Spin trapping of glutathiyl and protein radicals produced from nitric oxide-derived oxidants

Aromatic thiol-mediated cleavage of N-O bonds enables chemical ubiquitylation of folded proteins

Access to protein substrates homogenously modified by ubiquitin (Ub) is critical for biophysical and biochemical investigations aimed at deconvoluting the myriad biological roles for Ub. Current chemical strategies for protein ubiquitylation, however, employ temporary ligation auxiliaries that are removed under harsh denaturing conditions and have limited applicability. We report an unprecedented aromatic thiol-mediated N–O bond cleavage and its application towards native chemical ubiquitylation with the ligation auxiliary 2-aminooxyethanethiol. Our interrogation of the reaction mechanism suggests a disulfide radical anion as the active species capable of cleaving the N–O bond. The successful semisynthesis of full-length histone H2B modified by the small ubiquitin-like modifier-3 (SUMO-3) protein further demonstrates the generalizability and compatibility of our strategy with folded proteins.

Chemical approaches to site-specifically ubiquitylate a target protein allow investigation of the biochemical effects of this modification, but they often destabilize the protein. Here, the authors report on a synthetic conjugation strategy that leads to protein ubiquitylation in non-denaturing conditions.

The antioxidant mechanism of nitroxide TEMPO: scavenging with glutathionyl radicals

A rhodamine-nitroxide probe (R-NO˙) was introduced to probe glutathionyl radicals (GS˙) with high sensitivity and selectivity.

Redox control of enzymatic functions: The electronics of life's circuitry

The field of redox biology has changed tremendously over the past 20 years. Formerly regarded as bi-products of the aerobic metabolism exclusively involved in tissue damage, reactive oxygen species (ROS) are now recognized as active participants of cell signaling events in health and in disease. In this sense, ROS and the more recently defined reactive nitrogen species (RNS) are, just like hormones and second messengers, acting as fundamental orchestrators of cell signaling pathways. The chemical modification of enzymes by ROS and RNS (that result in functional enzymatic alterations) accounts for a considerable fraction of the transient and persistent perturbations imposed by variations in oxidant levels. Upregulation of ROS and RNS in response to stress is a common cellular response that foments adaptation to a variety of physiologic alterations (hypoxia, hyperoxia, starvation, and cytokine production). Frequently, these are beneficial and increase the organisms' resistance against subsequent acute stress (preconditioning). Differently, the sustained ROS/RNS-dependent rerouting of signaling produces irreversible alterations in cellular functioning, often leading to pathogenic events. Thus, the duration and reversibility of protein oxidations define whether complex organisms remain "electronically" healthy. Among the 20 essential amino acids, four are particularly susceptible to oxidation: cysteine, methionine, tyrosine, and tryptophan. Here, we will critically review the mechanisms, implications, and repair systems involved in the redox modifications of these residues in proteins while analyzing well-characterized prototypic examples. Occasionally, we will discuss potential consequences of amino acid oxidation and speculate on the biologic necessity for such events in the context of adaptative redox signaling. © 2014 IUBMB Life, 66(3):167-181, 2014.

Oxidation of thiamine on reaction with nitrogen dioxide generated by ferric myoglobin and hemoglobin in the presence of nitrite and hydrogen peroxide

