Factors regulating the substrate specificity of cytosolic phospholipase A2-alpha in vitro

Abstract Cytosolic phospholipase A 2 alpha (cPLA 2 α) plays a key role in signaling in mammalian cells by releasing arachidonic acid (AA) from glycerophospholipids (GPLs) but the factors determining the specificity of cPLA 2 α for AA-containing GPLs are not well understood. Accordingly, we investigated those factors by determining the activity of human cPLA 2 α towards a multitude of GPL species present in micelles or bilayers. Studies on isomeric PC sets containing a saturated acyl chain of 6 to 24 carbons in the sn 1 or sn 2 position in micelles showed an abrupt decrease in hydrolysis when the length of the sn 1 or sn 2 chain exceeded 17 carbons suggesting that the acyl binding cavity on the enzyme is of the corresponding length. Notably, the saturated isomer pairs were hydrolyzed identically in micelles as well as in bilayers suggesting promiscuous binding of acyl chains to the active site of cPLA 2 α. Such promiscuous binding would explain the previous finding that cPLA 2 α has both PLA 1 and PLA 2 activities. Interestingly, increasing the length of either the sn 1 or sn 2 acyl chain inhibited the hydrolysis in bilayers far more than that in micelles suggesting that with micelles (loosely packed) substrate accommodation at the active site of cPLA 2 α is rate-limiting, while with bilayers (tightly packed) upward movement of the substrate from the bilayer (efflux) is the rate-limiting step. With the AA-containing PCs, the length of the saturated acyl chain also had a much stronger effect on hydrolysis in bilayers vs . micelles in agreement with this model. In contrast to saturated PCs, a marked isomer preference was observed for AA-containing PCs both in micelles and bilayers. In conclusion, these data significantly help to understand the mode of action and specificity of cPLA 2 α.