The prerequisites for local lysolecithin formation in the human gallbladder. III. Demonstration of two different phospholipase A activities.
The positional specificity of the phospholipase A (EC 3.1.1.4) in human gallbladder epithelium has been studied using 14C-phosphatidylethanolamine radiolabeled either in the 1-acyl or in the 2-acyl position. After heating of homogenized epithelial cells at 70 degrees C for 2 min, their lysophospholipase activity was lost.  In contrast, the ability to hydrolyze 14C-phosphatidylethanolamine in biosynthetically radiolabeled Escherichia coli was largely retained. The amounts of radioactivity found in the products of hydrolysis under different conditions suggest that there are two different phospholipase A activities in the gallbladder epithelium: one, with optimal activity at pH 7, that requires Ca2+ and is specific for the 2-acyl position, and another, with optimal activity at pH 4, that does not require Ca2+ and that, apart from the 2-acyl position, attacks the 1-acyl position as well. It is possible, therefore, that a complete deacylation of diacylphosphoglycerides in the gallbladder wall is brought about in two different ways: at neutral pH through a combined action of phospholipase A2 and lysophospholipase, the latter being able to hydrolyze the 1-acyl-lysophosphoglyceride, and, at acid pH, through the action of phospholipase A1 and A2 activity, presuming 1-acyl- and 2-acyl-lysophosphoglycerides are also attacked. Both these processes have to be considered in order to explain a turnover of diacylphosphoglycerides that physiologically would prevent the accumulation of lytic lysophosphoglycerides. The possible relevance of these findings to the pathogenesis of aseptic cholecystitis is inferred.
D002118:Calcium / Q000378:metabolism; D002764:Cholecystitis / Q000209:etiology; D004848:Epithelium / Q000201:enzymology; D004926:Escherichia coli / Q000378:metabolism; D005704:Gallbladder / Q000201:enzymology* / Q000378:metabolism; D006358:Hot Temperature; D006801:Humans; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D008244:Lysophosphatidylcholines / Q000096:biosynthesis*; D010714:Phosphatidylethanolamines / Q000378:metabolism; D010740:Phospholipases / Q000378:metabolism*
D008244:Lysophosphatidylcholines; D010714:Phosphatidylethanolamines; D010740:Phospholipases; D002118:Calcium