Radiation chemistry applied to drug design

Peroxiredoxin 1 - Multifunctional antioxidant enzyme, protects from oxidative damages and increases the survival rate of mice exposed to total body irradiation

PURPOSE

Peroxiredoxin 1 (Prx1) is known to be a multifunctional antioxidant enzyme playing an essential role in protecting the organism against oxidative stress. We hypothesized that administration of exogenous recombinant Prx1 may provide additional protection of the mammalian organism during the development of acute oxidative stress induced by ionizing radiation. Hence, the aim of the present work was to study the radioprotective properties of exogenous Prx1.

MATERIALS AND METHODS

Recombinant Prx1 was obtained by genetic engineering. The properties of Prx1 were studied using physicochemical methods. An immunoblotting and ELISA were used for the determination of the level of endogenous and exogenous Prx1 in animal blood. The survival rate of irradiated animals was assessed for 30 days with various modes of administration (intraperitoneal, intramuscular, intravenously) Prx1. Using a hematological analyzer and microscopic analysis, the changes in the level of leukocytes and platelets were assessed in animals that received and did not receive an intravenous injection of Prx1 before irradiation. Genoprotective properties of Prx1 were confirmed by micronucleus test. Real-time PCR was used to investigate the effect of Prx1 on the expression of genes involved in response to oxidative stress.

RESULTS

Recombinant Prx1 was shown to significantly reduce oxidative damage to biological macromolecules. Prx1 is an effective radioprotector which decreases the severity of radiation-induced leuko- and thrombocytopenia, plus protects bone marrow cells from damage. The half-life of Prx1 in the bloodstream is more than 1 h, while within 1 h there is a loss of the antioxidant activity of Prx1 by almost 50%, which limits its use long (2 h) before irradiation. The introduction of Prx1 after irradiation has no significant radiomitigating effect. The most effective way of using Prx1 is intravenous administration shortly (15-30 min) before exposure to ionizing radiation, with a dose reduction factor of 1.3. Under the action of ionizing radiation a dose-dependent appearance of endogenous Prx1 in the bloodstream was also observed. The appearance of Prx1 in the bloodstream alters the expression of stress response genes (especial antioxidant response and DNA repair) in the cells of red bone marrow, promoting the activation of repair processes.

CONCLUSION

The recombinant Prx1 can be considered as an effective radioprotector for minimizing the risks of injury of animal's body by ionizing radiation.

Enzyme-Activated Generation of Reactive Oxygen Species from Heterocyclic N-Oxides under Aerobic and Anaerobic Conditions and Its Relevance to Hypoxia-Selective Prodrugs

Enzymatic one-electron reduction of heterocyclic N-oxides can lead to the intracellular generation of reactive oxygen species via several different chemical pathways. These reactions may be relevant to hypoxia-selective anticancer drugs, antimicrobial agents, and unwanted toxicity of heterocylic nitrogen compounds.

Persulfides: current knowledge and challenges in chemistry and chemical biology

Recent studies conducted in hydrogen sulfide (H2S) signaling have revealed potential importance of persulfides (RSSH) in redox biology. The inherent instability of RSSH makes these species difficult to study and sometimes controversial results are reported. In this review article we summarize known knowledge about both small molecule persulfides and protein persulfides. Their fundamental physical and chemical properties such as preparation/formation and reactivity are discussed. The biological implications of persulfides and their detection methods are also discussed.

Trisulfides in proteins

Trisulfides and other oligosulfides are widely distributed in the biological world. In plants, for example, garlic, trisulfides are associated with potentially beneficial properties. However, an extra neutral sulfur atom covalently bound between the two sulfur atoms of a pair of cysteines is not a common post-translational modification, and the number of proteins in which a trisulfide has been unambiguously identified is small. Nevertheless, we believe that its prevalence may be underestimated, particularly with the increasing evidence for significant pools of sulfides in living tissues and their possible roles in cellular metabolism. This review focuses on examples of proteins that are known to contain a trisulfide bridge, and gives an overview of the chemistry of trisulfide formation, and the methods by which it is detected in proteins.

Radiation chemistry comes before radiation biology

PURPOSE

This article seeks to illustrate some contributions of radiation chemistry to radiobiology and related science, and to draw attention to examples where radiation chemistry is central to our knowledge of specific aspects. Radiation chemistry is a mature branch of radiation science which is continually evolving and finding wider applications. This is particularly apparent in the study of the roles of free radicals in biology generally, and radiation biology specifically. The chemical viewpoint helps unite the spatial and temporal insight coming from radiation physics with the diversity of biological responses. While historically, the main application of radiation chemistry of relevance to radiation biology has been investigations of the free-radical processes leading to radiation-induced DNA damage and its chemical characterization, two features of radiation chemistry point to its wider importance. First, its emphasis on quantification and characterization at the molecular level helps provide links between DNA damage, biochemical repair processes, and mutagenicity and radiosensitivity. Second, its central pillar of chemical kinetics aids understanding of the roles of 'reactive oxygen species' in cell signalling and diverse biological effects more generally, and application of radiation chemistry in the development of drugs to enhance radiotherapy and as hypoxia-specific cytotoxins or diagnostic agents. The illustrations of the broader applications of radiation chemistry in this article focus on their relevance to radiation biology and demonstrate the importance of synergy in the radiation sciences.

