Intermediate spin-states in one-electron reduction of oxygen-hemoprotein complexes at low temperature

Transition metal complexes and the activation of dioxygen

In order to address how diverse metalloprotein active sites, in particular those containing iron and copper, guide O₂binding and activation processes to perform diverse functions, studies of synthetic models of the active sites have been performed. These studies have led to deep, fundamental chemical insights into how O₂coordinates to mono- and multinuclear Fe and Cu centers and is reduced to superoxo, peroxo, hydroperoxo, and, after O-O bond scission, oxo species relevant to proposed intermediates in catalysis. Recent advances in understanding the various factors that influence the course of O₂activation by Fe and Cu complexes are surveyed, with an emphasis on evaluating the structure, bonding, and reactivity of intermediates involved. The discussion is guided by an overarching mechanistic paradigm, with differences in detail due to the involvement of disparate metal ions, nuclearities, geometries, and supporting ligands providing a rich tapestry of reaction pathways by which O₂is activated at Fe and Cu sites.

Electron paramagnetic resonance and electron-nuclear double resonance studies of the reactions of cryogenerated hydroperoxoferric-hemoprotein intermediates

The fleeting ferric peroxo and hydroperoxo intermediates of dioxygen activation by hemoproteins can be readily trapped and characterized during cryoradiolytic reduction of ferrous hemoprotein–O₂ complexes at 77 K. Previous cryoannealing studies suggested that the relaxation of cryogenerated hydroperoxoferric intermediates of myoglobin (Mb), hemoglobin, and horseradish peroxidase (HRP), either trapped directly at 77 K or generated by cryoannealing of a trapped peroxo-ferric state, proceeds through dissociation of bound H₂O₂ and formation of the ferric heme without formation of the ferryl porphyrin π-cation radical intermediate, compound I (Cpd I). Herein we have reinvestigated the mechanism of decays of the cryogenerated hydroperoxyferric intermediates of α- and β-chains of human hemoglobin, HRP, and chloroperoxidase (CPO). The latter two proteins are well-known to form spectroscopically detectable quasistable Cpds I. Peroxoferric intermediates are trapped during 77 K cryoreduction of oxy Mb, α-chains, and β-chains of human hemoglobin and CPO. They convert into hydroperoxoferric intermediates during annealing at temperatures above 160 K. The hydroperoxoferric intermediate of HRP is trapped directly at 77 K. All studied hydroperoxoferric intermediates decay with measurable rates at temperatures above 170 K with appreciable solvent kinetic isotope effects. The hydroperoxoferric intermediate of β-chains converts to the S = 3/2 Cpd I, which in turn decays to an electron paramagnetic resonance (EPR)-silent product at temperature above 220 K. For all the other hemoproteins studied, cryoannealing of the hydroperoxo intermediate directly yields an EPR-silent majority product. In each case, a second follow-up 77 K γ-irradiation of the annealed samples yields low-spin EPR signals characteristic of cryoreduced ferrylheme (compound II, Cpd II). This indicates that in general the hydroperoxoferric intermediates relax to Cpd I during cryoanealing at low temperatures, but when this state is not captured by reaction with a bound substrate, it is reduced to Cpd II by redox-active products of radiolysis.

Spectroscopic studies of peroxo/hydroperoxo derivatives of heme proteins and model compounds

The spectroscopic characterization of peroxo- and hydroperoxo- intermediates of heme proteins and enzymes has long interested scientists studying the structure and function of these important biochemical systems. Until very recently, little progress had been made in studying these fleeting intermediates by vibrational spectroscopic methods. In this brief review, recent studies reporting the Resonance Raman and Infrared spectra of such intermediates and pertinent model compounds are reviewed and compared to corresponding studies utilizing electronic absorption and electron paramagnetic resonance spectrometric methods.

Cryoradiolysis and cryospectroscopy for studies of heme-oxygen intermediates in cytochromes p450

Cryogenic radiolytic reduction is one of the most straightforward and convenient methods of generation and stabilization of reactive iron-oxygen intermediates for mechanistic studies in chemistry and biochemistry. The method is based on one-electron reduction of the precursor complex in frozen solution via exposure to the ionizing radiation at cryogenic temperatures. Such approach allows for accumulation of the fleeting reactive complexes which otherwise could not be generated at sufficient amount for structural and mechanistic studies. Application of this method allowed for characterizing of peroxo-ferric and hydroperoxo-ferric intermediates, which are common for the oxygen activation mechanism in cytochromes P450, heme oxygenases, and nitric oxide synthases, as well as for the peroxide metabolism by peroxidases and catalases.

