[Show abstract][Hide abstract] ABSTRACT: Activation of the NADPH oxidase-derived oxidant burst of polymorphonuclear leukocytes (PMNs) is of critical importance in
inflammatory disease. PMN-derived superoxide (O⨪2) can be scavenged by nitric oxide (NO⋅) with the formation of peroxynitrite (ONOO−); however, questions remain regarding the effects and mechanisms by which NO⋅ and ONOO− modulate the PMN oxidative burst. Therefore, we directly measured the dose-dependent effects of NO⋅and ONOO− on O⨪2 generation from human PMNs stimulated with phorbol 12-myristate 13-acetate using EPR spin trapping. Pretreatment with low
physiological (μm) concentrations of NO⋅ from NO⋅ gas had no effect on PMN O⨪2 generation, whereas high levels (≥50 μm) exerted inhibition. With ONOO− pretreatment, however, a biphasic modulation of O⨪2 generation was seen with stimulation by μm levels, but inhibition at higher levels. With the NO⋅ donor NOR-1, which provides more sustained release of NO⋅ persisting at the time of O⨪2 generation, a similar biphasic modulation of O⨪2 generation was seen, and this was inhibited by ONOO− scavengers. The enhancement of O⨪2 generation by low concentrations of ONOO− or NOR-1 was associated with activation of the ERK MAPKs and was blocked by their inhibition. Thus, low physiological levels
of NO⋅present following PMN activation are converted to ONOO−, which enhances O⨪2 generation through activation of the ERK MAPK pathway, whereas higher levels of NO⋅ or ONOO−feed back and inhibit O⨪2 generation. This biphasic concentration-dependent regulation of the PMN oxidant burst by NO⋅-derived ONOO− may be of critical importance in regulating the process of inflammation.
[Show abstract][Hide abstract] ABSTRACT: Xanthine oxidase (XO) is a central mechanism of oxidative injury as occurs following ischemia. During the early period of reperfusion, both nitric oxide (NO(*)) and superoxide (O-*(2)) generation are increased leading to the formation of peroxynitrite (ONOO(-)); however, questions remain regarding the presence and nature of the interactions of NO(*) or ONOO(-) with XO and the role of this process in regulating oxidant generation. Therefore, we determined the dose-dependent effects of NO(*) and ONOO(-) on the O-*(2) generation and enzyme activity of XO, respectively, by EPR spin trapping of O-*(2) using 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline-N-oxide and spectrophotometric assay. ONOO(-) markedly inhibited both O-*(2) generation and XO activity in dose-dependent manner, while NO(*) from NO(*) gas in concentrations up to 200 microM had no effect. Furthermore, we observed that NO(*) donors such as NOR-1 also inhibited O-*(2) generation and XO activity; however, these effects were O-*(2)-dependent and blocked by superoxide dismutase or ONOO(-) scavengers. Finally, we found that ONOO(-) totally abolished the Mo(V) EPR spectrum. These changes were irreversible, suggesting oxidative disruption of the critical molybdenum center of the catalytic site. Thus, ONOO(-) formed in biological systems can feedback and down-regulate XO activity and O-*(2) generation, which in turn may serve to limit further ONOO(-) formation.