OH radical-induced crosslinks of methionine peptides

Peroxyl radical- and photo-oxidation of glucose 6-phosphate dehydrogenase generates cross-links and functional changes via oxidation of tyrosine and tryptophan residues

Protein oxidation is a frequent event as a result of the high abundance of proteins in biological samples and the multiple processes that generate oxidants. The reactions that occur are complex and poorly understood, but can generate major structural and functional changes on proteins. Current data indicate that pathophysiological processes and multiple human diseases are associated with the accumulation of damaged proteins. In this study we investigated the mechanisms and consequences of exposure of the key metabolic enzyme glucose-6-phosphate dehydrogenase (G6PDH) to peroxyl radicals (ROO^•) and singlet oxygen (¹O₂), with particular emphasis on the role of Trp and Tyr residues in protein cross-linking and fragmentation. Cross-links and high molecular mass aggregates were detected by SDS-PAGE and Western blotting using specific antibodies. Amino acid analysis has provided evidence for Trp and Tyr consumption and formation of oxygenated products (diols, peroxides, N-formylkynurenine, kynurenine) from Trp, and di-tyrosine (from Tyr). Mass spectrometric data obtained after trypsin-digestion in the presence of H₂¹⁶O and H₂¹⁸O, has allowed the mapping of specific cross-linked residues and their locations. These data indicate that specific Tyr-Trp and di-Tyr cross-links are formed from residues that are proximal and surface-accessible, and that the extent of Trp oxidation varies markedly between sites. Limited modification at other residues is also detected. These data indicate that Trp and Tyr residues are readily modified by ROO^• and ¹O₂ with this giving products that impact significantly on protein structure and function. The formation of such cross-links may help rationalize the accumulation of damaged proteins in vivo.

Singlet oxygen interaction with Ca(2+)-ATPase of cardiac sarcoplasmic reticulum

We investigated the role of singlet oxygen (generated from photoactivation of rose bengal) on the calcium transport and Ca(2+)-ATPase activity of cardiac sarcoplasmic reticulum (SR). Isolated cardiac SR exposed to rose bengal (10 nM) irradiated at 560 nm resulted in significant inhibition of Ca2+ uptake (from 2.27 +/- 0.05 to 0.62 +/- 0.05 mumol Ca2+/mg.min [mean +/- SEM], p less than 0.01) and Ca(2+)-ATPase activity (from 2.08 +/- 0.05 to 0.28 +/- 0.04 mumol Pi/min.mg [mean +/- SEM], p less than 0.01). The inhibition of calcium uptake and Ca(2+)-ATPase activity by rose bengal-derived activated oxygen (singlet oxygen) was dependent on the duration of exposure and intensity of light. Singlet oxygen scavengers ascorbic acid and histidine significantly protected SR Ca(2+)-ATPase against rose bengal-derived activated oxygen species, but superoxide dismutase and catalase did not attenuate the inhibition. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of SR exposed to photoactivated rose bengal for up to 14 minutes demonstrated complete loss of the Ca(2+)-ATPase monomer band, which was significantly protected by histidine. The addition of dithiothreitol (5 mM) had a slight protective effect, showing that new disulfide bond formation was not a major cause of aggregation. The results were also confirmed by high-performance liquid chromatography of the SR exposed to irradiated rose bengal. Irradiation of rose bengal also caused an 18% loss of total sulfhydryl groups of SR. On the other hand, superoxide radical (generated from xanthine oxidase action on xanthine) and hydroxyl radical (in the presence of Fe(3+)-EDTA or 0.5 mM H2O2 plus Fe(2+)-EDTA) as well as H2O2 (0.25-12 mM) were without any effect on the 97,000-d Ca(2+)-ATPase band of SR. Generation of radical species (superoxide and hydroxyl radical) from rose bengal was studied by electron paramagnetic resonance spectroscopy using the spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The results showed that irradiation of rose bengal formed a 1:2:2:1 quartet, characteristic of the DMPO-OH adduct, which was scavenged by ethanol but not by superoxide dismutase, catalase, or histidine. No radical species could be detected from irradiated rose bengal or irradiated DMPO under the assay conditions used. Peroxy adducts of DMPO might be produced but would be observed only at very low temperatures. Similarly, we could not detect any measurable.O2- anion from irradiation of rose bengal as indicated by either cytochrome c reduction at 550 nm or nitro blue tetrazolium reduction at 560 nm. These results show that SR is damaged most likely by singlet oxygen derived from rose bengal.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular proteolytic systems may function as secondary antioxidant defenses: an hypothesis

In recent years it has become clear that various free radicals and related oxidants can cause serious damage to intracellular enzymes and other proteins. Several investigators have shown that in extreme cases this can result in an accumulation of oxidatively damaged proteins as useless cellular debris. In other instances, proteins may undergo scission reactions with certain radicals/oxidants, resulting in the direct formation of potentially toxic peptide fragments. Data has also been gathered (recently) demonstrating that various intracellular proteolytic enzymes or systems can recognize, and preferentially degrade, oxidatively damaged proteins (to amino acids). In this hypothesis paper I present evidence to suggest that proteolytic systems (of proteinases, proteases, and peptidases) may function to prevent the formation or accumulation of oxidatively damaged protein aggregates. Proteolytic systems can also preferentially degrade peptide fragments and may thus prevent a wide variety of potentially toxic consequences. I propose that many proteolytic enzymes may be important components of overall antioxidant defenses because they can act to ameliorate the consequences of oxidative damage. A modified terminology is suggested in which the primary antioxidants are such agents as vitamin E, beta-carotene, and uric acid and such enzymes as superoxide dismutase, glutathione peroxidase, and DT-diaphorase. In this classification scheme, proteolytic systems, DNA repair systems, and certain lipolytic enzymes would be considered as secondary antioxidant defenses. As secondary antioxidant defenses, proteolytic systems may be particularly important in times of high oxidative stress, during periods of (primary) antioxidant insufficiency, or with advancing age.