Activation, inhibition, and regiospecificity of the lysophospholipase activity of the 85-kDa group IV cytosolic phospholipase A(2)

The 85-kDa Group IV calcium-dependent cytosolic phospholipase A(2) (cPLA(2)) catalyzes the hydrolysis of palmitoylglycero-3-phosphocholine to palmitic acid and glycero-3-phosphocholine. Palmitoylglycero-3-phosphocholine exists as a 9:1 equilibrium mixture of the sn-1 and sn-2 isomers, with the fatty acid predominately at the sn-1 position, We have monitored this reaction by P-31 NMR to determine which palmitoylglycero-3-phosphocholine isomer is processed by cPLA(2). When both lysophospholipid isomers are present in a 1:1 mixture under conditions in which acyl migration is minimized, cPLA(2) rapidly consumes both isomers, However, 1-palmitoylglycero-3-phosphocholine is consumed seven times faster than the 2-palmitoylglycero-3-phosphocholine isomer, We have previously reported that this lysophospholipase reaction is accelerated in the presence of glycerol, We now find that this apparent increase in activity is accounted for, in part, by glycerol acting as an alternative acceptor for the cleaved fatty acid, as is the case for this enzyme's phospholipase A(2) (PLA(2)) activity, In contrast, dioleoylglycerol, which accelerates the PLA(2) activity, does not act as an acceptor in either the lysophospholipase or the PLA(2) reaction, but can affect enzyme activities by altering substrate presentation, We also show that a known inhibitor of the PLA(2) activity of cPLA(2) is able to inhibit its lysophospholipase activity with a similar IC50 60 its PLA(2) activity, However the effect of inhibitors is dependent on the manner in which they are presented to the enzyme.