It is shown that nitrogen dioxide oxidizes thiamine to thiamine disulfide, thiochrome, and oxodihydrothiochrome (ODTch). The latter is formed during oxidation of thiochrome by nitrogen dioxide. Nitrogen dioxide was produced by incubation of nitrite with horse ferric myoglobin and human hemoglobin in the presence of hydrogen peroxide. After addition of tyrosine or phenol to aqueous solutions containing oxoferryl forms of the hemoproteins, thiamine, and nitrite, the yield of thiochrome greatly increased, whereas the yield of ODTch decreased. In the presence of high concentrations of tyrosine or phenol compounds ODTch was not formed at all. The neutral form of thiamine with the closed thiazole cycle and minor tricyclic form of thiamine do not enter the heme pocket of the protein and do not interact with the oxoferryl heme complex Fe(IV=O) or porphyrin radical. The tricyclic form of thiamine is oxidized to thiochrome by tyrosyl radicals located on the surface of the hemoprotein. The thiol form of thiamine is oxidized to thiamine disulfide by both hemoprotein tyrosyl radicals and oxoferryl heme complexes. Nitrite and also tyrosine, tyramine, and phenol readily penetrate into the heme pocket of the protein and reduce the oxyferryl complex to ferric cation. These reactions yield nitrogen dioxide as well as tyrosyl and phenoxyl radicals of tyrosine molecules and phenol compounds, respectively. Tyrosyl and phenoxyl radicals of low molecular weight compounds oxidize thiamine only to thiochrome and thiamine disulfide. The effect of oxoferryl forms of myoglobin and hemoglobin, nitrogen dioxide, and phenol on thiamine oxidative transformation as well as antioxidant properties of the hydrophobic thiamine metabolites thiochrome and ODTch are discussed.

Immuno-spin trapping of a post-translational carboxypeptidase B1 radical formed by a dual role of xanthine oxidase and endothelial nitric oxide synthase in acute septic mice

Post-translational modification of proteins due to exposure to radicals and other reactive species are markers of metabolic and inflammatory oxidative stress such as sepsis. This study uses the nitrone spin-trap DMPO and a combination of immuno-spin trapping and mass spectrometry to identify in vivo products of radical reactions in mice. We report the detection of dose-dependent production of DMPO-carboxypeptidase B1 (CPB1) adducts in the spleens of mice treated with lipopolysaccharide (LPS). Additionally, we report significant detection of DMPO-CPB1 adducts in mice experiencing normal physiological conditions. Treatments with inhibitors and experiments with knock-out mice indicate that xanthine oxidase and endothelial nitric oxide synthase are important sources of the reactive species that lead to CPB1 adduct formation. We also report a significant loss of CPB1 activity following LPS challenge in conjunction with an increase in CPB1 protein accumulation. This suggests the presence of a possible mechanism for CPB1 activity loss with compensatory protein production.

Nitric oxide induced oxidative changes in erythrocyte membrane components

The effects of NO in its environment may vary considerably depending on various factors. This study shows oxidative mechanism of cellular membrane alterations, which is not associated with triggering of ONOOH generation but is induced by pure NO. Our investigation examined the influence of low concentration of NO (0.1; 0.2 mmol/l) on the qualitative changes of structure and dynamics of erythrocyte membrane. NO causes a statistically significant increase in membrane fluidity on different depths of lipid bilayer that is correlated with increase of lipids peroxidation. Statistically significant changes in the conformational state of cytoskeleton proteins were also detected. NO can be considered as a molecule responsible for determining rheological properties of erythrocytes membrane. Therefore, we propose that NO acts as pro-oxidant molecule at concentrations for which membrane appeared to be the first target before it entered the cytosol.

A Role for Peroxymonocarbonate in the Stimulation of Biothiol Peroxidation by the Bicarbonate/Carbon Dioxide Pair

Peroxymonocarbonate (HCO4-) is an oxidant whose existence in equilibrium with hydrogen peroxide and bicarbonate has been known since the 1980s. More recently, peroxymonocarbonate has been proposed to mediate oxidative processes stimulated by the bicarbonate/carbon dioxide pair. To better understand this emerging biological oxidant, we re-examined the kinetics of its formation from hydrogen peroxide and bicarbonate/carbon dioxide by 13C NMR. Also, we studied its role in the accelerating effects of bicarbonate on biothiol (GSH and BSA-cysSH) peroxidation by kinetics and product analysis. The rate constants for peroxymonocarbonate formation and decay were estimated and Keq values determined (pH 7.2, at 25 and 37 degrees C; in the absence and presence of BSA and liposomes of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine phosphatidylcholine). Noteworthy is the fact the rate constant for peroxymonocarbonate formation estimated here (k1 approximately 10-2 M-1 s-1) was more than 1 order of magnitude higher than a previously reported value. Also, peroxymonocarbonate equilibrium was shown to be affected by BSA, liposomes, and a carbonic anhydrase mimetic. The Keq values determined in the absence and presence of BSA (0.35 and 0.48 M-1, respectively, at 37 degrees C) were employed to analyze the kinetics of BSA-cysSH and GSH peroxidation in the presence of bicarbonate (2-25 mM). A good fit of experimental data with simulations indicated that peroxymonocarbonate is the main species responsible for biothiol peroxidation in the presence of bicarbonate. The results indicate that peroxymonocarbonate is a feasible biological oxidant, in addition to supporting emerging data that the main physiological buffer is redox active.