CONCLUSIONS

The past contributions of radiation chemistry to radiation biology are evident, but there remains considerable potential to help advance future biological understanding using the knowledge and techniques of radiation chemistry.

Enzyme-Activated, Hypoxia-Selective DNA Damage by 3-Amino-2-quinoxalinecarbonitrile 1,4-Di-N-oxide

The compound 3-amino-2-quinoxalinecarbonitrile 1,4-dioxide (4) displays potent hypoxia-selective cytotoxicity in cell culture. This compound is structurally similar to the known hypoxia-selective DNA-damaging agent tirapazamine (1, TPZ), but the ability of 4 to cause DNA damage under low-oxygen conditions has not previously been characterized. The results presented here provide the first evidence that 4 causes reductively activated DNA damage under hypoxic conditions. The findings indicate that one-electron reduction of 4 by NADPH:cytochrome P450 reductase yields an oxygen-sensitive intermediate (5). This activated intermediate is rapidly destroyed by reaction with O2 under aerobic conditions, but goes forward to cause DNA damage under low-oxygen conditions. Analysis of the DNA damage indicates that reductive activation of 4 leads to production of a highly reactive, freely diffusible oxidizing radical that causes sequence-independent cleavage of the deoxyribose backbone and oxidative damage to the heterocyclic bases in duplex DNA. On the basis of the experiments reported here, the chemical nature of the DNA damage caused by redox-activated 4 is analogous to that reported previously for TPZ.

The reaction of superoxide radicals with S-nitrosoglutathione and the products of its reductive heterolysis

Generation of superoxide radicals (0.01-0.1 microm s(-1)) by radiolysis of aqueous solutions containing S-nitrosoglutathione (45-160 microm, pH 3.8-7.3) resulted in loss of this solute at rates varying with solute concentration, radical generation rate, and pH. The results were quantitatively consistent with the loss being attributed to competition between reaction of superoxide with S-nitrosoglutathione (rate constant 300 +/- 100 m(-1) s(-1)) and the pH-dependent disproportionation of superoxide/hydroperoxyl. This rate constant is much lower than previous estimates and seven orders of magnitude lower than the rate constants between superoxide and superoxide dismutase or superoxide and nitric oxide. This indicates that interaction between superoxide and S-nitrosoglutathione is unlikely to be biologically important, contrary to previous suggestions that reaction could serve to prevent the rapid reaction between superoxide and nitric oxide. Reductive homolysis of S-nitrosoglutathione by the carbon dioxide radical anion, a model for biological reductants such as disulfide radical anions, occurred with a rate constant of 7.4 x 10(8) m(-1) s(-1) and produced nitric oxide stoichiometrically. Thiyl radicals were not produced, indicating the alternative homolysis route to generate nitroxyl did not occur.

A labile sulfur in trisulfide affects cytochrome P-450 dependent lipid peroxidation in rat liver microsomes

The effects of trisulfide derivatives were studied on cytochrome P-450-dependent lipid peroxidation using rat liver microsomal systems. Cytochrome P-450-dependent lipid peroxidation was induced by carbon tetrachloride or tert-butylhydroperoxide and was evident by an increase in thiobarbituric acid-reactive substances (TBA-RS) and oxygen consumption. In these cytochrome P-450-dependent lipid peroxidation systems, pretreatment of microsome with trisulfide derivatives (cystine trisulfide and thiocyclam) significantly inhibited TBA-RS formation and oxygen consumption compared with disulfide or thiol analogs (cystine, nereistoxin, or cysteine). The labile sulfur contained in trisulfide disappeared during incubation with liver microsomes. In the CCl4-induced lipid peroxidation system, the cytochrome P-450 level and NAD(P)H-cytochrome P-450 reductase activity were significantly decreased by the addition of trisulfide derivatives. Therefore, in cytochrome P-450-dependent lipid peroxidation system, the potential effects of trisulfide appear to be mediated via enzyme inhibition. These results suggest that pretreatment of the trisulfide derivatives may affect the toxic function of exogenous xenobiotics or drugs, which are reduced by the liver enzyme cytochrome P-450 to radical species.

Comparative mechanisms and rates of free radical scavenging by carotenoid antioxidants

The comparative mechanisms and relative rates of nitrogen dioxide (NO2.), thiyl (RS.) and sulphonyl (RSO2.) radical scavenging by the carotenoid antioxidants lycopene, lutein, zeaxanthin, astaxanthin and canthaxanthin have been determined by pulse radiolysis. All the carotenoids under study react with the NO2. radical via electron transfer to generate the carotenoid radical cation (Car.+). In marked contrast the glutathione and 2-mercaptoethanol thiyl radicals react via a radical addition process to generate carotenoid-thiyl radical adducts [RS-Car].. The RSO2. radical undergoes both radical addition, [RSO2-Car]. and electron abstraction, Car.+. Both carotenoid adduct radicals and radical cations decay bimolecularly. Absolute rate constants for radical scavenging were in the order of approximately 10(7)-10(9) M(-1) s(-1) and follow the sequence HO(CH2)2S. > RSO2. > GS. > NO2.. Although there were some discernible trends in carotenoid reactivity for individual radicals, rate constants varied by no greater than a factor of 2.5. The mechanism and rate of scavenging is strongly dependent on the nature of the oxidising radical species but much less dependent on the carotenoid structure.

