A rapid phospholipase D assay using zirconium precipitation of anionic substrate phospholipids: application to N-acylethanolamine formation in vitro

Activation of phospholipase D (PLD) is involved in a number of signal trans duction pathways in eukaryotic cells, The most common method for determinat ion of PLD activity in vitro involves incubation with a radiolabeled substr ate and lipid extraction followed by thin-layer chromatography in order to separate and quantify substrate and product(s). A more rapid assay can be u sed when utilizing phosphatidylcholine as a substrate because one of the pr oducts, choline, is water soluble and therefore easily separated from the s ubstrate, However, this separation principle is not applicable in evaluatin g N-acylphosphatidylethanolamine (NAPE)-hydrolizing PLD activity, which pro duces two lipophilic products, N-acylethanolamine (NAE) and phosphatidic ac id, Therefore, we developed a rapid assay for the routine detection of NAPE -hydrolyzing PLD activity. This assay is based on precipitation of radiolab eled substrate (NAPE) in the presence of ZrOCl2, followed by quantification of radiolabeled NAE released into a methanolic supernatant, The precipitat ion involves a chemical reaction of the zirconyl cation with the phosphate anion, Conditions were optimized for the complete precipitation of NAPE, wh ereas N-acyllysophosphatidylethanolamine and glycerophospho(N-acyl)ethanola mine were precipitated at least 95%. Furthermore, this precipitation method can be extended to assays of other anionic phospholipid-hydrolyzing PLD ac tivities by selecting an optimal pH of the precipitation solution, For exam ple, 98-99% precipitation of phosphatidylethanolamine, phosphatidylglycerol , and phosphatidylserine was achieved. Consequently this new assay allows f or a convenient examination of PLD activities toward a variety of phospholi pid substrates, and in particular allows for the analysis of NAE formation from NAPE in vitro, a feature that will facilitate a more complete biochemi cal characterization of this anandamide-generating enzyme.