The critical iron-oxygen intermediate in human aromatase

Aromatase (CYP19) is the target of several therapeutics used for breast cancer treatment and catalyzes the three-step conversion of androgens to estrogens, with an unusual C-C cleavage reaction in the third step. To better understand the CYP19 reaction, the oxy-ferrous complex of CYP19 with androstenedione substrate was cryotrapped, characterized by UV-vis spectroscopy, and cryoreduced to generate the next reaction cycle intermediate. EPR analysis revealed that the initial intermediate observed following cryoreduction is the unprotonated g(1)=2.254 peroxo-ferric intermediate, which is stable up to 180K. Upon gradual cryoannealing, the low-spin (g(1)=2.39) product complex is formed, with no evidence for accumulation of the g(1)=2.30 hydroperoxo-ferric intermediate. The relative stabilization of the peroxo-ferric heme and the lack of observed hydroperoxo-ferric heme distinguish CYP19 from other P450s, suggesting that the proton delivery pathway is more hindered in CYP19 than in most other P450s.

The influence of X-rays on the structural studies of peroxide-derived myoglobin intermediates

In recent years, the awareness of potential radiation damage of metal centers in protein crystals during crystallographic data collection has received increasing attention. The radiation damage can lead to radiation-induced changes and reduction of the metal sites. One of the research fields where these concerns have been comprehensively addressed is the study of the reaction intermediates of the heme peroxidase and oxygenase reaction cycles. For both the resting states and the high-valent intermediates, the X-rays used in the structure determination have given undesired side effects through radiation-induced changes to the trapped intermediates. However, X-rays have been used to generate and trap the peroxy/hydroperoxy state in crystals. In this review, the structural work and the influence of X-rays on these intermediates in myoglobin are summarized and viewed in light of analogous studies on similar intermediates in peroxidases and oxygenases.

Reaction of the Co(II)-substrate radical pair catalytic intermediate in coenzyme B12-dependent ethanolamine ammonia-lyase in frozen aqueous solution from 190 to 217 K

The decay kinetics of the aminoethanol-generated Co(II)-substrate radical pair catalytic intermediate in ethanolamine ammonia-lyase from Salmonella typhimurium have been measured on timescales of <10(5) s in frozen aqueous solution from 190 to 217 K. X-band continuous-wave electron paramagnetic resonance (EPR) spectroscopy of the disordered samples has been used to continuously monitor the full radical pair EPR spectrum during progress of the decay after temperature step reaction initiation. The decay to a diamagnetic state is complete and no paramagnetic intermediate states are detected. The decay exhibits three kinetic regimes in the measured temperature range, as follows. i), Low temperature range, 190 < or = T < or = 207 K: the decay is biexponential with constant fast (0.57 +/- 0.04) and slow (0.43 +/- 0.04) phase amplitudes. ii), Transition temperature range, 207 < T < 214 K: the amplitude of the slow phase decreases to zero with a compensatory rise in the fast phase amplitude, with increasing temperature. iii), High temperature range, T > or = 214 K: the decay is monoexponential. The observed first-order rate constants for the monoexponential (k(obs,m)) and the fast phase of the biexponential decay (k(obs,f)) adhere to the same linear relation on an lnk versus T(-1) (Arrhenius) plot. Thus, k(obs,m) and k(obs,f) correspond to the same apparent Arrhenius prefactor and activation energy (logA(app,f) (s(-1)) = 13.0, E(a,app,f) = 15.0 kcal/mol), and therefore, a common decay mechanism. We propose that k(obs,m) and k(obs,f) represent the native, forward reaction of the substrate through the radical rearrangement step. The slow phase rate constant (k(obs,s)) for 190 < or = T < or = 207 K obeys a different linear Arrhenius relation (logA(app,s) (s(-1)) = 13.9, E(a,app,s) = 16.6 kcal/mol). In the transition temperature range, k(obs,s) displays a super-Arrhenius increase with increasing temperature. The change in E(a,app,s) with temperature and the narrow range over which it occurs suggest an origin in a liquid/glass or dynamical transition. A discontinuity in the activation barrier for the chemical reaction is not expected in the transition temperature range. Therefore, the transition arises from a change in the properties of the protein. We propose that a protein dynamical contribution to the reaction, which is present above the transition temperature, is lost below the transition temperature, owing to an increase in the activation energy barrier for protein motions that are coupled to the reaction. For both the fast and slow phases of the low temperature decay, the dynamical transition in protein motions that are obligatorily coupled to the reaction of the Co(II)-substrate radical pair lies below 190 K.

