A new role for nicotine: selective inhibition of thromboxane formation by direct interaction with thromboxane synthase in human promyelocytic leukaemia cells differentiating into macrophages

Thromboxane, one of the major oxygenated arachidonic acid metabolites of human macrophages, is the most potent vasoconstricting and proaggregatory molecule known. In addition, thromboxane has been shown to be related to host defence mechanisms. We studied the effects of nicotine and its major metabolites on thromboxane formation using cultured macrophage-like cells (HL-60), microsomal assays and purified thromboxane synthase. In intact cells, nicotine, cotinine and methylnicotine at submicromolar concentrations inhibited the rate of conversion of both arachidonic acid and the unstable endoperoxide prostaglandin H2 into thromboxane but not into other eicosanoids. This indicates that nicotine selectively inhibits thromboxane synthase at concentrations that are readily observed in the circulation of smokers. Microsomal assays revealed that nicotine decreased the maximal velocity of thromboxane synthase without affecting the apparent affinity of the enzyme for its substrate. In contrast, no effect of nicotine on kinetic parameters of prostaglandin H synthase or prostacyclin synthase could be observed. Difference spectra, using purified thromboxane synthase, revealed that nicotine directly interacts with the enzyme, presumably by binding the nitrogen of the nicotine ring structure to the iron of the cytochrome P-450 component of thromboxane synthase.