The role of CO2 in cobalt-catalyzed peroxidations

A ditryptophan cross-link is responsible for the covalent dimerization of human superoxide dismutase 1 during its bicarbonate-dependent peroxidase activity

Unlike intermolecular disulfide bonds, other protein cross-links arising from oxidative modifications cannot be reversed and are presumably more toxic to cells because they may accumulate and induce protein aggregation. However, most of these irreversible protein cross-links remain poorly characterized. For instance, the antioxidant enzyme human superoxide dismutase 1 (hSod1) has been reported to undergo non-disulfide covalent dimerization and further oligomerization during its bicarbonate-dependent peroxidase activity. The dimerization was shown to be dependent on the oxidation of the single, solvent-exposed Trp(32) residue of hSod1, but the covalent dimer was not isolated nor was its structure determined. In this work, the hSod1 covalent dimer was isolated, digested with trypsin in H(2)O and H(2)(18)O, and analyzed by UV-Vis spectroscopy and mass spectrometry (MS). The results demonstrate that the covalent dimer consists of two hSod1 subunits cross-linked by a ditryptophan, which contains a bond between C3 and N1 of the respective Trp(32) residues. We further demonstrate that the cross-link cleaves under usual MS/MS conditions leading to apparently unmodified Trp(32), partially hinders proteolysis, and provides a mechanism to explain the formation of hSod1 covalent trimers and tetramers. This characterization of the covalent hSod1 dimer identifies a novel oxidative modification of protein Trp residues and provides clues for studying its occurrence in vivo.

Buffer modulation of menadione-induced oxidative stress in Saccharomyces cerevisiae

The objective of this study was to compare, in vivo, the effects of bicarbonate and phosphate buffers on survival and menadione-induced oxidative stress in yeast cells. The latter were treated with different concentrations of menadione in the presence of these two buffers. At 25 mM concentration of buffers, menadione only slightly reduced yeast surviving; at 50 mM concentration, cell killing by menadione was much more pronounced in bicarbonate than in phosphate buffer. Although the content of protein carbonyl groups did not show development of oxidative stress under menadione-induced stress, inactivation of aconitase and decrease in glutathione level mirrored its induction. However, cellular glutathione and aconitase activity decrease did not correlate with yeast survival. In vitro, aconitase was more quickly inactivated in 50 mM carbonate, than in 50 mM phosphate buffer. The possible involvement of the carbonate radical in these processes is discussed.

EPR spin-trapping of protein radicals to investigate biological oxidative mechanisms

Presently, free radicals and oxidants are considered to mediate from signaling circuits involved in physiology and pathology to cell and tissue injury. The elucidation of these many inter-related processes requires a better understanding of cellular oxidative mechanisms many of which are mediated by protein radicals. Here, we will discuss the potentialities of EPR spin-trapping of protein radicals to unravel oxidative mechanisms. An overview of the methodology and its application to identify protein residues that are the target of specific oxidants, characterize emerging oxidants, and discriminate radical from non radical mechanisms will be presented. The examples are based on work developed in our laboratories but will be discussed in a broad scenario to emphasize that simple experiments can provide relevant insights into the biological reactivity of known and emerging biological oxidants and into signaling mechanisms.

The role of CO2 in metal-catalyzed peroxidations

Transition metals, such as Cu(+2), Mn(+2), and Co(+2), have been seen to catalyze the bicarbonate enhanced oxidation of a variety of substrates by H(2)O(2). In several of these cases it has been demonstrated that CO(2), rather than bicarbonate, is the enhancing species. Mechanisms that are in accord with the data involve a hypervalent state that may be written (MO)(+n), or (MOH)(+n+1), or (M)(+n+2). This metal centered oxidant then oxidizes CO(2) to the carbonate radical; that is then the proximal oxidant of the various substrates. Whether a similar process has in vivo reality remains to be demonstrated.

Responses to Nickel in the Proteome of the Hyperaccumulator Plant Alyssum lesbiacum

A proteomic analysis of the Ni hyperaccumulator plant Alyssum lesbiacum was carried out to identify proteins that may play a role in the exceptional degree of Ni tolerance and accumulation characteristic of this metallophyte. Of the 816 polypeptides detected in root tissue by 2D SDS-PAGE, eleven increased and one decreased in abundance relative to total protein after 6-week-old plants were transferred from a standard nutrient solution containing trace concentrations of Ni to a moderately high Ni treatment (0.3 mM NiSO4) for 48 h. These polypeptides were identified by tandem mass spectrometry and the majority were found to be involved in sulphur metabolism (consistent with a re-allocation of sulphur towards cysteine and glutathione), protection against reactive oxygen species, or heat-shock response. In contrast, very few polypeptides were found to change in abundance in root or shoot tissue after plants were exposed for 28 days to 0.03 mM NiSO4, a concentration representing the optimum for growth of this species but sufficient to lead to hyperaccumulation of Ni in the shoot. Under these conditions, constitutively expressed genes in this highly Ni-tolerant species may be sufficient to allow for effective chelation and sequestration of Ni without the need for additional protein synthesis.