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.

Glutathione and the redox control system trypanothione/trypanothione reductase are involved in the protection of Leishmania spp. against nitrosothiol-induced cytotoxicity

Glutathione is the major intracellular antioxidant thiol protecting mammalian cells against oxidative stress induced by oxygen- and nitrogen-derived reactive species. In trypanosomes and leishmanias, trypanothione plays a central role in parasite protection against mammalian host defence systems by recycling trypanothione disulphide by the enzyme trypanothione reductase. Although Kinetoplastida parasites lack glutathione reductase, they maintain significant levels of glutathione. The aim of this study was to use Leishmania donovani trypanothione reductase gene mutant clones and different Leishmania species to examine the role of these two individual thiol systems in the protection mechanism against S-nitroso-N-acetyl-D,L-penicillamine (SNAP), a nitrogen-derived reactive species donor. We found that the resistance to SNAP of different species of Leishmania was inversely correlated with their glutathione concentration but not with their total low-molecular weight thiol content (about 0.18 nmol/10(7) parasites, regardless Leishmania species). The glutathione concentration in L. amazonensis, L. donovani, L. major, and L. braziliensis were 0.12, 0.10, 0.08, and 0.04 nmol/10(7) parasites, respectively. L. amazonensis, that have a higher level of glutathione, were less susceptible to SNAP (30 and 100 microM). The IC50 values of SNAP determined to L. amazonensis, L. donovani, L. major, and L. braziliensis were 207.8, 188.5, 160.9, and 83 microM, respectively. We also observed that L. donovani mutants carrying only one trypanothione reductase allele had a decreased capacity to survive (approximately 40%) in the presence of SNAP (30-150 microM). In conclusion, the present data suggest that both antioxidant systems, glutathione and trypanothione/trypanothione reductase, participate in protection of Leishmania against the toxic effect of nitrogen-derived reactive species.

New approach for the detection of peptide- and protein-based radicals using a pre-fluorescent probe

A novel application for pre-fluorescent probes in the detection of peptide- and protein-based radicals is proposed. Pre-fluorescent probes combine a fluorescent moiety labeled with a paramagnetic nitroxide that acts as a fluorescence quencher. Trapping of a radical by the nitroxide group restores the fluorescence properties. The increase in fluorescence intensity with time reflects the formation and quenching of free radicals and can be employed for the quantitative evaluation of yields and kinetics. In this test system, the pre-fluorescent probe 4-(9-acridinecarbonate)-2,2,6,6-tetramethylpiperidinyl-1-oxyl radical (Ac-Tempo), in which an acridine moiety was labeled with 2,2,6,6-tetramethylpiperidinyl-1-oxy (Tempo), was employed to probe peptide- and protein-based radicals. Peptide-based radicals were generated through the reaction between horseradish peroxidase (HRP)/H(2)O(2) and a derivative of tyrosine. Protein-based radicals were generated through the reaction between myoglobin (Mb) and H(2)O(2). In both cases the Ac-Tempo was found, using a combination of high-performance liquid chromatography (HPLC) and mass spectrometry, to be converted into fluorescent acridine (Ac)-piperidine (4-(9-acridinecarbonate)-2,2,6,6-tetramethylpiperidine).