Resonance Raman spectroscopic studies of hydroperoxo derivatives of cobalt-substituted myoglobin

Recent progress in generating and stabilizing reactive heme protein enzymatic intermediates by cryoradiolytic reduction has prompted application of a range of spectroscopic approaches to effectively interrogate these species. The impressive potential of resonance Raman spectroscopy for characterizing such samples has been recently demonstrated in a number of studies of peroxo- and hydroperoxo-intermediates. While it is anticipated that this approach can be productively applied to the wide range of heme proteins whose reaction cycles naturally involve these peroxo- and hydroperoxo-intermediates, one limitation that sometimes arises is the lack of enhancement of the key intraligand nu(O-O) stretching mode in the native systems. The present work was undertaken to explore the utility of cobalt substitution to enhance both the nu(Co-O) and nu(O-O) modes of the CoOOH fragments of hydroperoxo forms of heme proteins bearing a trans-axial histidine linkage. Thus, having recently completed RR studies of hydroperoxo myoglobin, attention is now turned to its cobalt-substituted analogue. Spectra are acquired for samples prepared with (16)O(2) and (18)O(2) to reveal the nu(M-O) and nu(O-O) modes, the latter indeed being observed only for the cobalt-substituted proteins. In addition, spectra of samples prepared in deuterated solvents were also acquired, providing definitive evidence for the presence of the hydroperoxo-species.

The crystal structure of peroxymyoglobin generated through cryoradiolytic reduction of myoglobin compound III during data collection

Myoglobin has the ability to react with hydrogen peroxide, generating high-valent complexes similar to peroxidases (compounds I and II), and in the presence of excess hydrogen peroxide a third intermediate, compound III, with an oxymyoglobin-type structure is generated from compound II. The compound III is, however, easily one-electron reduced to peroxymyoglobin by synchrotron radiation during crystallographic data collection. We have generated and solved the 1.30 Å (1 Å=0.1 nm) resolution crystal structure of the peroxymyoglobin intermediate, which is isoelectric to compound 0 and has a Fe–O distance of 1.8 Å and O–O bond of 1.3 Å in accordance with a FeII–O–O− (or FeIII–O–O2−) structure. The generation of the peroxy intermediate through reduction of compound III by X-rays shows the importance of using single-crystal microspectrophotometry when doing crystallography on metalloproteins. After having collected crystallographic data on a peroxy-generated myoglobin crystal, we were able (by a short annealing) to break the O–O bond leading to formation of compound II. These results indicate that the cryoradiolytic-generated peroxymyoglobin is biologically relevant through its conversion into compound II upon heating. Additionally, we have observed that the Xe1 site is occupied by a water molecule, which might be the leaving group in the compound II to compound III reaction.

Thirty years of microbial P450 monooxygenase research: Peroxo-heme intermediates—The central bus station in heme oxygenase catalysis

Oxygen has always been recognized as an essential element of many life forms, initially through its role as a terminal electron acceptor for the energy-generating pathways of oxidative phosphorylation. In 1955, Hayaishi et al. [Mechanism of the pyrocatechase reaction, J. Am. Chem. Soc. 77 (1955) 5450-5451] presented the most important discovery that changed this simplistic view of how Nature uses atmospheric dioxygen. His discovery, the naming and mechanistic understanding of the first "oxygenase" enzyme, has provided a wonderful opportunity and scientific impetus for four decades of researchers. This volume provides an opportunity to recognize the breakthroughs of the "Hayaishi School." Notable have been the prolific contributions of Professor Ishimura et al. [Oxygen and life. Oxygenases, Oxidases and Lipid Mediators, International Congress Series, Elsevier, Amsterdam, 2002], a first-generation Hayaishi product, to characterization of the cytochrome P450 monooxygenases.

