ESR investigations of the reactions of radiation-produced thiyl and DNA peroxyl radicals: formation of sulfoxyl radicals

Vitamin C: antioxidant or pro-oxidant in vivo?

Ascorbic acid has a multiplicity of antioxidant properties, but it can exert pro-oxidant effects in vitro, usually by interaction with transition metal ions. It is as yet uncertain that these pro-oxidant effects have any biological relevance: some of the available data are summarized.

Reactions of sulphonyl peroxyl radicals with DNA and its components: hydrogen abstraction from the sugar backbone versus addition to pyrimidine double bonds

The reactions of sulphonyl peroxyl radicals with DNA and its components at low temperatures are investigated through use of electron spin resonance spectroscopy. Radiolysis of cysteine sulphinic acid solution in oxygenated aqueous 5 mol dm-3 LiCl glass has been employed to generate sulphonyl peroxyl radicals, CysSO2OO.. The mechanism of production is a two-step process involving the oxidation of the sulphinate (CysSO2-) to the sulphonyl radical (CysSO2.) by radiolytically produced Cl2- followed by oxygen addition to CysSO2.. Sulphonyl peroxyls are shown to be reactive toward DNA and nucleotides in low temperature glasses, giving rise to carbon-based peroxyl radicals. However, of the DNA bases only thymine was found to react readily. The thymine methyl group contains a relatively weak C-H bond which makes it easily abstractable. DNA nucleosides and nucleotides are likely attacked by CysSO2OO. by abstraction from the sugar C-H bonds. The C(1) position of deoxyribose ring of model nucleosides is suggested to be a preferred site for attack at low temperatures. The reaction with double-stranded DNA at higher temperatures may occur at the more accessible C(4) position. CysSO2OO. is also shown to react with a number of methyl substituted uracils, producing the highly persistent 5-oxouracil-6-yl radicals. The reaction mechanism proposed involves the addition of CysSO2OO. to pyrimidine 5,6-double bonds followed by elimination of cysteine sulphonic acid.

The characterization of antioxidants

The role of antioxidants in nutrition is an area of increasing interest. Antioxidants are used (1) to prolong the shelf life and maintain the nutritional quality of lipid-containing foods, and (2) to modulate the consequences of oxidative damage in the human body. This review discusses what an antioxidant is and how the properties of antioxidants may be characterized.

Influence of oxygen on the repair of direct radiation damage to DNA by thiols in model systems

Here the reactions of thiols with DNA primary radical intermediates formed after gamma-irradiation of frozen (77 K) anoxic and oxic solutions of DNA/thiol mixtures are investigated. Through analysis of the experimental composite spectra at each annealing temperature, the relative concentrations of individual radicals present are estimated and reaction sequences inferred. In all samples the primary DNA radical anions and cations (DNA.+ and DNA.-) are suggested to be the predominant radicals at low temperatures. In anoxic samples, TH. (5,6-dihydrothym-5-yl radical), .RSSR.- and, in glutathione samples, .GSH [gamma-glu-NHC(CH2SH)CO-gly] radicals are observed as the temperature is increased. The presence of oxygen efficiently suppresses the formation of RSSR.- and .GSH; instead, in oxic samples, O2.-, DNAOO., RSOO. and RSO. are observed at higher temperatures. The photolytic conversion of RSOO. to RSO2. is used to verify the presence of RSOO. in gamma-irradiated DNA/thiol systems and confirm that the computer analysis employed yields reasonable estimates of the relative DNAOO. and RSOO. concentrations. From the relative concentrations of radicals present, it is clear that the radicals observed at higher temperatures originate from the radical reactions of the primary DNA.+ and DNA.- radicals. Based on the reaction sequences inferred and previous work with thiols alone, it is concluded that TH., DNAOO. and RSOO. (in part) originate largely with DNA.-, whereas RSSR.-, .GSH and RSOO. (in part) originate largely with DNA.+. The possible roles of DNAOO., RSOO., RSO., RSO2. and .OOGSH in the chemical oxygen enhancement effect at biologically realistic temperatures are discussed.

