Covalent binding of arachidonic acid metabolites to human platelet proteins. Identification of prostaglandin H synthase as one of the modified substrates.

The covalent modification of proteins by metabolites of arachidonic acid (AA) was investigated in human platelets. Following incubation of washed human platelets with radiolabeled AA, ethanol precipitation of the proteins, and lipid extraction by organic solvents, a small fraction of the radioactivity added (0.3%) was tightly bound to the protein pellet. A dozen labeled protein bands were resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Exhaustive hydrolysis of platelet proteins by proteases released an amphipathic radiolabeled material which had a chromatographic behavior similar to that of a known peptidolipid, leukotriene C4. These findings suggest a covalent nature for the observed binding. This binding was specific for AA since palmitate, myristate, or linoleate did not bind to a significant extent. It involved products of both cyclooxygenase and lipoxygenase pathways: it was indeed inhibited to a greater extent by eicosatetraynoic acid than by indomethacin. The protein-associated radioactivity was increased by the thromboxane synthase inhibitor dazoxiben. Indomethacin completely abolished this increase in binding, which could not be reproduced by exogenous prostaglandin (PG) E2, F2 alpha, or D2, and might thus involve PGG2 and/or PGH2. Diamide, an agent known to inhibit the reduction of 12-hydroperoxyeicosatetraenoic acid in platelets, produced an increase of the covalent binding, which was abolished by eicosatetraynoic acid but not by indomethacin: this suggests that the lipoxygenase product bound was 12-hydroperoxyeicosatetraenoic acid or a by-product. Dazoxiben and diamide produced distinct patterns of protein labeling after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. One labeled band had a Mr of 70,000 as the PGH synthase monomer. Addition of AA at 17 microM enhanced the labeling of this band, while 100 microM was inhibitory. Labeling of this band was also induced by thrombin in prelabeled platelets. Two monoclonal antibodies against PGH synthase caused immune precipitation of a 70-kDa labeled protein in homogenates of [3H]AA-labeled platelets. PGH synthase, purified from ram seminal vesicles, was covalently modified after incubation with [3H]AA: this labeling was almost completely abolished by indomethacin. As much as 40% of platelet PGH synthase was covalently modified after incubation with 17 microM AA. It can be concluded that in intact platelets PGH synthase is covalently modified by an eicosanoid following incubation with exogenous AA or after AA mobilization from phospholipids by thrombin.