Study of the two pathways for arachidonate oxygenation in blood platelets

Abstract

During collagen-induced blood platelet aggregation, arachidonic acid is set free from membrane phospholipids and subsequently converted into 12-hydroxyeicosatetraenoic acid by arachidonate lipoxygenase and into thromboxane A2, 12-hydroxyheptadecatrienoic acid (HETE) and malondialdehyde by cyclooxygenase and thromboxane synthase. Lipoxygenase and cyclooxygenase have optimal activity at neutral to basic pH, while the thromboxane synthase is pH-independent between 5 and 9. These enzymes are membrane-bound. The cyclooxygenase is rapidly inactivated upon membrane disruption by nonionic detergents or phospholipid degradation with phospholipase A2. It was found that platelet phospholipase A2 preferentially splits off fatty acid with four double bonds. Eicosatetraynoic acid was used to investigate the physiological function of the arachidonate lipoxygenase during collagen-induced aggregation of rat blood platelets. This fatty acid is a more efficient inhibitor of lipoxygenase than of cyclooxygenase. At an inhibitor concentration of 0.6 microgram/ml, platelet aggreation, 12-hydroxyeicosatetraenoic acid production as well as 15-hydroxytryptamine release are completely inhibited, while there is an apparent stimulation of the cyclooxygenase. These results indicate that arachidonate lipoxygenase is essential for irreversible blood platelet aggregation.

Protease-Activated Receptor Signaling in Platelets Activates Cytosolic Phospholipase A 2α Differently for Cyclooxygenase-1 and 12-Lipoxygenase Catalysis

“…Maximal formation of TxA 2 is reached in ≤15 seconds after the PAR1 peptide, whereas formation of 12-HETE increases gradually over 120 seconds. These differences between the rates of formation and the time at which maximal formation is achieved are very similar to those previously reported for the formation of TxA 2 and 12-HETE following collagen-induced platelet activation 29. It is clear from the persisting 12-HETE formation long after TxA 2 production has ceased that the AA substrate is not provided by the same mechanisms to COX-1 and to 12-LOX after PAR1 activation.…”

The effect of dietary arachidonic acid on plasma lipoprotein distributions, apoproteins, blood lipid levels, and tissue fatty acid composition in humans

“…As AA in the plasma FFA is a substrate for cyclooxygenase (40,41), it is probably beneficial that AA level in the plasma FFA is only marginally influenced by dietary AA. Here we found that AA level in the FFA in plasma was 2% in subjects on the low-AA diet and increased to 3% when they consumed to high-AA diet.…”

Modulation of the Eosinophil Chemotactic Factor (ECF) Release from Various Cells and Their Subcellular Components by Phospholipids

“…Ad dition of indomethacin at concentrations which do inhibit the prostaglandin synthesis enhanced the ECF generation from human PMN:ETYA, an inhibitor of the cycloxy-genase and lipoxygenase pathways, signifi cantly impairs the ECF release induced by arachidonic acid, ionophore and phospholi pase Ao at a concentration which does not interfere with the chemotactic response of eosinophils. It has also been described that ETYA inhibits the release of histamine from human basophil granulocytes, rat mast cells [32,43]; and further the release of ly sosomal enzymes from human PMNs [34], These observations strongly suggest a role for arachidonic acid metabolites in cell phy siology [4,12,13].…”