Reactions of the Protein Radical in Peroxide-treated Myoglobin

Stimulation by nitroxides of catalase-like activity of hemeproteins. Kinetics and mechanism

The ability of stable nitroxide radicals to detoxify hypervalent heme proteins such as ferrylmyoglobin (MbFeIV) produced in the reaction of metmyoglobin (MbFeIII) and H2O2 was evaluated by monitoring O2 evolution, H2O2 depletion, and redox changes of the heme prosthetic group. The rate of H2O2 depletion and O2 evolution catalyzed by MbFeIII was enhanced by stable nitroxides such as 4-OH-2,2,6,6-tetramethyl-piperidinoxyl (TPL) in a catalytic fashion. The reduction of MbFeIV to MbFeIII was the rate-limiting step. Excess TPL over MbFeIII enhanced catalase-like activity more than 4-fold. During dismutation of H2O2, [TPL] and [MbFeIV] remained constant. NADH caused: (a) inhibition of H2O2 decay; (b) progressive reduction of TPL to its respective hydroxylamine TPL-H; and (c) arrest/inhibition of oxygen evolution or elicit consumption of O2. Following depletion of NADH the evolution of O2 resumed, and the initial concentration of TPL was restored. Kinetic analysis showed that two distinct forms of MbFeIV might be involved in the process. In summary, by shuttling between two oxidation states, namely nitroxide and oxoammonium cation, stable nitroxides enhance the catalase mimic activity of MbFeIII, thus facilitating H2O2 dismutation accompanied by O2 evolution and providing protection against hypervalent heme proteins.

An EPR investigation of human methaemoglobin oxidation by hydrogen peroxide: methods to quantify all paramagnetic species observed in the reaction

The method of Electron Paramagnetic Resonance (EPR) spectroscopy was used to study the reaction of human methaemoglobin (metHb) with hydrogen peroxide. The samples for EPR measurements were rapidly frozen in liquid nitrogen at different times after H2O2 was added at 3- and 10-fold molar excess to 100 microM metHb in 50 mM phosphate buffer, pH 7.4, 37 degrees C. Precautions were taken to remove all catalase from the haemoglobin preparation and no molecular oxygen evolution was detected during the reaction. On addition of H2O2 the EPR signals (-196 degrees C) of both high spin and low spin metHb rapidly decreased and free radicals were formed. The low temperature (-196 degrees C) EPR spectrum of the free radicals formed in the reaction has been deconvoluted into two individual EPR signals, one being an anisotropic signal (g parallel = 2.035 and g perpendicular = 2.0053), and the other an isotropic singlet (g = 2.0042, delta H = 20 G). The former signal was assigned to peroxyl radicals. As the kinetic behaviour of both peroxyl (ROO.) and non-peroxyl (P.) free radicals were similar, we concluded that ROO. radicals are not formed from P. radicals by addition of O2. The time courses for both radicals showed a steady state during the time required for H2O2 to decompose. Once all peroxide was consumed, the radical decayed with a first order rate constant of 1.42 x 10(-3) s-1 (1:3 molar ratio). The level of the steady state was higher and its duration shorter at lower initial concentration of H2O2. The formation of the rhombic Fe(III) non-haem centres with g = 4.35 was found. Their yield was proportional to the H2O2 concentration used and the centres were ascribed to haem degradation products. The reaction was also monitored by EPR spectroscopy at room temperature. The kinetics of the free radicals measured in the reaction mixture at room temperature was similar to that observed when the fast freezing method and EPR measurement at -196 degrees C were used.

Covalent crosslinking of the heme prosthetic group to myoglobin by H2O2: toxicological implications

It is known that treatment of myoglobin with H2O2 leads to covalent alteration of the heme prosthetic group with concomitant formation of a protein bound heme adduct and transforms myoglobin from an oxygen storage protein to an oxidase. In the current study it was shown, with the use of 14C-labeled heme reconstituted into apomyoglobin, that up to 88% of the oxidatively altered heme can be accounted for by the protein bound product. Furthermore, a partially purified preparation of the protein bound heme adduct was introduced into human fibroblasts using the method of osmotic lysis of pinosomes and found to cause cell death (40%) within 1 h, as evidenced by trypan blue exclusion. Native myoglobin introduced into cells in the same manner or extracellular treatment by the protein bound heme adduct had no effect on cell viability. The extent of cell death could be decreased (50%) by N-acetyl-L-cysteine, indicating a potential role for reactive oxygen intermediates in this process. These results show that the covalently altered myoglobin can elicit cellular damage and suggests that similar processes may occur in vivo in pathologic conditions such as that involving cardiac ischemia and reperfusion injury, where covalently altered myoglobin may form.

