Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the progressive dysfunction and death of motor neurons by mechanisms that remain unclear. Evidence indicates that oxidative mechanisms contribute to ALS pathology, but classical antioxidants have not performed well in clinical trials. Cyclic nitroxides are an alternative worth exploring because they are multifunctional antioxidants that display low toxicity in vivo. Here, we examine the effects of the cyclic nitroxide tempol (4-hydroxy-2,2,6,6-tetramethyl piperidine-1-oxyl) on ALS onset and progression in transgenic female rats over-expressing the mutant hSOD1G93A . Starting at 7 weeks of age, a high dose of tempol (155 mg/day/rat) in the rat´s drinking water had marginal effects on the disease onset but decelerated disease progression and extended survival by 9 days. In addition, tempol protected spinal cord tissues as monitored by the number of neuronal cells, and the reducing capability and levels of carbonylated proteins and non-native hSOD1 forms in spinal cord homogenates. Intraperitoneal tempol (26 mg/rat, 3 times/week) extended survival by 17 days. This group of rats, however, diverted to a decelerated disease progression. Therefore, it was inconclusive whether the higher protective effect of the lower i.p. dose was due to higher tempol bioavailability, decelerated disease development or both. Collectively, the results show that tempol moderately extends the survival of ALS rats while protecting their cellular and molecular structures against damage. Thus, the results provide proof that cyclic nitroxides are alternatives worth to be further tested in animal models of ALS.

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.Keywords: ditryptophan, cross-links, protein oxidation, free radicals Protein OxidationFree radicals and other oxidants are formed in living organisms by oxidation-reduction processes, which occur continuously during metabolism and through interactions with the environment. Organisms evolved enzymatic antioxidant defenses and the capability to use antioxidants from the diet to control the levels of these species. 1,2 Nevertheless, under certain circumstances, the levels of radicals and oxidants increase, promoting the oxidation of biomolecules. Among them, proteins are the major targets, due to their high biological abundance and reactivity towards one-and two-electron oxidants. [3][4][5] Protein residues most susceptible to oxidation are the sulfur-containing residues Cys and Met, and the aromatic residues His, Phe, Tyr, and Trp. In vivo, the oxidation of Cys and Met residues can be reversed by biological reductants with the assistance of enzymatic systems. In fact, the reversible oxidation of protein-Cys residues is emerging as a fundamental cell regulatory mechanism. 2,[6][7][8] The oxidation of all the other protein residues is irreversible, and includes several covalent modifications, such as protein cleavage, carbonylation, nitration, hydroxylation, halogenation and protein cross-linking with other proteins, lipids, carbohydrates and nucleic acids. [3][4][5][9][10][11] These modifications result in protein fragmentation, loss of protein function, protein aggregation and/or altered protein turnover, leading to cell and tissue dysfunction and various human pathologies. To maintain cellular homeostasis, proteins irreversibly oxidized are targeted for degradation by the proteasomal and lysosomal degradation pathways.11,12 During aging and pathological conditions associated with oxidative stress, however, the levels of over-oxidized and cross-linked proteins accumulate because these species are poor substrates for the intracellular proteolytic systems. This may lead to protein aggregation, which is a hallmark of age-related diseases, such as neurodegenerative diseases, atherosclerosis and cataract.11,12 Therefore, the study of irreversible protein oxidation in vitro and in vivo is relevant to the understanding o...