The Prerequisites for Local Lysolecithin Formation in the Human Gallbladder

Abstract

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 °C for 2 min, their lysophospholipase activity was lost. In contrast, the ability to hydrolyze ¹⁴C-phosphatidylethanolamine in biosynthethically 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 diacylphos-phoglycerides that physiologically would prevent the accumulation of lytic lysophos-phoglycerides. The possible relevance of these findings to the pathogenesis of aseptic cholecystitis is inferred.

Key-words:

• Cholecystitis
• lysolecithin
• lysophospholipase
• phospholipase A₁
• phospholipase A₂