The effect of substrate availability on the metabolism of arachidonic acid in human platelets

Abstract

Washed human platelets transform 14C-arachidonic acid in vitro to HETE, HHT and thromboxane B 2 via the 12-lipoxygenase and cyclooxygenase pathways, respectively. At low (1 μ m) substrate concentration, the cyclooxygenase predominates with formation of HHT and thromboxane B 2. Not until the substrate level reaches 10 μ m do substantial amounts of HETE appear. Between substrate levels of 10–100 μ m, the cyclooxygenase gradually becomes saturated and lipoxygenase activity increases 20-fold. Cyclooxygenase inhibition by indomethacin increases the formation of HETE by shifting substrate to the lipoxygenase pathway. Lipoxygenase inhibitors, acetone phenylhydrazone or nordihydroguaiaretic acid selectively block HETE formation at 10 μ m. At higher concentrations 100 μ m or more, they act nonselectively against both enzyme pathways. The results suggest that the platelet cyclooxygenase has a higher affinity for arachidonic acid than the lipoxygenase and competes more favorably for substrate in the same pool. Lipoxygenase activity only becomes significant when massive amounts of arachidonic acid are released.