospholipids of erythrocytes

Abstract

The problem of whether human pancreatic phospholipase A 2 (PLA 2) can really hydrolyze membrane phospholipids in vitro was studied to understand pathophysiology of acute pancreatitis. Total amount of lysophospholipids generated in erythrocytes by exogenously added human pancreatic PLA 2 (2 μg/ml) was only 12% of the amount of sphingomyelin, which was not decomposed by the enzyme. About fivefold the amount of lysophospholipids was generated in ghost membranes during one-sixth of the incubation time compared to that in intact erythrocyte membranes. Escherichia coli lipopolysaccharide (LPS) (10 μg/ml) was able to stimulate membrane-associated PLA 2 of erythrocytes, the amount of lysophospholipids generated being 12.5% of that of sphingomyelin without adding the exogenous PLA 2. The stimulation of membrane-associated PLA 2 in erythrocytes was inhibited by pretreatment of lipopolysaccharide with polymyxin-B sulfate. When intact erythrocytes were incubated with human pancreatic PLA 2 and LPS, the amount of generated lysophospholipids was 24% of that of sphingomyelin. These results suggested that the exogenously added human pancreatic PLA 2 cannot degrade phospholipids of intact erythrocytes so extensively under physiological conditions, and, in acute pancreatitis, unknown factors may be involved in the hydrolysis of phospholipids. LPS, which activates membrane-associated PLA 2, may be one of the factors, and thus membrane phospholipids are hydrolyzed in the disease.