A New Role for Nicotine: Selective Inhibition of Thromboxane Formation by Direct Interaction with Thromboxane Synthase

Summary

Animal cells can convert arachidonic acid into prostaglandins, thromboxane, and leukotrienes. These locally produced mediators of inflammatory and hypersensitivity reactions have been implicated in several clinically important disease processes. Thromboxane, the major oxygenated arachidonic acid metabolite of macrophages, is the most potent vasoconstricting and proaggregatory molecule known and has also been related to host defense mechanisms. We have studied effects of nicotine on thromboxane formation using cultured macrophage-like cells, microsomal assays, and purified thromboxane synthase. In intact macrophage-like cells nicotine, cotinine, and methylnicotine, at submicromolar concentrations, inhibited the rate of conversion of endoperoxide prostaglandin H2 into thromboxane but not into prostacyclin. This indicated that nicotine selectively inhibited thromboxane synthase at concentrations that are readily observed in the circulation of smokers. Microsomal assays revealed that nicotine decreased the maximal velocity (V_max) of thromboxane synthase without affecting the apparent affinity of the enzyme for its substrate (K_m). No effect of nicotine on kinetic parameters of prostacyclin synthase could be observed. To elucidate the molecular mechanism of nicotine’s effect we obtained difference spectra using purified thromboxane synthase. The difference spectra revealed that nicotine directly interacted with the enzyme presumably by binding of the nitrogen of the nicotine ring structure to the iron of the cytochrome p-450 component of thromboxane synthase.