Mechanisms of inhibition of phospholipase A2

Differential scanning calorimetry (d.s.c.) and assays of phospholipase A₂ activity were used as tools to distinguish between drugs which interact with the phospholipids and those which interact directly with the enzyme. Cholesterol lowered the transition temperature (tc) and reduced the heat absorbed at transition and inhibited phospholipase A₂ activity on liposomes prepared from dipalmitoyllecithin-cholesterol. Subsequent addition of filipin to these liposomes overcame the inhibitory effect. Cholesterol therefore inhibits phospholipase A₂ by interacting with the membrane phospholipids. Mepacrine and phentermine did not interact with dipalmitoyl-lecithin (DPL) as determined by d.s.c., but reduced the rate of hydrolysis induced by purified phospholipase A₂ by a direct interaction with the enzyme. The anaesthetics, ethrane, halothane and trichloroethylene, inhibited phospholipase A₂ more than 90 per cent and were found to interact with DPL to modify membrane fluidity and lower the transition temperature. However, they also appeared to interact directly with the enzyme because the inhibitory effect was not overcome either by assaying phospholipase A₂ at the new tc or by a ten-fold increase in Ca²⁺ concentration. The anti-inflammatory steroids hydrocortisone, dexamethasone and betamethasone did not affect the rate of hydrolysis of DPL liposomes induced by phospholipase A₂ even at 2:1 w/w steroid/lipid. Furthermore, these steroids were found to be without any effect on membrane fluidity as examined by d.s.c. and microviscosimetry.