Cyclic nitroxides inhibit the toxicity of nitric oxide-derived oxidants: mechanisms and implications

The substantial therapeutic potential of tempol (4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy) and related cyclic nitroxides as antioxidants has stimulated innumerous studies of their reactions with reactive oxygen species. In comparison, reactions of nitroxides with nitric oxide-derived oxidants have been less frequently investigated. Nevertheless, this is relevant because tempol has also been shown to protect animals from injuries associated with inflammatory conditions, which are characterized by the increased production of nitric oxide and its derived oxidants. Here, we review recent studies addressing the mechanisms by which cyclic nitroxides attenuate the toxicity of nitric oxidederived oxidants. As an example, we present data showing that tempol protects mice from acetaminophen-induced hepatotoxicity and discuss the possible protection mechanism. In view of the summarized studies, it is proposed that nitroxides attenuate tissue injury under inflammatory conditions mainly because of their ability to react rapidly with nitrogen dioxide and carbonate radical. In the process the nitroxides are oxidized to the corresponding oxammonium cation, which, in turn, can be recycled back to the nitroxides by reacting with upstream species, such as peroxynitrite and hydrogen peroxide, or with cellular reductants. An auxiliary protection mechanism may be down-regulation of inducible nitric oxide synthase expression. The possible therapeutic implications of these mechanisms are addressed.

O considerável potencial terapêutico de tempol (4-hidroxi-2,2, 6,6-tetrametil-1piperiniloxila) e nitróxidos cíclicos relacionados como antioxidantes tem estimulado inúmeros estudos de suas reações com espécies reativas derivadas de oxigênio. Em comparação, as reações de nitróxidos com oxidantes derivados do óxido nítrico têm sido investigadas menos frequentemente. Todavia, essas reações são relevantes porque o tempol é também capaz de proteger animais de injúrias associadas a condições inflamatórias, as quais são caracterizadas por uma aumentada produção de óxido nítrico e derivados oxidantes. Aqui, discutimos estudos recentes abordando os mecanismos pelos quais nitróxidos cíclicos atenuam a toxicidade de oxidantes derivados do óxido nítrico. Como um exemplo, apresentamos dados que demonstram que o tempol protege camundongos do dano hepatotóxico promovido por altas doses de acetaminofeno e discutimos o possível mecanismo de proteção. Com base nos estudos sumarizados, é proposto que nitróxidos atenuam a injúria tecidual em condições inflamatórias devido principalmente a sua capacidade de reagir rapidamente com ambos, dióxido de nitrogênio e radical carbonato. Em conseqüência, os nitróxidos são oxidados ao cátion oxamônio correspondente, o qual, por sua vez, pode ser reciclado ao nitróxido através de reações com espécies precursoras, como peroxinitrito e peróxido de hidrogênio, ou com redutores celulares. Um possível mecanismo auxiliar de proteção é a regulação negativa da expressão da sintase do óxido nítrico induzível. As possíveis implicações terapêuticas desses mecanismos são abordadas.

Active oxygen species as signaling mediators in the vascular system

There is increasing evidence that active oxygen species (AOS) can act as autocrine or paracrine regulatory mediators, perhaps as part of a more general cellular redox messenger system. This signaling role of AOS has been particularly studied in the vascular system, considering that: 1) Nitric oxide, a gaseous free radical, or a related compound, is a major endothelium-derived vasodilator which is scavenged by superoxide. This interaction not only promotes vasoconstriction, but can generate peroxynitrite, which may be toxic to cells; 2) AOS modulate cytoplasmic calcium and hydrogen concentration and are related to other messenger systems such as membrane G-proteins and protein kinase C; 3) through EPR techniques we showed recently that in vitro and in vivo release of vascular free radicals (probably superoxide) can be triggered by physiological stimuli such as shear stress increases. Flow-induced spin adduct signals were abolished by endothelium removal; 4) we showed previously that superoxide dismutase completely prevents vasoconstriction soon after vascular injury induced by angioplasty, thus implicating the superoxide radical in this acute phenomenon. In addition, we showed that allopurinol or N-acetylcysteine markedly increases vascular diameter up to 7 days after vascular injury, suggesting that redox phenomena may be involved in sustained vessel recoil; 5) endothelial free radical production is increased in atherosclerosis; likewise, lipoprotein oxidation within the vascular wall appears to be a critical in vivo step for atherogenesis. In addition, oxidative stress contributes to dysfunction of endothelium-dependent relaxation in hypertension and diabetes; 6) AOS were shown to mediate growth-related responses in vascular cells. Recently, we showed that in vivo administration of oxidized glutathione markedly enhances vascular proliferation after injury. Therefore , it is suggested that AOS may work as vascular mediators that may exert useful physiological roles; derangements of redox signaling mechanisms are likely involved in endothelial dysfunction and pathological vascular responses.

Free radical reactions: formation of adducts with biomolecules and their biological significance

Inherent in oxygen utilization by aerobic organisms is the possibility of damage to biomolecules by oxidative reactions, a phenomenon that has come to be called oxidative stress. The most extensively studied deleterious reactions in biological systems have been free radical chain reactions which lead to degradation of biomolecules and cellular structures. As discussed in this review, however, elucidation of the mechanisms by which free radicals play their physiological and pathological roles may require a better understanding of the addition reactions between free radicals and biomolecules.