Discovery of an arachidonoyl coenzyme A synthetase in human platelets

Platelets contain small amounts of a variety of free fatty acids but essentially no free arachidonate. When free fatty acids are incubated with platelets, there is preferential incorporation of arachidonic acid and 8,-11,14-eicosatrienoic acid compared to other fatty acids. We now explain these findings by the discovery that platelets contain two long chain acyl-CoA synthetases. One shows activity with a range of different fatty acids, similar to long chain acyl-CoA synthetases studied previously. A crude platelet membrane preparation contains this enzyme that catalyzes the formation of 0.75 nmol of oleoyl-CoA/min/10⁹ plateles. The other enzyme is specific for the prostaglandin precursor arachidonic acid and 8,11,14-eicosatrienoic acid. Based on the ability of fatty acids to inhibit arachidonate and 8,11,14-eicosatrienoate activatio, we conclude that other fatty acids including linoleic, 5,8,11-eicosatrienoic, and oleic acids are not substrates for the enzyme. Platelet membranes catalyze formation of 2.9 nmol of arachidonoyl-CoA/min/10⁹ platelets and 2.5 nmol of 8,11,14-eicosatrienoyl-CoA/min/10⁹ platelets. Arachidonoyl-CoA synthetase has optimal activity at pH 8 and requires ATP (Km=0.5 mM), Mg² (Km=2.5 mM), CoA(Km=0.13 mM), and arachidonic acid (Km=0.03 mM). We propose that the arachidonate-specific acyl-CoA synthetase may control the level of free arachidonic acid in platelets, limiting prostaglandin synthesis by the unstimulated cell and capturing free arachidonate from extracellular sources.