Absence of a detectable intermediate in the compound I formation of horseradish peroxidase at ambient temperature

A microsecond-resolved absorption spectrometer was developed to investigate the elementary steps in hydrogen peroxide (H(2)O(2)) activation reaction of horseradish peroxidase (HRP) at ambient temperature. The kinetic absorption spectra of HRP upon the mixing with various concentrations of H(2)O(2) (0.5-3 mm) were monitored in the time range from 50 to 300 mus. The time-resolved spectra in the Soret region possessed isosbestic points that were close to those between the resting state and compound I. The kinetic changes in the Soret absorbance could be well fitted by a single exponential function. Accordingly, no distinct spectrum of the putative intermediate between the resting state and compound I was identified. These results were consistent with the proposal that the O-O bond activation in heme peroxidases is promoted by the imidazolium form of the distal histidine that exists only transiently. It was estimated that the rate constant for the breakage of the O-O bond in H(2)O(2) by HRP is significantly faster than 1 x 10(4) s(-1).

Models and mechanisms of O-O bond activation by cytochrome P450. A critical assessment of the potential role of multiple active intermediates in oxidative catalysis

Cytochrome P450 enzymes promote a number of oxidative biotransformations including the hydroxylation of unactivated hydrocarbons. Whereas the long-standing consensus view of the P450 mechanism implicates a high-valent iron-oxene species as the predominant oxidant in the radicalar hydrogen abstraction/oxygen rebound pathway, more recent studies on isotope partitioning, product rearrangements with 'radical clocks', and the impact of threonine mutagenesis in P450s on hydroxylation rates support the notion of the nucleophilic and/or electrophilic (hydro)peroxo-iron intermediate(s) to be operative in P450 catalysis in addition to the electrophilic oxenoid-iron entity; this may contribute to the remarkable versatility of P450s in substrate modification. Precedent to this mechanistic concept is given by studies with natural and synthetic P450 biomimics. While the concept of an alternative electrophilic oxidant necessitates C-H hydroxylation to be brought about by a cationic insertion process, recent calculations employing density functional theory favour a 'two-state reactivity' scenario, implicating the usual ferryl-dependent oxygen rebound pathway to proceed via two spin states (doublet and quartet); state crossing is thought to be associated with either an insertion or a radicalar mechanism. Hence, challenge to future strategies should be to fold the disparate and sometimes contradictory data into a harmonized overall picture.

Conformational substates of the oxyheme centers in alpha and beta subunits of hemoglobin as disclosed by EPR and ENDOR studies of cryoreduced protein

Exposure of frozen solutions of oxyhemoglobin to gamma-irradiation at 77 K yields EPR- and ENDOR-active, one-electron-reduced oxyheme centers which retain the conformation of the diamagnetic precursor. EPR spectra have been collected for the centers produced in human HbO(2) and isolated alphaO(2) and betaO(2) chains, as well as alphaO(2)beta(Zn), alpha(Zn)betaO(2), and alphaO(2)beta(Fe(3+)) hybrids, each in frozen buffer and in frozen glasses that form in the presence of glycols and sugars and also in the presence of IHP. These reveal two spectroscopically distinct classes of such ferriheme centers (g(1) <or= 2.25), denoted A and B. Averaged over many similar sites, the A-center has a rhombic EPR signal with a g-tensor, g(A) = [2.248(4), 2.146(1), 1.966(1)]; the B-center exhibits a less anisotropic EPR signal, g(B) = [2.216(3), 2.118(2), 1.966(1)]. Early measurement had suggested that, in the cryoreduced HbO(2) tetramer, the two centers corresponded to the two different chains [Symons, M. C. R., and Petersen, R. L. (1978) Proc. R. Soc. London, Ser. B 201, 285-300]. However, the present EPR and ENDOR results show that the two signals instead reflect the fact that the parent oxyhemes exist in two major conformational substates and that this is true for both alphaO(2) and betaO(2) subunits: alphaO(2)(A) (minor species) and alphaO(2)(B) (major species); betaO(2)(A)(major species) and betaO(2)(B) (minor species). Similar behavior is seen for MbO(2) [Kappl, R., Höhn-Berlage, M., Hüttermann, J., Bartlett, N., and Symons, M. C. R. (1985) Biochim. Biophys. Acta 827, 327-343]. The A/B g-tensors of alphaO(2) and betaO(2) chains vary little with the environment of the chains, while the relative populations of the substates depend greatly on glycols and IHP. These results suggest a quaternary influence on the oxyheme distal pocket of alpha chains and that the glycol-induced changes in the substate populations of the R-state HbO(2) tetramer are largely associated with the alphaO(2) subunit. (1)H ENDOR spectra from the distal histidine proton hydrogen-bonded to the peroxo ligand show very different isotropic coupling for the A- and B-centers. Analysis of the spectroscopic data suggests that the A- and B-centers represent different orientations of the oxyheme O(2) ligand relative to the distal histidine. It is likely that the A and B conformational substates in the alphaO(2) and betaO(2) subunits differ not only in their tertiary structures but in their affinities for O(2).

