In vitro: MethylArachidonyl Fluorophosphonate (MAFP) was reported to be a selective, active-site directed, irreversible inhibitor of cytosolic phospholipase A2 (cPLA2). MAFP could also potently inhibit the Ca2+-independent cytosolic phospholipase A2 (iPLA2) in a concentration-dependent manner. Such inhibition was not reversed upon extensive dilution of the enzyme into the assay mixture. The Preincubation of iPLA2 with MAFP led to a linear and time-dependent inactivation of enzyme activity, and the enzyme was protected from inactivation by the reversible inhibitor. The ability of MAFP to inhibit the iPLA 2 indicated that this enzyme proceeded via an acyl-enzyme intermediate. Further testing showed that MAFP could not inhibit the CoA-dependent acyltransferase, arachidonoyI-CoA synthetase, or CoA-independent transacylase activities. Therefore, MAFP was not a general inhibitor for enzymes acting on arachidonoyi substrates [1].