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Tempol (4-hydroxy-2,2,6,6-tetramethyl piperidine-1-oxyl) reduces tissue injury in animal models of various diseases via mechanisms that are not completely understood. Recently, we reported that high doses of tempol moderately increased survival in a rat model of ALS (amyotrophic lateral sclerosis) while decreasing the levels of oxidized hSOD1 (human Cu,Zn-superoxide dismutase) in spinal cord tissues. To better understand such a protective effect in vivo, we studied the effects of tempol on hSOD1 oxidation in vitro. The chosen oxidizing system was the bicarbonate-dependent peroxidase activity of hSOD1 that consumes H2O2 to produce carbonate radical, which oxidizes the enzyme. Most of the experiments were performed with 30 μM hSOD1, 25 mM bicarbonate, 1 mM H2O2, 0.1 mM DTPA (diethylenetriaminepenta-acetic acid) and 50 mM phosphate buffer at a final pH of 7.4. The results showed that tempol (5-75 μM) does not inhibit hSOD1 turnover, but decreases its resulting oxidation to carbonylated and covalently dimerized forms. Tempol acted by scavenging the carbonate radical produced and by recombining with hSOD1-derived radicals. As a result, tempol was consumed nearly stoichiometrically with hSOD1 monomers. MS analyses of turned-over hSOD1 and of a related peptide oxidized by the carbonate radical indicated the formation of a relatively unstable adduct between tempol and hSOD1-Trp32•. Tempol consumption by the bicarbonate-dependent peroxidase activity of hSOD1 may be one of the reasons why high doses of tempol were required to afford protection in an ALS rat model. Overall, the results of the present study confirm that tempol can protect against protein oxidation and the ensuing consequences.

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...

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Protein disulfide isomerase (PDI) is a dithiol-disulfide oxidoreductase that has essential roles in redox protein folding. PDI has been associated with protective roles against protein aggregation, a hallmark of neurodegenerative diseases. Intriguingly, PDI has been detected in the protein inclusions found in the central nervous system of patients of neurodegenerative diseases. Oxidized proteins are also consistently detected in such patients, but the agents that promote these oxidations remain undefined. A potential trigger of protein oxidation is the bicarbonate-dependent peroxidase activity of the human enzyme superoxide dismutase 1 (hSOD1). Therefore, we examined the effects of this activity on PDI structure and activity. The results showed that PDI was oxidized to radicals that lead to PDI inactivation and aggregation. The aggregates are huge and apparently produced by covalent cross-links. Spin trapping experiments coupled with MS analysis indicated that at least 3 residues of PDI are oxidized to tyrosyl radicals (Y(63), Y(116) and Y(327)). Parallel experiments showed that PDI is also oxidized to radicals, inactivated and aggregated by the action of photolytically generated carbonate radical and by UV light. PDI is prone to inactivation and aggregation by one-electron oxidants and UV light probably because of its high content of aromatic amino acids.

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