Groups I,II and III extracellular phospholipases A2: Selective inhibition of group II enzymes by indomethacin but not other NSAIDs

The three types (groups I, II and III) of stable extracellular 14 kDa phospholipase A₂ enzymes differ in their primary amino acid sequences and their properties. It may thus be possible to design low-molecular weight inhibitors targeted to the secretory form of mammalian PLA₂. this enzyme has been implicated in inflammatory disorders. We have studied the inhibition of four distinct PLA₂ enzymes by a range of NSAIDs, using³H-oleate release from prelabelled membranes ofE. coli for assay. The enzymes used were cobra venom PLA₂ (Naja naja, a group I enzyme), bee venom PLA₂ (Apis mellifera, group III), recombinant human synovial PLA₂ (group II) and rat peritoneal PLA₂ (group II). Under the conditions of the³H-oleateE. coli assay, 1 mM concentrations of aspirin, sodium salicylate, paracetamol (acetaminophen), oxphenbutazone, ibuprofen, flurbiprofen and nabumetone failed to inhibit significantly any of the four enzymes. However, indomethacin inhibited all four enzymes, although effects were greatest on the two group II enzymes (rat peritoneal and human synovial PLA₂). Approximate IC₅₀ values were 28 and 35 M, respectively. Inhibition by indomethacin was not time dependent and was greater at micromolar rather than millimolar levels of calcium. We conclude that indomethacin but not the other tested classes of NSAID inhibits the group II PLA₂ enzyme in a selective manner and suggest that this may be relevant both to its clinical spectrum and to the design of novel pharmaceutical leads.