Synthesis and Evaluation of Phospholipid Analogues as Inhibitors of Cobra Venom Phospholipase A2

Article abstract of DOI:10.1021/ja00260a020

Analogues of phospholipids that contain fluoro ketone, ketone, and alcohol replacements for the ester at the 2-position of the glycerol backbone have been prepared and analyzed as inhibitors of phospholipase A₂ from Naja naja naja venom.Phospholipid analogues were studied that contain two alkyl chains as well as single chain compounds that lack carbon-1 of the glycerol backbone and the attached acyl unit.Compounds that contain both long and medium length alkyl chains were studied.All of the potential inhibitors were tested in a well-defined mixed micelle system in which both the substrates and the inhibitors have been incorporated into Triton X-100 micelles.Surprisingly, the best inhibitors studied were the single chain fluoro ketones despite the fact that the enzyme has a strong preference for two-chain lipids.The most potent compound was found to have a dissociation constant some 600-3000-fold lower than the Michaelis constant for dipalmitoyl phosphatidylcholine substrate. 19F NMR studies of the fluoro ketone phospholipid analogues in micelles show that whereas the single chain compounds are partially in the hydrated-ketone form, the two-chain compounds are less than 0.1percent hydrated.In every case studied, potent inhibition of phospholipase A₂ was observed only with those compounds that are significantly hydrated in the micelle, and it is suggested that the hydrated fluoro ketone containing phospholipid analogues are the species responsible for the inhibition.In addition, the single chain fluoro ketones were better inhibitors than single and double chain alcohol and ketone analogues.Previous studies have been show that the cobra venom enzyme is activated by choline-containing lipids, and evidence is presented for the binding of the hydrated fluoro ketone inhibitors selectively to the activated enzyme.