HYDROGEN-PEROXIDE INCREASES THE AVAILABILITY OF ARACHIDONIC-ACID FOR OXIDATIVE-METABOLISM BY INHIBITING ACYLATION INTO PHOSPHOLIPIDS IN THE ALVEOLAR MACROPHAGE

Reactive oxygen species stimulate metabolism of arachidonic acid (AA) to eicosanoids in a variety of cells and tissues, yet the pathway(s) by which oxidants increase the availability of AA for oxidative metabolism are not known. Thus, we explored the effects of hydrogen peroxide (H2O2) on deacylation and reacylation of AA to determine the enzymatic mechanism(s) by which this oxidant increases levels of free, unesterified AA, and thereby its oxidative metabolism to eicosanoids, in the rat alveolar macrophage (AM). Over the range from 0.1 to 0.5 mM, H2O2 caused marked time- and dose-dependent inhibition of incorporation of [H-3]AA into macrophage phospholipids, whereas calcium ionophore A23187 and zymosan particles did not cause such inhibition. Within this concentration range, there was an almost exact reciprocal correlation between inhibition of [H-3]AA acylation and H2O2-stimulated accumulation of free [H-3]AA in prelabeled AM cultures. Thimerosal, which blocks AA reacylation but spares deacylation via phospholipase A2 (PLA2), did not affect accumulation of free [H-3]AA in prelabeled cells stimulated with H2O2, while markedly augmenting [H-3]AA release in response to A23187 and to zymosan. Despite its ability to block AA acylation almost completely, H2O2 did not directly inhibit arachidonoyl CoA synthetase or arachidonoyl CoA:lysophosphatide acyltransferase, which catalyze AA incorporation into phospholipids. However, H2O2 (0.1 to 0.5 mM) markedly depleted AMs of ATP, required for synthesis of the acylation intermediate arachidonoyl CoA, suggesting that this was the means by which H2O2 inhibited acylation. Notably, H2O2 (0.03 to 3 mM) failed to stimulate macrophage PLA2 activity. We conclude that H2O2, in contrast to A23187 and zymosan, inhibits incorporation of AA into phospholipids, and that this represents the major mechanism by which the oxidant increases the availability of free AA for oxidative metabolism in the AM. This may be an important basis for release of eicosanoids in oxidant-induced inflammation and injury of the lung.