Resonance Raman spectroscopic studies of hydroperoxo-myoglobin at cryogenic temperatures

In agreement with previous reports (Gasyna, Z. FEBS Lett. 1979, 106, 213-218 and Leibl, W.; Nitschke, W.; Huettermann, J. Biochim. Biophys. Acta 1986, 870, 20-30) radiolytically reduced samples of oxygenated myoglobin at cryogenic temperatures have been shown by optical absorption and EPR studies to produce directly the peroxo-bound myoglobin at 77 K. Annealing to temperatures near 185 K induces proton transfer, resulting in the formation of the hydroperoxo heme derivative. Resonance Raman studies of the annealed samples has permitted, for the first time, the direct observation of the key nu(Fe-O) stretching mode of the physiologically important Fe-OOH fragment of this ubiquitous intermediate. The assignment of this mode to a feature appearing at 617 cm(-1) is strongly supported by documentation of a 25 cm(-1) shift to lower energy upon substitution with (18)O(2) and by a 5 cm(-1) shift to lower energy for samples prepared in solutions of deuterated solvent.

Kinetic and spectroscopic characterization of a hydroperoxy compound in the reaction of native myoglobin with hydrogen peroxide

The reaction of metmyoglobin with H2O2 was investigated in a pH range between 8.5 and 6.0 with the aid of stopped flow-rapid scan and rapid freezing-EPR techniques. Singular value decomposition analyses of the stopped flow data at pH 8.5 revealed that a spectral species previously unknown accumulated during the reaction and exhibited a Soret absorption maximum at >/=423 nm. In the EPR experiments, the new species exhibited a set of g values at 2.32, 2.19, and 1.94, indicating that the species was assignable to a ferric hydroperoxy (Fe(III)[O-O-H]-) compound. In contrast, the hydroperoxy compound scarcely accumulated in the reaction at pH 6.0, and the dominant intermediate species accumulated was compound I, which was derived from the oxygen-oxygen bond cleavage of the hydroperoxy compound. The accumulated amount of the hydroperoxy compound relative to compound I showed a pH dependence with an apparent pKa (pKaapp) from 6.95 to 7.27 depending on the metmyoglobins examined. This variation in pKaapp paralleled that in pKa of the acid-alkaline transition (pKaAB) of metmyoglobins, suggesting that the accumulation of hydroperoxy compound is controlled by the distal histidine. We propose that the H2O2 activation by metmyoglobin is promoted at the acidic condition due to the imidazolium form of the distal histidine, and we further propose that the controlled protonation state of the distal histidine is important for the facile O-O bond cleavage in heme peroxidases.

Cryogenic absorption spectra of hydroperoxo-ferric heme oxygenase, the active intermediate of enzymatic heme oxygenation

Using radiolysis with (32)P enriched phosphate as an internal source of ionizing radiation, the formation of hydroperoxo-ferric complex from oxy-ferrous precursor with a high yield was monitored at 77 K in heme oxygenase (HO) by means of optical absorption spectroscopy. Well-resolved absorption spectra (maxima at 421 nm, 530 nm, 557 nm) of hydroperoxo-ferric intermediate of this heme enzyme were measured in 70% glycerol/buffer frozen glasses. After annealing at 210-215 K this complex converts to the product complex, alpha-meso hydroxyheme-HO. No heme degradation products were formed in control experiments with ferric HO or other heme proteins.

