Pro-oxidant effects of cross-linked haemoglobins explored using liposome and cytochrome c oxidase vesicle model membranes

A biomimetic assay for antioxidant reactivity, based on liposomes and myoglobin

Antioxidant assays are typically based on non-physiologically relevant reagents. We describe here a quantitative assay based on the inhibition of the liposome autooxidation in the presence of myoglobin (ILA-Mb), an oxidative process with direct biomedical relevance. Additional advantages of the assay include the use of standard and readily available reagents (lecithin and myoglobin) and the applicability to lipophilic antioxidants. The ILA-Mb assay is based on previously reported qualitative or semi-quantitative ones that employed cytochrome c instead of myoglobin. A number of antioxidants are tested, and their IC50 parameters are discussed and interpreted to involve direct interaction with both myoglobin and the liposomes.

On the Apparent Redox Reactivity of "Oxygen-Enriched Water"

Molecular oxygen-enriched water (OxEW) is advocated in popular media as useful for various health issues, presumably due to involvement of a purported antioxidant activity and to such notions as "active oxygen." To our knowledge, there are no explicit reports in the scientific literature where such redox reactivity would be described and explained. Reported here are data showing that a commercial preparation of OxEW does display a measurable, albeit very small, antioxidant activity as monitored by reaction with a standard reagent, DPPH. Moreover, OxEW also displays an apparent pro-oxidant reactivity, against caffeic acid. This does not correlate with any UV-vis-detectable contents of chemical substances in the water, nor can it be explained by typical chemical impurities (e.g., hydrogen peroxide or molecular hydrogen) that would arise upon enrichment with molecular oxygen of pure water by the two most common procedures: purging with gaseous O₂ or electrolysis. Instead, this apparent redox reactivity is revealed to be due to differences in pH and in chemical content - and the differences in turn are most likely due to the trace amounts of inorganic ions/elements in the OxEW; importantly, electrolysis, which is often employed as a means to generate O₂ in OxEW preparation, is also found to enhance the redox effect of OxEW-like preparations. Thus, in line with expectations, the herein-reported data show that there are no long-lived reactive oxygen species, no activated oxygen, and no extra reducing agents in OxEW - but that an apparent weak redox reactivity can still be measured and assigned to simple side effects of the electrolysis procedure presumably performed in order to enrich the sample in oxygen.

Role of gap junctions in early brain injury following subarachnoid hemorrhage

Gap junction inhibition has been demonstrated to reverse the vascular contraction that follows experimental subarachnoid hemorrhage. This study hypothesizes that the use of established gap junction inhibitors: octonal and carbenoxolone, to interrupt cell to cell communication will provide neuroprotection against early brain injury after SAH. The filament perforation model of SAH was performed in male Sprague-Dawley rats weighing between 300 and 380 g. Octanol (260.46 mg or 781.38 mg/kg), carbenoxolone (100 mg/kg), or vehicles were given via intraperitoneal injection 1 h after SAH. Neurologic deficits and cerebral apoptosis were assessed 24 and 72 h after SAH. In addition, Western blot analysis was performed to confirm the in vivo inhibition of CNS gap junctions. The administration of octanol and carbenoxolone both failed to attenuate the neurological deficits induced by SAH, and they did not reduce neuronal apoptosis. Additionally, carbenoloxone increased post SAH mortality and exacerbated SAH-induced apoptosis. Despite previous studies that show gap junction inhibitors reverse vasospasm following experimental SAH, they failed to improve clinical outcomes or provide neuroprotection in this study.

A new sensitive assay reveals that hemoglobin is oxidatively modified in vivo

Free radical formation in heme proteins is recognised as a factor in mediating the toxicity of peroxides in oxidative stress. As well as initiating free radical damage, heme proteins damage themselves. Under extreme conditions, where oxidative stress and low pH coincide (e.g., myoglobin in the kidney following rhabdomyolysis and hemoglobin in the CSF subsequent to subarachnoid hemorrhage), peroxide can induce covalent heme to protein cross-linking. In this paper we show that, even at neutral pH, the heme in hemoglobin is covalently modified by oxidation. The product, which we term OxHm, is a "green heme" iron chlorin with a distinct optical spectrum. OxHm formation can be quantitatively prevented by reductants of ferryl iron, e.g., ascorbate. We have developed a simple, robust, and reproducible HPLC assay to study the extent of OxHm formation in the red cell in vivo. We show that hemoglobin is oxidatively damaged even in normal blood; approximately 1 in 2,000 heme groups exist as OxHm in the steady state. We used a simple model (physical exercise) to demonstrate that OxHm increases significantly during acute oxidative stress. The exercise-induced increase is short-lived, suggesting the existence of an active mechanism for repairing or removing the damaged heme proteins.