Reaction of ferric leghemoglobin with H2O2: formation of heme-protein cross-links and dimeric species

Ferric leghemoglobin in the presence of H2O2 is known to give rise to protein radicals, at least one of which is centred on a tyrosine residue. These radicals are quenched by at least two processes. The first one involves an intramolecular heme-protein cross-link probably involving the tyrosine radical; this leads to the formation of a green compound with spectral characteristics differing markedly from those of ferryl and ferric leghemoglobin. This green compound cannot be reduced by dithionite or ascorbate, precluding any role for this species as an oxygen carrier. It exhibits modified EPR and pyridine haemochromogen spectra, indicating that alterations occur at the porphyrin macrocycle level. The additional compound previously described [Puppo, A., Monny, C. and Davies, M.J. (1993) Biochem. J. 289, 435-438] appears to be a mixture of ferry Lb and this green compound. The second quenching route results in the formation of intermolecular cross-links and hence dimeric forms of the protein. Ascorbate and glutathione inhibit both this intermolecular dimer formation and the formation of the intramolecular haem-protein cross-links and are likely to play a protective role in vivo.

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.

Reduction of ferrylmyoglobin and ferrylhemoglobin by nitric oxide: a protective mechanism against ferryl hemoprotein-induced oxidations

The reactions of metmyoglobin (metMb) and methemoglobin (metHb), oxidized to their respective oxoferryl free radical species (.Mb-FeIV = O/.Hb-4FeIV = O) by tert-butyl hydroperoxide (t-BuOOH), with nitric oxide (NO.) were studied by a combination of optical, electron spin resonance (ESR), ionspray mass (MS), fluorescence, and chemiluminescence spectrometries to gain insight into the mechanism by which NO. protects against oxidative injury produced by .Mb-FeIV = O/.Hb-4FeIV = O. Oxidation of metMb/metHb by t-BuOOH in a nitrogen atmosphere proceeded via the formation of two protein electrophilic centers, which were heme oxoferryl and the apoprotein radical centered at tyrosine (for the .Mb-FeIV = O form, the g value was calculated to be 2.0057), and was accompanied by the formation of t-BuOOH-derived tert-butyl(per)oxyl radicals. We hypothesized that NO. may reduce both oxoferryl and apoprotein free radical electrophilic centers of .Mb-FeIV = O/.Hb-4FeIV = O and eliminate tert-butyl(per)oxyl radicals, thus protecting against oxidative damage. We found that NO. reduced .Mb-FeIV = O/.Hb-4FeIV = O to their respective ferric (met) forms and prevented the following: (i) oxidation of cis-parinaric acid (PnA) in liposomes, (ii) oxidation of luminol, and (iii) formation of the tert-butyl(per)oxyl adduct with the spin trap DMPO. NO. eliminated the signals of tyrosyl radical detected by ESR and oxoferryl detected by MS in the reaction of t-BuOOH with metMb. As evidenced by MS of apomyoglobin, this effect was due to the two-electron reduction of .Mb-FeIV = O by NO. at the oxoferryl center rather than to nitrosylation of the tyrosine residues. Results of our in vitro experiments suggest that NO. exhibits a potent, targetable antioxidant effect against oxidative damage produced by oxoferryl Mb/Hb.

The role of H2O2-generated myoglobin radical in crosslinking of myosin

The mechanism of myoglobin/H2O2 derived peroxidation of myosin was studied by comparing the catalytic activity of myoglobin and horseradish peroxidase using O-dianisidine, N-acetyl tyrosine and myosin as substrates. It was found that both hemoproteins induced myosin crosslinking and concomitant tyrosines oxidation to bityrosines, suggesting inter-molecular coupling of tyrosines in the crosslinking. The enzymatic activity of both hemoproteins on myosin was weak compared to small substrates. While horseradish peroxidase was much more active than myoglobin on small substrates, the reverse was true for myosin peroxidation. Since the suicidal interaction of myoglobin with H2O2 forms unstable tyrosine radicals, we suggest that the increased activity of myoglobin on myosin results from an efficient electron transfer between surface tyrosines of myosin and myoglobin but not horseradish peroxidase. These conclusions were supported by evidence that sperm whale myoglobin, which contains two active tyrosines--the heme-adjacent (tyrosine-103) and the surface (tyrosine-151), is more active as a mediator of myosin peroxidation than horse heart myoglobin which is devoid of the surface tyrosine.

