Reduction of sperm whale ferrylmyoglobin by endogenous reducing agents: potential reducible loci of ferrylmyoglobin

Understanding mechanisms of antioxidant action in health and disease

Several different reactive oxygen species (ROS) are generated in vivo. They have roles in the development of certain human diseases whilst also performing physiological functions. ROS are counterbalanced by an antioxidant defence network, which functions to modulate ROS levels to allow their physiological roles whilst minimizing the oxidative damage they cause that can contribute to disease development. This Review describes the mechanisms of action of antioxidants synthesized in vivo, antioxidants derived from the human diet and synthetic antioxidants developed as therapeutic agents, with a focus on the gaps in our current knowledge and the approaches needed to close them. The Review also explores the reasons behind the successes and failures of antioxidants in treating or preventing human disease. Antioxidants may have special roles in the gastrointestinal tract, and many lifestyle features known to promote health (especially diet, exercise and the control of blood glucose and cholesterol levels) may be acting, at least in part, by antioxidant mechanisms. Certain reactive sulfur species may be important antioxidants but more accurate determinations of their concentrations in vivo are needed to help assess their contributions.

The mechanism underlying nitroxyl and nitric oxide formation from hydroxamic acids

BACKGROUND

The pharmacological effects of hydroxamic acids (RC(O)NHOH, HX) are partially attributed to their ability to serve as HNO and/or NO donors under oxidative stress. Given the development and use of HXs as therapeutic agents, elucidation of the oxidation mechanism is needed for more educated selection of HX-based drugs.

METHODS

Acetohydroxamic and glycine-hydroxamic acids were oxidized at pH 7.0 by a continuous flux of radiolytically generated (·)OH or by metmyoglobin and H(2)O(2) reactions system. Gas chromatography and spectroscopic methods were used to monitor the accumulation of N(2)O, N(2), nitrite and hydroxylamine.

RESULTS

Oxidation of HXs by (·)OH under anoxia yields N(2)O, but not nitrite, N(2) or hydroxylamine. Upon the addition of H(2)O(2) to solutions containing HX and metmyoglobin, which is instantaneously and continuously converted into compound II, nitrite and, to a lesser extent, N(2)O are accumulated under both anoxia and normoxia.

CONCLUSIONS

Oxidation of HXs under anoxia by a continuous flux of (·)OH, which solely oxidizes the hydroxamate moiety to RC(O)NHO(·), forms HNO. This observation implies that bimolecular decomposition of RC(O)NHO(·) competes efficiently with unimolecular decomposition processes such as internal disproportionation, hydrolysis or homolysis. Oxidation by metmyoglobin/H(2)O(2) involves relatively mild oxidants (compounds I and II). Compound I reacts with HX forming RC(O)NHO(·) and compound II, which oxidizes HX, RC(O)NHO(·), HNO and NO. The latter reaction is the main source of nitrite.

GENERAL SIGNIFICANCE

HXs under oxidative stress release HNO, but can be considered as NO-donors provided that HNO oxidation is more efficient than its reaction with other biological targets.

Analysis of the oxidase activity induced by CCl(4) and H(2)O(2) in different recombinant myoglobins

Hemoproteins may present several functions due to their prosthetic groups. After a long time, well-studied proteins such as myoglobin have surprised us with new functions. Myoglobin is a hemoprotein which has some well described and unexpected functions within the organism. Oxidase activity in standard myoglobins has been described and this activity was attributed to a covalent linkage between heme and some amino acid residues such as histidine, when myoglobins are treated with alkyl halides, and tyrosine, and when myoglobins are treated with H(2)O(2). We have found that the oxidase activity, due to H(2)O(2) treatment, can appear in different myoglobins, which presents no key residue, such as Tyr 103, for the oxidase activity previously described in the literature.

Control of oxidative reactions of hemoglobin in the design of blood substitutes: role of the ascorbate-glutathione antioxidant system

Uncontrolled oxidative reactions of hemoglobin (Hb) are still the main unresolved problem for Hb-based blood substitute developers. Spontaneous oxidation of acellular ferrous Hb into a nonfunctional ferric Hb generates superoxide anion. Hydrogen peroxide, formed after superoxide anion dismutation, may react with ferrous/ferric Hb to produce toxic ferryl Hb, fluorescent heme degradation products, and/or protein-based free radicals. In the presence of free iron released from heme, superoxide anion and hydrogen peroxide might react via the Haber-Weiss and Fenton reactions to generate the hydroxyl radical. These highly reactive oxygen and heme species may not only be involved in shifting the cellular redox balance to the oxidized state that facilitates signal transduction and pro-inflammatory gene expression, but could also be involved in cellular and organ injury, and generation of vasoactive compounds such as isoprostanes and angiotensins. It is believed that these toxic species may be formed after administration of Hb-based blood substitutes, particularly in ischemic patients with a diminished ability to control oxidative reactions. Although varieties of antioxidant strategies have been suggested, this in vitro study examined the ability of the ascorbate-glutathione antioxidant system in preventing Hb oxidation and formation of its ferryl intermediate. The results suggest that although ascorbate is effective in reducing the formation of ferryl Hb, glutathione protects heme against excessive oxidation. Ascorbate without glutathione failed to protect the red blood cell membranes against Hb/hydrogen peroxide-mediated peroxidation. This study provides evidence that the ascorbate-glutathione antioxidant system is essential in attenuation of the pro-oxidant potential of redox active acellular Hbs, and superior to either ascorbate or glutathione alone.

