Protein oxidation is an unavoidable consequence of aerobic metabolism. The oxidation of most proteins residues is non-repairable and may affect protein structure and function. In particular, protein cross-links arising from oxidative modifications are presumably toxic to cells because they may accumulate and induce protein aggregation. However, most of these irreversible protein cross-links remain partially characterized. Up to very recently, ditryptophan cross-links (Trp-Trp), in particular, have been largely disregarded in the literature. Here, we briefly review studies showing that Trp-Trp cross-links can be formed in proteins exposed to a variety of oxidants. The challenges to fully characterize Trp-Trp cross-links are discussed as well as their potential roles in protein dysfunction and aggregation.

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.

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.

The generation of reactive oxygen and nitrogen species (ROS and RNS) during metabolism is capable of damaging cellular biomolecules. To be protected against oxidative injury, cells evolved complex cellular defense mechanisms and the capability to use exogenous antioxidants to eliminate ROS/RNS. The potential role of micronutrients as antioxidants (vitamin C, vitamin E, carotenoids and poliphenols) has stimulated intense research efforts. In various human supplementation studies, however, these compounds presented pro-oxidant effects at high doses for most risk groups. Therefore, more studies about the bioavailability, tissue uptake, metabolism and biological activities should be performed before establishing recommendations for disease prevention.

Accumulated evidence indicates that oxidative stress plays a role in neurodegenerative diseases, such as Alzheimer, Parkinson and Amyotrophic Lateral Sclerosis. Here, we emphasize the results provided by the technology of genetically modified animals. Studies with transgenic and knockout mice have allowed great advances in the research of oxidative stress in general and in the central nervous system, and are pointing to potential targets for the development of new drugs and therapies to disrupt the cycle of events that lead to neuronal death. Thus, genetically modified animals are a valuable tool for the comprehension of human diseases, including neurodegenerative ones.