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Inhibition of human phospholipases A2 by cis-unsaturated fatty acids and their oxidative metabolites and/or polymers was studied using partially purified human phospholipases A2 and [1-14C]oleate labelled, autoclaved E. coli as substrate. As previously reported for other phospholipases A2, oleic and arachidonic acids inhibited human synovial fluid phospholipase A2 with IC50s of 15 and 30 microM respectively. Air oxidation of arachidonic acid or hydroxylation of oleic acid (12-hydroxy-oleate) substantially relieved that inhibition. Similarly, the enzymatically oxidatized metabolite of arachidonate, prostaglandin B1 (PGB1), did not inhibit enzymatic activity. However, prostaglandin Bx (PGBx), an oligomer (n = 6) of PGB1, was a potent inhibitor of Ca(++)-dependent, neutral-active phospholipase A2 activities. Enzymatic activity in acid extracts from human neutrophils, platelets, sperm, plasma, synovial fluid, endometrium, degenerative disc, and snake venom was inhibited by PGBx with IC50s ranging from 0.5-7.0 microM. Inhibition was independent of substrate phospholipid concentration over a 24-fold range (5-120 microM) and PGBx quenched the tryptophan fluorescence of snake venom phospholipase A2 in a dose-dependent manner. Agonist-induced (A23187) release of arachidonic acid from prelabelled human neutrophils and cultured human endothelial cells was also inhibited by PGBx with IC50s of 3 and 20 microM, respectively. These results illustrate that oxidative reactions of cis-unsaturated fatty acids relieve their natural inhibitory activity, and polymerization of an inactive fatty acid metabolite yields a potent inhibitor of in vitro and in situ phospholipase A2 activity.
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