Formation and decay of hydroperoxo-ferric heme complex in horseradish peroxidase studied by cryoradiolysis

Using radiolytic reduction of the oxy-ferrous horseradish peroxidase (HRP) at 77 K, we observed the formation and decay of the putative intermediate, the hydroperoxo-ferric heme complex, often called "Compound 0." This intermediate is common for several different enzyme systems as the precursor of the Compound I (ferryl-oxo pi-cation radical) intermediate. EPR and UV-visible absorption spectra show that protonation of the primary intermediate of radiolytic reduction, the peroxo-ferric complex, to form the hydroperoxo-ferric complex is completed only after annealing at temperatures 150-180 K. After further annealing at 195-205 K, this complex directly transforms to ferric HRP without any observable intervening species. The lack of Compound I formation is explained by inability of the enzyme to deliver the second proton to the distal oxygen atom of hydroperoxide ligand, shown to be necessary for dioxygen bond heterolysis on the "oxidase pathway," which is non-physiological for HRP. Alternatively, the physiological substrate H2O2 brings both protons to the active site of HRP, and Compound I is subsequently formed via rearrangement of the proton from the proximal to the distal oxygen atom of the bound peroxide.

Characterization of the oxygenated intermediate of the thermophilic cytochrome P450 CYP119

Using UV-Vis, resonance Raman, and EPR spectroscopy we have studied the properties of the oxygenated ferrous cytochrome P450 from Sulfolobus solfataricus, (CYP119). The recently determined crystal structure of CYP119 is compared with other available structures of P450s, and detailed structural and spectroscopic analyses are reported. With several structural similarities to CYP102, such as in-plane iron position and a shorter iron-proximal ligand bond, CYP119 shows low-spin conformation preference in the ferric form and partially in the ferrous form at low temperatures. These structural features can explain the fast autoxidation of the oxyferrous complex of CYP119. Finally, we report the first UV-Vis and EPR spectra of the cryoradiolytically reduced oxygenated intermediate of CYP119. The primary reduced intermediate, a hydroperoxo-ferric complex of CYP119, undergoes a 'peroxide shunt' pathway during gradual annealing at 170-195 K and returns to the low-spin ferric form.

Cryotrapped reaction intermediates of cytochrome p450 studied by radiolytic reduction with phosphorus-32

Unstable reaction intermediates of the cytochrome P450 catalytic cycle have been prepared at cryogenic temperatures using radiolytic one-electron reduction of the oxy-P450 CYP101 complex. Since a rate-limiting step in the catalytic cycle of the enzyme is the reduction of the ferrous oxygenated heme protein, subsequent reaction intermediates do not normally accumulate. Using (60)Co gamma-irradiation, the primary reduced oxy-P450 species at 77 K has been identified as a superoxo- or hydroperoxo-Fe(3+)-heme complex (Davydov, R., Macdonald, I. D. G., Makris, T. M., Sligar, S. G., and Hoffman, B. M. (1999) J. Am. Chem. Soc. 121, 10654-10655). The electronic absorption spectroscopy is an essential tool to characterize cytochrome P450 intermediates and complements paramagnetic methods, which are blind to important diamagnetic or antiferromagnetically coupled states. We report a method of trapping unstable states of redox enzymes using phosphorus-32 as an internal source of electrons. We determine the UV-visible optical spectra of the reduced oxygenated state of CYP101 and show that the primary intermediate, a hydroperoxo-P450, is stable below 180 K and converts smoothly to the product complex at approximately 195 K. In the course of the thermal annealing, no spectral changes indicating the presence of oxoferryl species (the so-called compound I type spectrum) was observed.

