Methylene blue inhibits the arachidonic acid metabolism in human blood platelets.

When human blood platelets are incubated with methylene blue (MB) that oxidizes cellular NADPH, a marked inhibition of platelet aggregation and of the metabolism of arachidonic acid (AA) in both the cyclooxygenase and lipoxygenase pathway is found. The inhibition of aggregation and AA metabolism is prevented, at least partially, by the thiol-oxidizing agent diamide. The results indicate an involvement of the reduced coenzymes NADPH and glutathione (GSH) in the regulation of cyclooxygenase and lipoxygenase activity in platelets.