Synthesis of mixed-chain phosphatidylcholines including coumarin fluorophores for FRET-based kinetic studies of phospholipase A2 enzymes
Phospholipase A2 (PLA2) enzymes catalyze the hydrolysis of the sn-2 ester linkage of glycerophospholipids to produce fatty acids and lysophospholipids. A significant number of mammalian phospholipases comprise a family of secreted PLA2 enzymes, found in specific tissues and cellular locations, exhibiting unique enzymatic properties and distinct biological functions. Development of new real-time spectrofluorimetric PLA2 assays should facilitate the kinetic characterization and mechanistic elucidation of the isozymes in vitro, with the potential applicability to detect and measure catalytic PLA2 activity in tissues and cellular locations. Here we report a new synthesis of double-labeled phosphatidylcholine analogs with chain-terminal reporter groups including coumarin fluorophores for fluorescence resonance energy transfer (FRET)-based kinetic studies of PLA2 enzymes. The use of coumarin derivatives as fluorescent labels provides reporter groups with substantially decreased size compared to the first generation of donor-acceptor pairs of fluorescent phospholipids. The key advantage of the design is to interfere less with the physicochemical properties of the acyl chains, thereby improving the substrate quality of the synthetic probes. In order to assess the impact of the fluorophore substituents on the catalytic hydrolysis and on the phospholipid packing in the lipid-water interface of the assay, we used the experimentally determined specific activity of bee-venom phospholipase A2 as a model for the secretory PLA2 enzymes. Specifically, the rate of PLA2 hydrolysis of the coumarin labeled phosphatidylcholine analogs was less than three times slower than the natural substrate dipalmitoyl phosphatidylcholine (DPPC) under the same experimental conditions. Furthermore, variation of the mole fraction of the synthetic phosphatidylcholine vs. that of the natural DPPC substrate showed nearly ideal mixing behavior in the phospholipid-surfactant aggregates of the assay. The synthesis provides a rapid and efficient method for preparation of new synthetic phosphatidylcholines with the desired target structures for enzymatic and physicochemical studies.