The effect of acid and alkali unfolding and subsequent refolding on the pro-oxidative activity of trout hemoglobin

The pro-oxidative activity of trout hemoglobin was significantly increased at low pH (2.5-3.5) in a washed fish muscle (WFM) system. It was found that the more unfolded the hemoglobin was the more exposed its heme group was, which increased its pro-oxidative activity. The amount of oxidation products produced (TBARS) were, however, lower at low pH vs neutral pH. At pH 10.5-11, the pro-oxidative activity of hemoglobin was greatly suppressed. The conformation of hemoglobin was significantly more stable at high pH as compared to pH 7 as judged by its visible absorption spectrum. Hemoglobin readjusted from low pH to pH 7 had a higher pro-oxidative activity (i.e., more rapid oxidation) in WFM than native hemoglobin at pH 7, even though TBARS values were lower than in the untreated sample at pH 7. The results suggest that the WFM becomes slightly more susceptible to oxidation after low pH treatment but also produces less TBARS. The increased pro-oxidative activity after pH readjustment correlated well with an incomplete recovery in the native structure on pH readjustment. A longer unfolding time and a lower pH led to a less refolded hemoglobin with increased pro-oxidative activity. Hemoglobin was less pro-oxidative at low pH in the presence of 500 mM NaCl. The presence of salt did, however, increase the pro-oxidative properties of hemoglobin after readjustment to pH 7. The treatment of washed fish muscle at alkaline pH followed by adjustment to pH 7 led to a slight delay in hemoglobin-mediated lipid oxidation in WFM as compared to native hemoglobin at pH 7. The results suggest that WFM becomes less susceptible toward oxidation after pH readjustment from alkaline pH. These results clearly show that for muscle protein extraction/isolation processes requiring highly alkaline or acidic conditions, alkaline conditions are preferred if the lipid oxidation originating from hemoglobin is to be minimized.

The effects of pH on the mechanism of hydrogen peroxide and lipid hydroperoxide consumption by myoglobin: a role for the protonated ferryl species

Myoglobin catalyses the breakdown of lipid hydroperoxides (e.g., HPODE) during which the absorption band of the lipid conjugated diene (234 nm) is partially bleached. The constant for this process is strongly pH-dependent (k = 9.5 x 10(-3)s(-1), pH 7: k = 2.3 x 10(-1)s(-1), pH 5). This rate enhancement is not due to acid-induced changes in protein conformation or the involvement of protein-based radical species, as demonstrated by an almost identical pH dependence of the same reaction catalyzed by ferric haemin. The rate constants for ferryl formation and auto-reduction show different pH dependencies, with a pK of 8.3 for ferryl formation and a projected pK of 3.5 for ferryl auto-reduction. The pH dependence for the auto-reduction of the ferryl species is the same as that of the myoblobin catalyzed breakdown of HPODE. We propose that the protonated form of ferryl myoglobin (Fe(4+) - OH(-)) is the reactive species regulating the peroxidatic activity of myoglobin. The protonated ferryl species abstracts an electron from either the protein or porphyrin, allowing fast regeneration of the ferric species. Alkaline conditions stabilize the ferryl species, making myoglobin considerably less reactive towards lipids and lipid hydroperoxides. These findings are significant for understanding myoglobin-induced oxidative stress in vivo and the development of therapies.

Redox reactions of hemoglobin and myoglobin: biological and toxicological implications

Direct cytotoxic effects associated with hemoglobin (Hb) or myoglobin (Mb) have been ascribed to redox reactions (involving either one- or two-electron steps) between the heme group and peroxides. These interactions are the basis of the pseudoperoxidase activity of these hemoproteins and can be cytotoxic when reactive species are formed at relatively high concentrations during inflammation and typically lead to cell death. Peroxides relevant to biological systems include hydrogen peroxide, lipid hydroperoxides, and peroxynitrite. Reactions between Hb/Mb and peroxides form the ferryl oxidation state of the protein, analogous to compounds I and II formed in the catalytic cycle of many peroxidase enzymes. This higher oxidation state of the protein is a potent oxidant capable of promoting oxidative damage to most classes of biological molecules. Free iron, released from Hb, also has the potential to promote oxidative damage via classical "Fenton" chemistry. It has become increasingly evident that Hb/Mb redox reactions or their by-products play a critical role in the pathophysiology of some disease states. This review briefly discusses the reactions of Hb/Mb with biological peroxides, potential cytotoxicity and the impact of these interactions on modulation of cell signaling pathways regulated by these reactive species. Also discussed in this article is the role of heme-protein chemistry in relation to the toxicity of hemoproteins.

Site-specific modifications and toxicity of blood substitutes. The case of diaspirin cross-linked hemoglobin

Safe and effective hemoglobin-based blood substitutes may be advantageous over conventional therapies for certain clinical settings requiring short term blood replacement such as emergency resuscitation and hemodilution in surgery. Many advances have been made in developing these oxygen therapeutics, however safety concerns continue to slow their clinical progress. An important and often overlooked consideration in evaluating the safety of modified hemoglobins is the impact of chemical and/or genetic modifications on the redox chemistry of these proteins. Diaspirin cross-linked hemoglobin (DBBF-Hb) has been extensively evaluated in vitro and in animal models, and thus represents a useful model to explore possible correlations between structural-functional alterations and toxicity of hemoglobin-based blood substitutes.

