PHOSPHOLIPASE A(2) AS A MECHANOSENSOR

Osmotic swelling of large unilamellar vesicles (LUVs) causes membrane stretching and thus reduces the lateral packing of lipids. This is dem onstrated to modulate strongly the catalytic activity of phosphotipase A(2) (PLA(2)) toward a fluorescent phospholipid, 6-pyren-1-yl)]decano yl-sn-glycero-3-phosphocholine (PPDPC) residing in LUVs composed of di fferent unsaturated and saturated phosphatidylcholines. The magnitude of the osmotic pressure gradient Delta Omega required for maximal PLA( 2) activity as well as the extent of activation depend on the degree o f saturation of the membrane phospholipid acyl chains. More specifical ly Delta Omega needed for maximal hydrolytic activity increases in the sequence DOPC < SOPC < DMPC in accordance with the increment in the i ntensity of chain-chain van der Waals interactions. Previous studies o n the hydrolysis of substrate monolayers by C. adamanteus and N. naja PLA(2) revealed maximal hydrolytic rates for these two enzymes to be a chieved at lipid packing densities corresponding to surface pressures of 12 and 18 mN m(-1), respectively. In keeping with the above the mag nitudes of Delta Omega producing maximal activity of Crotalus adamante us and Naja naja toward PPDPC/DMPC LUVs were 40 and 20 mOsm/kg, respec tively. Our findings suggest a novel possibility of regulating the act ivity of PLA(2) and perhaps also other lipid packing density-dependent enzymes in vivo by osmotic forces applied on cellular membranes. Impo rtantly, our results reveal serendipitously that the responsiveness of membranes to osmotic stress is modulated by the acyl chain compositio n of the lipids.