PRESENCE OF GLYCEROL MASKS THE EFFECTS OF PHOSPHORYLATION ON THE CATALYTIC EFFICIENCY OF CYTOSOLIC PHOSPHOLIPASE A(2)

Cytosolic phospholipase A(2) catalyzes the selective release of arachi donic acid from the sn-2 position of phospholipids and is believed to play a key cellular role in the generation of arachidonic acid. The en zymatic activity of cPLA(2) is affected by several mechanisms, includi ng substrate presentation and the phosphorylation state of the enzyme. Using covesicles of -[arachidonoyl-1-C-14]-Sn-glycero-3-phosphocholin e and 1,2-dimyristoyl-phosphatidylmethanol as substrate, the effects o f phosphorylation on the interfacial binding and catalytic constants w ere investigated. Phosphorylated and dephosphorylated enzyme forms wer e shown to have identical values of 2.6 mu M for K-M(app), an equilibr ium dissociation constant which consists of the intrinsic dissociation constant from the lipid/water interface (K-s) and the dissociation co nstant for phospholipid from the active site (K-M). Moreover, the val ues of K-M for phosphorylated and dephosphorylated enzyme did not dif fer significantly (0.4 +/- 0.1 and 0.2 +/- 0.1, respectively), However , dephosphorylation of the enzyme reduced the value of k(cat) by 39%. The phosphorylation state of the enzyme had no effect on either the co operativity shown by this enzyme or the thermal stability of the enzym e. Surprisingly, the presence of glycerol (4 M) masks the effect of ph osphorylation on k(cat). Instead, glycerol increased the value of k(ca t) by 440% for the phosphorylated enzyme and by 760% for the dephospho rylated form. Moreover, addition of glycerol had only small effects on K-M(app). The increase in the k(cat) upon addition of glycerol result s from a substantial decrease in the activation energy from 29.4 to 14 .8 kcal mol(-1). To determine whether the effects of phosphorylation o f the enzyme or addition of glycerol are unique to this artificial sub strate, membranes from U937 cells were isolated and used as substrate. With these membranes, the dephosphorylated enzyme was only 21% less a ctive than the phosphorylated enzyme. In the presence of glycerol, the re was no detectable difference the two enzyme forms, and the rate of hydrolysis was increased by 300-390% over that measured in the absence of glycerol. These results suggest that the catalytic efficiency of t he phosphorylated enzyme is not particularly relevant to its activatio n in vivo. Moreover, it may be that glycerol is mimicking the effect o f some unidentified factor which greatly enhances the catalytic effici ency of the enzyme. (C) 1997 Academic Press.