Study of a Fenton type reaction: effect of captopril and chelating reagents

The purpose of this study was to determine if captopril, an angiotensin-converting enzyme inhibitor, could interact with iron ions and so modify a Fenton type reaction. Results indicate that different degrees of thiobarbituric acid-reactive substance from deoxyribose are obtained in an ascorbate-driven Fenton system depending on the order of addition of captopril and iron to the incubation medium. Similar results were obtained with the chelating reagents ethylenediaminetetraacetic acid and diethylenetriaminepentaacetic acid, indicating that the buffer solution plays a relevant role when a particular iron complex is formed with a chelating agent. These metal complexes produce oxidizing species in a Fenton type system whose nature is discussed.

Oxidation of polyunsaturated fatty acids and lipids through thiyl and sulfonyl radicals: reaction kinetics, and influence of oxygen and structure of thiyl radicals

Thiyl free radicals have been shown to react with polyunsaturated fatty acids via abstraction of bisallylic hydrogen, forming pentadienyl radicals, and via addition to the double bonds. In the absence of oxygen, the latter pathway leads to regeneration of thiyl radicals through beta-elimination or "repair" of the adduct radicals by thiols. In the presence of oxygen, fixation of thiyl-induced damage occurs through reaction of O2 with the pentadienyl radical (yielding conjugated dienyl peroxyl radicals) and also with the thiyl-to-double bond adduct radical. A quantitative reaction scheme evaluated from these data considers abstraction, addition, rearrangement, and repair reactions, and the evaluation of rate constants for the individual steps. Absolute rate constants have been measured, in particular, for reactions of thiyl free radicals from glutathione, cysteine, homocysteine, N-acetylcysteine, cysteine ethyl ester, penicillamine, captopril, mercaptoethanol, and dithiothreitol with polyunsaturated fatty acids (PUFAs) ranging from 18:2 to 22:6, and the lipids trilinolein and trilinolenin. The rate constants for hydrogen abstraction were found to be typically of the order of 10(7) mol-1 dm3 s-1 and to increase with increasing lipophilicity of the attacking thiyl radical. Thioperoxyl radicals, RSOO., were found to be rather unreactive toward PUFAs, in contrast to the isomer sulfonyl radicals, RSO2., which not only abstract hydrogen from the bisallylic methylene groups of the PUFAs (although only at relatively small yield) but also readily add to the PUFA double bonds (major pathway). Specific information was obtained on the optical properties of the thiyl radical derived from the ACE inhibitor captopril, CpS. (lambda max = 340 nm, epsilon = 460 +/- 50 mol-1 dm3 cm-1), and its conjugate disulfide radical anion (CpS:.SCp) (lambda max = 420 nm).

Drug antioxidant effects. A basis for drug selection?

A free radical is any species capable of independent existence that contains one or more unpaired electrons. Free radical reactions have been implicated in the pathology of more than 50 human diseases. Radicals and other reactive oxygen species are formed constantly in the human body, both by deliberate synthesis (e.g. by activated phagocytes) and by chemical side-reactions. They are removed by enzymic and nonenzymic antioxidant defence systems. Oxidative stress, occurring when antioxidant defences are inadequate, can damage lipids, proteins, carbohydrates and DNA. A few clinical conditions are caused by oxidative stress, but more often the stress results from the disease. Sometimes it then makes a significant contribution to the disease pathology, and sometimes it does not. Several antioxidants are available for therapeutic use. They include molecules naturally present in the body [superoxide dismutase (SOD), alpha-tocopherol, glutathione and its precursors, ascorbic acid, adenosine, lactoferrin and carotenoids] as well as synthetic antioxidants [such as thiols, ebselen (PZ51), xanthine oxidase inhibitors, inhibitors of phagocyte function, iron ion chelators and probucol]. The therapeutic efficacy of SOD, alpha-tocopherol and ascorbic acid in the treatment of human disease is generally unimpressive to date although dietary deficiencies of the last two molecules should certainly be avoided. Xanthine oxidase inhibitors may be of limited relevance as antioxidants for human use. Exciting preliminary results with probucol (antiatherosclerosis), ebselen (anti-inflammatory), and iron ion chelators (in thalassaemia, leukaemia, malaria, stroke, traumatic brain injury and haemorrhagic shock) need to be confirmed by controlled clinical trials. Clinical testing of N-acetylcysteine in HIV-1-positive subjects may also be merited. A few drugs already in clinical use may have some antioxidant properties, but this ability is not widespread and drug-derived radicals may occasionally cause significant damage.