Hydrogen peroxide-mediated alteration of the heme prosthetic group of metmyoglobin to an iron chlorin product: evidence for a novel oxidative pathway

Treatment of metmyoglobin with H2O2 is known to lead to the crosslinking of an active site tyrosine residue to the heme [Catalano, C. E., Y. S. Choe, and P. R. Ortiz de Montellano (1989) J. Biol. Chem. 264, 10534-10541]. We have found in this study that this reaction also leads to an altered heme product not covalently bound to the protein. This product was characterized by visible absorption, infrared absorption, and mass and NMR spectrometry as an iron chlorin product formed from the saturation of the double bond between carbon atoms at positions 17 and 18 of pyrrole ring D with concomitant addition of a hydroxyl group on the carbon atom at position 18 and lactonization of the propionic acid to the carbon atom at position 17. Studies with the use of (18)O-labeled H2O2, O2, and H2O clearly indicate that the source of the added oxygen on the heme is water. Evidently, water adds regiospecifically to a cationic site formed on a carbon atom at position 18 after oxidation of the ferric heme prosthetic group with peroxide. Prolonged incubation of the reaction mixture containing the iron hydroxychlorin product led to the formation of an iron dihydroxychlorin product, presumably from a slow addition of water to the initial iron hydroxychlorin. The iron chlorin products characterized in this study are distinct from the meso-oxyheme species, which is thought to be formed during peroxide-mediated degradation of metmyoglobin, cytochrome P450, ferric heme, and model ferric hemes, and give further insight into the mechanism of H2O2-induced heme alterations.

Self-peroxidation of metmyoglobin results in formation of an oxygen-reactive tryptophan-centered radical

In the reaction between hydrogen peroxide and metmyoglobin, the heme iron is oxidized to its ferryl-oxo form and the globin to protein radicals, at least one of which reacts with dioxygen to form a peroxyl radical. To identify the residue(s) that forms the oxygen-reactive radical, we utilized electron spin resonance (ESR) spectroscopy and the spin traps 2-methyl-2-nitrosopropane and 3,5-dibromo-4-nitrosobenzenesulfonic acid (DB-NBS). Metmyoglobin radical adducts had spectra typical of immobilized nitroxides that provided little structural information, but subsequent nonspecific protease treatment resulted in the detection of isotropic three-line spectra, indicative of a radical adduct centered on a tertiary carbon with no bonds to nitrogen or hydrogen. Similar isotropic three-line ESR spectra were obtained by spin trapping the oxidation product of tryptophan reacting with catalytic metmyoglobin and hydrogen peroxide. High resolution ESR spectra of DBNBS/.trp and of the protease-treated DBNBS/.metMb were simulated using superhyperfine coupling to a nitrogen and three non-equivalent hydrogens, consistent with a radical adduct formed at C-3 of the indole ring. Oxidation of tryptophan by catalytic metMb and hydrogen peroxide resulted in spin trap-inhibitable oxygen consumption, consistent with formation of a peroxyl radical. The above results support self-peroxidation of a tryptophan residue in the reaction between metMb and hydrogen peroxide.

Acid-catalysed reduction of ferrylmyoglobin: product distribution and kinetics of autoreduction and reduction by NADH

The pH dependence of iron(II)/iron(III) product distribution, following reduction of the hypervalent iron in equine ferrylmyoglobin by the protein moiety of the pigment (so-called autoreduction) and by NADH (nicotinamide adenine dinucleotide, reduced) and the rate of reduction was found to depend different on pH. Autoreduction is specific acid catalysed and has a more modest temperature dependence than autoxidation of oxymyoglobin, with the activation parameters delta H# = 58.5 +/- 0.4 kJ.mol-1 and delta S# = 2.7 +/- 0.1 J.mol-1.K-1 in 0.16 mol.l-1 NaCl. The product of autoreduction is the iron(III) pigment metmyoglobin, which is slightly modified in the protein moiety. The reaction has a positive kinetic salt effect from which it is deduced that the reactive centre of ferrylmyoglobin has a charge of +1 in agreement with the structure Fe(IV) = O. Reduction by NADH involves parallel reactions of two pigment forms in acid/base equilibrium with each other with a pKa equal to 4.9, both forms yielding metmyoglobin as well as the iron(II) pigment, oxymyoglobin, as products. The protonated form reacts faster than the deprotonated form, and two-electron transfer has greater importance for the protonated form with a limiting Fe(II)/Fe(III) product ratio of 0.6 in acidic solution compared to 0.12 in alkaline solution. A square root dependence of rate on NADH concentration suggests involvement of NAD.radicals with a disproportionation as the termination reaction.