The effects of xenobiotics on erythrocytes

1. Methemoglobin formation was observed when erythrocytes were incubated with xenobiotics such as hydroxylamines or phenols, other metabolites resulting from the interaction of these compounds with erythrocytes being reactive free radicals derived from the respective xenobiotic, and a ferryl-heme oxo-complex. 2. Steady-state levels of these reaction products depended on the permeability of the erythrocyte membrane for the various methemoglobin (MetHb) generators and the presence of antioxidants that downregulate the radicals formed. 3. Electron spin resonance (ESR) spectra of xenobiotic-derived free radicals could be obtained only from the readily water soluble hydroxylamines, whereas the poorly water soluble phenolic compounds did not allow the use of concentrations required for the generation of detectable amounts of ESR-sensitive metabolites in erythrocytes. 4. Previous investigations with oxyhemoglobin solutions and with the MetHb/H2O2 model systems have shown that, apart from ESR-sensitive radical species, excited reaction intermediates such as compound 1 ferryl hemoglobin can be detected as well by using chemiluminescence measurements. 5. A strong correlation was found between the intensity of the emitted light and the MetHb formation rate, indicating that the production of compound 1 ferryl hemoglobin is closely related to the MetHb formation step. 6. The sensitivity of the photon-counting method allowed measurements of excited species in intact erythrocytes not only with the readily soluble hydroxylamines, but also with the less soluble phenolic compounds. 7. In addition, parameters indicative of xenobiotic-induced oxidative alterations were found: a significant decrease in intraerythrocytic thiol levels was a result of all compounds that initiate MetHb formation, as also described for slowly reacting xenobiotics. 8. With the most reactive compound investigated, unsubstituted hydroxylamine, a significant release of iron from the oxidatively modified hemoglobin was detected, facilitated by binding of this transition metal to hydroxylamine and its final oxidation product, nitric oxide. 9. The use of the ESR spin-labeling technique revealed membrane alterations of erythrocytes exposed to the reducing MetHb generators presented in this study. 10. A direct action of BHA and BHT on the integrity of the erythrocyte membrane was observed, leading to hemolysis independent of the formation of prooxidant species. 11. The presence of strong prooxidants (radicals) was indicated both by fluidity changes in the membrane and by an oxidative decrease in cytosolic thiol levels.

Hydroxylamine and phenol-induced formation of methemoglobin and free radical intermediates in erythrocytes

As previously shown with isolated oxyhemoglobin, methemoglobin formation can also be induced in intact erythrocytes by hydroxylamine compounds and substituted phenols such as butylated hydroxyanisole (BHA). Electron spin resonance investigations revealed that, accordingly, free radical intermediates were formed in erythrocytes from hydroxylamine, N,N-dimethylhydroxylamine, and N-hydroxyurea. Due to the low stability of the dihydronitroxyl radicals, their detection required the use of a continuous flow system and relatively high amounts of the reactants. As has already been demonstrated with the solubilized hemoglobin system, hemoglobin of intact erythrocytes also reacts with the more hydrophilic xenobiotics such as hydroxylamine. However, the reaction rate was slightly reduced, indicating the existence of an incomplete permeability barrier for these compounds. The limited solubility of phenolic compounds in the aqueous buffer of suspended erythrocytes (in combination with the strict requirement of osmolarity in order to prevent hemolysis) impeded the direct detection of the respective phenoxyl radicals previously reported in hemoglobin solutions. However, in accordance with earlier findings in homogeneous reaction systems, chemiluminescence was observed as well, indicating the existence of a further reaction intermediate, which was also obtained in pure hemoglobin solutions when mixed with the respective reactants. As has recently been demonstrated, this light emission is indicative of the existence of highly prooxidative compound I intermediates during methemoglobin formation. Prooxidant formation in erythrocytes is reflected by a significant decrease in thiol levels even with those compounds where free radical formation was not directly detectable by ESR spectroscopy. The use of the spin-labeling technique revealed membrane effects as a result of oxidative stress. Oxidative metabolism of hemoglobin with hydroxylamine caused a release of low molecular weight iron. The marked hemolysis observed in the presence of BHA results from a direct membrane effect of this compound rather than a consequence of free radical-induced oxidative stress. A correlation of the different results is discussed in terms of possible toxicological consequences.