Scavenging of hypochlorous acid and of myoglobin-derived oxidants by the cardioprotective agent mercaptopropionylglycine

Mercaptopropionylglycine (MPG) has a marked cardioprotective action in several model systems of ischaemia-reoxygenation injury. Suggested mechanisms of action include scavenging of hydroxyl radical and of hypochlorous acid and reacting with an oxidant formed by reaction of myoglobin with H2O2, thereby slowing lipid peroxidation stimulated by myoglobin-H2O2 mixtures. This oxidant seems not to be singlet O2 or hydroxyl radical. Studies in vitro show that scavenging of hypochlorous acid is a feasible mechanism of cardioprotective action for MPG in vivo in ischaemia/reperfusion systems to which neutrophil-mediated injury contributes. However, the poor ability of MPG to inhibit lipid peroxidation stimulated by myoglobin/H2O2 mixtures and its ability to increase iron ion release from myoglobin in the presence of a large excess of H2O2 suggests that MPG is unlikely to protect the myocardium by interfering with oxidants produced by the myoglobin-H2O2 system.

The formation and structure of the sulfoxyl radicals RSO(.), RSOO(.), RSO2(.), and RSO2OO(.) from the reaction of cysteine, glutathione and penicillamine thiyl radicals with molecular oxygen

This work reports an electron spin resonance study of the reactions of cysteine, glutathione and penicillamine thiyl radicals with molecular oxygen in frozen aqueous solutions at low temperatures. For all three thiols, the thiyl radical, RS., is found to react with oxygen to form the thiol peroxyl radical, RSOO(.). On the absorption of visible light, RSOO(.) photoisomerizes to the sulfonyl radical, RSO2(.), which subsequently reacts with molecular oxygen to form RSO2OO(.), the sulfonyl peroxyl radical. The identities of the sulfonyl and sulfonyl peroxyl radicals were confirmed by their production by a different route, from sulfinic acid. Sulfinyl radicals, RSO(.), are found as the final radical species in the reactions of thiyl radicals and oxygen. Parallel 17O hyperfine couplings (A parallel) are reported for each sulfoxyl radical and a correlation between the spin density on oxygen and the reactivity of the radical is suggested. As a result of this correlation sulfonyl peroxyl radicals are predicted to be far more reactive than thiol peroxyl radicals. We also report molecular orbital calculations on the nature of the spin density distribution and the molecular geometry of the model radicals CH3SO2(.) and CH3SO2OO(.).

Apparent inactivation of alpha 1-antiproteinase by sulphur-containing radicals derived from penicillamine

alpha 1-Antiproteinase is the major inhibitor of proteolytic enzymes, such as elastase, in human plasma. Its elastase-inhibitory capacity can be inactivated by exposure to hydroxyl radicals (.OH) generated either by pulse radiolysis or by an Fe3+-EDTA/H2O2/ascorbic acid system. Inactivation of alpha 1-antiproteinase by radiolytically-generated .OH under anoxic conditions was decreased by adding a range of anti-inflammatory drugs to the reaction mixtures, including the thiol compound penicillamine. However, under conditions favouring formation of oxysulphur radicals, protection by thiols such as penicillamine was much decreased. It is proposed that sulphur-containing radicals resulting from attack of biologically-produced oxidants upon penicillamine in the presence of O2 can themselves inactivate alpha 1-antiproteinase, and that such radicals might contribute to the side-effects produced by penicillamine or gold thiol therapy in rheumatoid arthritis.