NADPH binding and control of catalase compound II formation: comparison of bovine, yeast, and Escherichia coli enzymes

1. NADPH binds to bovine catalase and to yeast catalases A and T, but not to Escherichia coli catalase HPII. The association was demonstrated using chromatography and fluorimetry. Bound NADPH fluoresces in a similar way to NADPH in solution. 2. Bound NADPH protects bovine and yeast catalases against forming inactive peroxide compound II either via endogenous reductant action or by ferrocyanide reduction during catalytic activity in the presence of slowly generated peroxide. 3. Bound NADPH reduces neither compound I nor compound II of catalase. It apparently reacts with an intermediate formed during the decay of compound I to compound II; this postulated intermediate is an immediate precursor of stable compound II either when the latter is formed by endogenous reductants or when ferrocyanide is used. It represents therefore a new type of hydrogen donor that is not included in the original classification of Keilin and Nicholls [Keilin, D. and Nicholls, P. (1958) Biochim. Biophys. Acta 29, 302-307] 4. A model for NADPH action is presented in which concerted reduction of the ferryl iron and of a neighbouring protein free radical is responsible for the observed NADPH effects. The roles of migrant radical species in mammalian and yeast catalases are compared with similar events in metmyoglobin and cytochrome c peroxidase reactions with peroxides.

Oxidation reactions of human, opossum (Didelphis virginiana) and spot (Leiostomus xanthurus) hemoglobins: a search for a correlation with some structural-functional properties

1. Relative to human HbA, opossum (Didelphis virginiana) hemoglobin was found to be more susceptible to autoxidation. While the initial rate of autoxidation of spot (Leiostomus xanthurus) hemoglobin is close to that of HbA, complete oxidation occurs in 50 hr. 2. Direct addition of hydrogen peroxide (H2O2) induced oxidation of hemoglobins in a definite order: spot Hb > HbA > opossum Hb. Excess H2O2 led to heme degradation and precipitation that occurred much faster for spot Hb than the case with other proteins. 3. Exposure of hemoglobins to a continuous flux of H2O2, generated by the glucose/glucose oxidase system, induced the formation of heterogeneous protein-associated oxidation products. 4. Differential reactivity among these hemoglobins under the same or different oxidative conditions, with respect to methemoglobin formation and stability of the ferric form, may reflect the differences in the local heme environment of these proteins.

Ferrous-iron-induced oxidation in chicken liver slices as measured by hemichrome formation and thiobarbituric acid-reactive substances: effects of dietary vitamin E and beta-carotene

Hemichrome formation in chicken liver slices was determined by employing a Heme Protein Spectra Analysis Program (HPSAP) on the visible spectrum of the liver tissue. Relative hemichrome formation (RHF) in liver tissue exposed to ferrous iron for 1 h at 37 degrees C could be predicted according to the general catalytic equation RHF = k.[Fe2+]/(Ap + [Fe2+]), with k = 132 +/- 30, where the factor Ap represents the additive antioxidative potential in the liver tissue. RHF in Fe2+ exposed liver slices incubated at 37 degrees C for 1 h correlated significantly with formation of thiobarbituric acid-reactive substances (TBARS) (r = .77, P < .0001). RHF was found to decrease significantly with increasing vitamin E concentration in liver tissue exposed to ferrous iron (1 h, 37 degrees C). However, the influence of beta-carotene on RHF in ferrous-iron exposed liver slices (1 h, 37 degrees C) was less evident, as the concentration of Fe2+ was found to be decisive for whether beta-carotene acted as an antioxidant or a prooxidant under the conditions in question. Results in the liver slice model system regarding the effect of vitamin E and beta-carotene on iron overload were supported in a subsequent in vivo iron injection experiment with chicks. These observations indicate that RHF is a sensitive marker for ferrous-iron-induced oxidative damage in the present tissue slice system.

Towards the physiological function of uric acid

Uric acid, or more correctly (at physiological pH values), its monoanion urate, is traditionally considered to be a metabolically inert end-product of purine metabolism in man, without any physiological value. However, this ubiquitous compound has proven to be a selective antioxidant, capable especially of reaction with hydroxyl radicals and hypochlorous acid, itself being converted to innocuous products (allantoin, allantoate, glyoxylate, urea, oxalate). There is now evidence for such processes not only in vitro and in isolated organs, but also in the human lung in vivo. Urate may also serve as an oxidisable cosubstrate for the enzyme cyclooxygenase. As shown for the coronary system, a major site of production of urate is the microvascular endothelium, and there is generally a net release of urate from the human myocardium in vivo. In isolated organ preparations, urate protects against reperfusion damage induced by activated granulocytes, cells known to produce a variety of radicals and oxidants. Intriguingly, urate prevents oxidative inactivation of endothelial enzymes (cyclooxygenase, angiotensin converting enzyme) and preserves the ability of the endothelium to mediate vascular dilatation in the face of oxidative stress, suggesting a particular relationship between the site of urate formation and the need for a biologically potent radical scavenger and antioxidant.