Reactions of reducing xenobiotics with oxymyoglobin. Formation of metmyoglobin, ferryl myoglobin and free radicals: an electron spin resonance and chemiluminescence study

The oxygen-haem centre of oxymyoglobin reacts with reducing xenobiotics such as hydroxylamines and phenols with the concomitant formation of metmyoglobin and oxidation of the respective xenobiotic. Metmyoglobin formation rates were measured by visible spectroscopy with xenobiotic concentrations ranging from 100 microM to 30 mM. Analogous to previous results obtained with oxyhaemoglobin, the first step in the reaction of hydroxylamines with oxymyoglobin leads to the formation of the one-electron oxidation product of hydroxylamine, a nitroxyl radical detectable by electron spin resonance. A variety of paramagnetic secondary products were also found. The terminal oxidation product of hydroxylamine and hydroxyurea was the myoglobin-nitric oxide complex, one showing similar spectral characteristics to the analogous haemoglobin-nitric oxide adduct found in our previous experiments. On the other hand, the amount of low-spin ferric complexes obtained from metmyoglobin and an excess of the respective hydroxylamine was considerably lower than the corresponding results with methaemoglobin. A second important reaction intermediate was the compound I-type ferryl haem-species detected by a recently-published chemiluminescence assay. Partial spectral resolution of the emitted light using a set of cut-off filters indicated that maximum light emission occurred above 600 nm, most probably involving excited porphyrin states. The intensity of oxymyoglobin-related light emission was considerably higher than that reported earlier with oxyhaemoglobin. This indicates a difference in the excitation mechanism which leads to the formation of the compound I-type ferry haem species.

Chemiluminescence from activated heme compounds detected in the reaction of various xenobiotics with oxyhemoglobin: comparison with several heme/hydrogen peroxide systems

Chemiluminescence was detected in the reaction of oxyhemoglobin with various hydroxylamines and phenols, which have previously been shown to produce free radicals. The emitted light intensity correlated roughly with the methemoglobin formation rate, indicating the involvement of a photoemissive species as a reaction intermediate. In our previous work, we postulated the involvement of a catalase-insensitive, heme-bound hydrogen peroxide species in the methemoglobin formation reaction. In a series of experiments, we showed that intensive chemiluminescence occurred when hydrogen peroxide was mixed with either methemoglobin or metmyoglobin but not with hematin, which lacks the globin moiety. This suggests the involvement of the globin moiety in the light-emitting reaction sequence. The detection of paramagnetic globin species exhibiting similar kinetics as the corresponding light-emitting compound demonstrated that the assumed H2O2-heme compound has strong oxidizing properties. Accordingly, addition of bovine serum albumin to the hematin-hydrogen peroxide system also resulted in a strong chemiluminescence due to the formation of a paramagnetic transient species which could be detected by electron spin resonance (ESR). Several other heme compounds, such as cytochrome c or cytochrome c oxidase which have no vacant ligand site, did not show any light emission under similar conditions. This means that hydrogen peroxide must have access to a free-binding position on the heme. Chemiluminescence most probably stems from the transition of the initially formed heme-H2O2 adduct to the compound II type species. Due to their oxidizing nature, these species might be responsible for deleterious toxic effects such as lipid peroxidation and protein degradation.

One-electron reduction of chloroperoxidase by radiolytically generated electrons

Upon irradiation of aqueous ethylene glycol/water solutions of native chloroperoxidase (CPO) with 60Co-gamma rays at 77K one observes the one-electron reduction of the enzyme active site by radiolytically generated thermolyzed electrons. In the present study the first absorption spectrum of a low-spin ferrous form of CPO is reported which has peaks at 438, 532 and 563 nm, similar to those observed previously for cytochrome P-450. All previously described ferrous forms of CPO are high spin. In order to observe the final results of the CPO reaction with electrons, the spectral changes of native enzyme after room temperature-gamma-irradiation have also been investigated. Evidence of changes is also presented probably connected with disruption of the tertiary structure of enzyme, correlated with decrease of enzyme activity.

A possible model of hemoprotein-peroxide complexes formed in an iron-tetraphenylporphyrin system

A ferric low-spin species with an anomalously small g3-g1 separation generated by the reaction of Fe(III)tetraphenylporphyrin with t-butylhydroperoxide in the presence of tatramethylammonium hydroxide was characterized by ESR spectroscopy. The reaction kinetics, investigated by monitoring ESR intensity, indicated that this low-spin complex is highly reactive and easily changed to non-heme type iron complexes. The rhombic parameters of this complex are very similar to those of heme-peroxide adducts such as [Fe(II)-hemoglobin-O2]- and [Fe(II)-horseradish peroxidase-O2]-.