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Received for publication August 18, 2004.
Revised September 28, 2004.
Accepted for publication October 5, 2004.
Smoking causes a dysfunction in endothelial NO synthase (eNOS), which is ameliorated, in part, by administration of tetrahydrobiopterin (BH4). The exact mechanism by which the NO deficit occurs is unknown. We have previously shown that aqueous extracts of chemicals in cigarettes (CE) cause the suicide inactivation of neuronal NO synthase (nNOS) by interacting at the substrate binding site (Drug Metab. Dispos. 31:932-937, 2003). In the current study, we have found that CE directly inactivates eNOS by a process that is not affected by the natural substrate, L-arginine, and is distinct from the mechanism of inactivation of nNOS. We discovered that CE causes a time-, concentration-, and NADPH- dependent inactivation of eNOS in an in vitro system containing the purified enzyme, indicating a metabolic component to the inactivation. The CE-treated eNOS, but not nNOS, was nearly fully reactivated upon incubation with excess BH4 suggesting that BH4-depletion is a potential mechanism of inactivation. Moreover in the presence of CE, eNOS catalyzed the oxidation of BH4 to dihydrobiopterin and biopterin by a process attenuated by high concentrations of superoxide dismutase but not catalase. We speculate that a redox active component in CE, perhaps a quinone compound, causes oxidative uncoupling of eNOS to form superoxide, which in turn oxidizes BH4. The discovery of a direct inactivation of eNOS by a compound(s) present in tobacco provides a basis for further study of not only the mechanisms responsible for the biological effects of tobacco, but also a search for a potentially novel inactivator of eNOS.
Key words:
inhibition, nitric oxide synthase, reactive metabolites
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