Acellular hemoglobin-mediated oxidative stress toward endothelium: a role for ferryl iron

We tested the hypothesis that chemical modifications used to produce stable, oxygen-carrying, Hb-based blood substitutes can induce cytotoxicity in endothelial cells in culture because of altered redox activity. We examined the interaction of hydrogen peroxide with nonmodified hemoglobin (HbA0) and two chemically modified hemoglobins, alpha-cross-linked hemoglobin (alpha-DBBF) and its polymerized form (poly-alpha-DBBF). Hydrogen peroxide-induced cell death (as assessed by lactate dehydrogenase release) in bovine aortic endothelial cells (BAEC) was completely inhibited by all three hemoglobin preparations, consistent with their known pseudoperoxidase activity [hemoglobin consumes peroxide as it cycles between ferric (Fe3+) and ferryl (Fe4+) hemes]. However, reaction of the modified hemoglobins, but not HbA0, with hydrogen peroxide induced apoptotic cell death (as assessed by morphological changes and DNA fragmentation) that correlated with the formation of a long-lived ferrylhemoglobin. A preparation of ferryl-alpha-DBBF free of residual peroxide rapidly induced morphological changes and DNA fragmentation in BAEC, indicative of apoptotic cell death. Redox cycling of chemically modified hemoglobins by peroxide yielded a persistent ferryl iron that was cytotoxic to endothelial cells.

Mechanism of reaction of myoglobin with the lipid hydroperoxide hydroperoxyoctadecadienoic acid

The reaction between myoglobin and the lipid hydroperoxide 13(S)-hydroperoxy-9,11(cis,trans)-octadecadienoic acid (HPODE) was studied kinetically by spectrophotometric, polarographic and analytical methods. Metmyoglobin catalysed the decomposition of HPODE, resulting in peroxide, oxygen and conjugated diene depletion, together with the transient production of ferryl myoglobin. The reaction stoichiometry was 2:1:1 for peroxide to oxygen to conjugated diene, whereas the myoglobin remained generally intact. This stoichiometry and the rates of change of conjugated diene and ferryl myoglobin concentrations were not completely consistent with previously proposed mechanisms. We propose a novel mechanism in which HPODE reacts with both ferric myoglobin and ferryl myoglobin to form a redox cycle. Both peroxyl and alkoxyl radicals are produced, explaining the observed stoichiometry of peroxide, oxygen and conjugated diene depletion and the transient appearance of ferryl myoglobin. Computer simulation shows that this mechanism is fully capable of reproducing the observed time courses of all components.

Calcium- and pH-linked oligomerization of sorcin causing translocation from cytosol to membranes

Sorcin, a cytosolic calcium-binding protein containing a pair of EF-hand motifs, undergoes a Ca2(+)-dependent translocation to the cell membrane. The underlying conformational change is similar at pH 6.0 and 7.5 and consists in an increase in overall hydrophobicity that involves the aromatic residues and in particular the two tryptophan residues which become less exposed to solvent. The concomitant association from dimers to tetramers indicates that the tryptophan residues, which are located between the EF-hand sites, become buried at the dimer-dimer interface. Ca2(+)-bound sorcin displays a striking difference in solubility as a function of pH that has been ascribed to the formation of calcium-stabilized aggregates.

Reduction of hydroxyl radical generation in a rat hindlimb model of ischemia-reperfusion injury using crosslinked hemoglobin-superoxide dismutase-catalase

The effects of PolyHb (intermolecularly crosslinked hemoglobin) and PolyHb-SOD-CAT (intermolecularly crosslinked hemoglobin, superoxide dismutase and catalase) on the production of hydroxyl radical was studied using a rat hindlimb model of ischemia-reperfusion. Hydroxyl radical generation was assessed by an indirect assay based on the hydroxylation of 4-hydroxybenzoate. The hydroxylation product, 3,4 dihydroxybenzoate (3,4 DHBA), was analyzed by high performance liquid chromatography and electrochemical detection. The identification of 3,4 DHBA was confirmed by analysis of authentic standard and an in vitro hydroxyl radical generation system. Ischemia was induced in both hindlimbs by ligation of the infrarenal aorta. After a 4hr ischemic period, hindlimbs were simultaneously perfused with PolyHb-SOD-CAT (5 g/dl) into one limb and PolyHb (5 g/dl) into the other limb via femoral arterial catheters. Each perfusate also contained the hydroxyl radical trap, 4-hydroxbenzoate (5 mM). Femoral venous effluents were analyzed for the presence of the 3,4 DHBA. Data indicates that greater 3,4 DHBA production occurs during PolyHb perfusion as compared to PolyHb-SOD-CAT. These preliminary results show that perfusion with PolyHb-SOD-CAT may alleviate oxidative stress in a model of ischemia-reperfusion.