Oxidative denitrification of N omega-hydroxy-L-arginine by the superoxide radical anion

The superoxide radical anion (O2-.) produced during the catalytic activity of nitric oxide synthase (NOS) and cytochrome P-450 has been implicated in the oxidative denitrification of hydroxyguanidines ( > C = NOH). The reactivity of the radiolytically generated O2-. radical with N omega-hydroxy-L-arginine (NHA) is pH dependent and appears to parallel the prototropic equilibrium of the hydroxyguanidino group ( > C = NOH reversible > C = NO(-)+H+; pK = 8). The N omega-hydroxyguanidino group is more reactive towards O2-. when deprotonated but exhibits negligible reactivity when protonated. Based on a model, the rate constant for the reaction of the O2-. with NHA was estimated as kappa (O2-.+ > C = NO-) approximately 200-500 M-1.s-1, which is probably too low to compete with O2-. reactions with NO- or superoxide dismutase, which occur many orders of magnitude faster. The oxidative elimination of NO from NHA by O2-. was not accompanied by the formation of L-citrulline. Since only 21% of NHA will exist in the deprotonated > C = NO- form at physiological pH, it is unlikely that oxidative denitrification of NHA by cytochrome P-450 or NOS-derived O2-. radicals will prove a major free-radical pathway to NO. and L-citrulline.

Scavenging of nitrogen dioxide, thiyl, and sulfonyl free radicals by the nutritional antioxidant beta-carotene

Mechanisms of free radical scavenging by the nutritional antioxidant beta-carotene have been investigated by pulse radiolysis. Free radicals, which can initiate the chain of lipid peroxidation, including nitrogen dioxide (NO2.), thiyl (RS.), and sulfonyl (RSO2.) radicals, are rapidly scavenged by beta-carotene. Absolute rate constant k[NO2. + beta-carotene] = (1.1 +/- 0.1) x 10(8) m-1 s-1 and for the glutathione thiyl radical k[GS. + beta-carotene] = (2.2 +/- 0.1) x 10(8) m-1 s-1 have been determined. The mechanisms however are mutually exclusive, the former involving electron transfer to generate the radical-cation [ beta-carotene]+. and the latter by radical-addition to generate an adduct-radical [RS... beta-carotene].. Rate constants for thiyl radical-addition reactions vary from 10(6) to 10(9) m-1 s-1 and correlate with the lipophilicity of the thiyl radical under study. Sulfonyl radicals undergo both electron abstraction, [ beta-carotene]+. and radical-addition, [RSO2... beta-carotene]. in an approximate 3:1 ratio. The beta-carotene radical-cation and adduct-radicals are highly resonance stabilized and undergo slow bimolecular decay to non-radical products. These carotenoid-derived radicals react differently with oxygen, a factor which is expected to influence the antioxidant activity of beta-carotene within tissues of varying oxygen tension in vivo.

Free radicals in biology: oxidative stress and the effects of ionizing radiation

The most important electron acceptor in the biosphere is molecular oxygen which, by virtue of its bi-radical nature, readily accepts unpaired electrons to give rise to a series of partially reduced species collectively known as reduced (or 'reactive') oxygen species (ROS). These include superoxide (O.2-), hydrogen peroxide (H2O2), hydroxyl radical (HO.) and peroxyl (ROO.) and alkoxyl (RO.) radicals which may be involved in the initiation and propagation of free radical chain reactions and which are potentially highly damaging to cells. Mechanisms have evolved to restrict and control such processes, partly by compartmentation, and partly by antioxidant defences such as chain-breaking antioxidant compounds capable forming stable free radicals (e.g. ascorbate, alpha-tocopherol) and the evolution of enzyme systems (e.g. superoxide dismutase, catalase, peroxidases) that diminish the intracellular concentration of the ROS. Although some ROS perform useful functions, the production of ROS exceeding the ability of the organism to mount an antioxidant defence results in oxidative stress and the ensuing tissue damage may be involved in certain disease processes. Evidence that ROS are involved in primary pathological mechanisms is a feature mainly of extraneous physical or chemical perturbations of which radiation is perhaps the major contributor. One of the important radiation-induced free-radical species is the hydroxyl radical which indiscriminately attacks neighbouring molecules often at near diffusion-controlled rates. Hydroxyl radicals are generated by ionizing radiation either directly by oxidation of water, or indirectly by the formation of secondary partially ROS. These may be subsequently converted to hydroxyl radicals by further reduction ('activation') by metabolic processes in the cell. Secondary radiation injury is therefore influenced by the cellular antioxidant status and the amount and availability of activating mechanisms. The biological response to radiation may be modulated by alterations in factors affecting these secondary mechanisms of cellular injury.