A hydrogen-donating monohydroxamate scavenges ferryl myoglobin radicals

The addition of 25 microM hydrogen peroxide to 20 microM metmyoglobin produces ferryl (FeIV = O) myoglobin. Optical spectroscopy shows that the ferryl species reaches a maximum concentration (60-70% of total haem) after 10 minutes and decays slowly (hours). Low temperature EPR spectroscopy of the high spin metmyoglobin (g = 6) signal is consistent with these findings. At this low peroxide concentration there is no evidence for iron release from the haem. At least two free radicals are detectable by EPR immediately after H2O2 addition, but decay completely after ten minutes. However, a longer-lived radical is observed at lower concentrations that is still present after 90 minutes. The monohydroxamate N-methylbutyro-hydroxamic acid (NMBH) increases the rate of decay of the fenyl species. In the presence of NMBH, none of the protein-bound free radicals are detectable; instead nitroxide radicals produced by oxidation of the hydroxamate group are observed. Similar results are observed with the trihydroxamate, desferrioxamine. "Ferryl myoglobin" is still able to initiate lipid peroxidation even after the short-lived protein free radicals are no longer detectable (E.S.R. Newman, C.A. Rice-Evans and M.J. Davies (1991) Biochemical and Biophysical Research Communications 179, 1414-1419). It is suggested that the longer-lived protein radicals described here may be partly responsible for this effect. The mechanism of inhibition of initiation of lipid peroxidation by hydroxyamate drugs, such as NMBH, may therefore be due to reduction of the protein-derived radicals, rather than reduction of ferryl haem.

Differential susceptibilities of the prosthetic heme of hemoglobin-based red cell substitutes. Implications in the design of safer agents

One approach to the development of an effective red cell substitute has been chemical modification of human hemoglobin to optimize oxygen transport and plasma half-life. Human hemoglobin A0 and two of these modified hemoglobins, one prepared from the cross-linking of the alpha-chains at lysine residue 99 by bis(3,5-dibromosalicyl)fumarate (Hb-DBBF) and the other by acylation of lysine residue 82 of the beta-chain by mono-(3,5-dibromosalicyl)fumarate (Hb-FMDA), were tested by HPLC for their susceptibility to oxidative damage caused by H2O2. Such oxidative insult may occur during ischemia and reperfusion of tissues after transfusion of red cell substitutes to patients with hypovolemic shock and trauma. Hb-DBBF was extremely susceptible to damage of its heme and protein moieties with stoichiometric amounts of H2O2, whereas Hb-FMDA was highly resistant, even at 10-fold molar excess and at an acidic pH of 4.7. Hemoglobin A0 was of intermediate susceptibility, exhibiting alteration of heme and protein moieties at acidic but not neutral pH. Since the degradation of heme can release the potentially toxic agent iron, Hb-FMDA may be a more promising candidate than Hb-DBBF for development as a red cell substitute. A similar approach may be used to assess the susceptibility of other hemoglobin-based red cell substitutes to oxidative damage in order to determine the molecular basis of heme and protein alteration.

Detection and reactions of the globin radical in haemoglobin

The reaction of methaemoglobin with hydrogen peroxide and other oxidants has been studied using both electron paramagnetic resonance (EPR) and optical spectroscopy. The results obtained are consistent with the formation of an iron(IV)-oxo species (which is one oxidising equivalent above the initial level) and rapid transfer of the second oxidising equivalent into the surrounding globin generating a protein radical; this species has been observed by stopped-flow EPR. The partially resolved hyperfine splittings of the EPR signal (a2H 0.66, a2H 0.17, aH 1.157, aH 0.203 mT), together with its g value (2.0044) suggest that this species is a sterically-constrained tyrosine phenoxyl radical. Experiments with inhibitors and chemically-modified haemoglobins are in agreement with this assignment. This radical is not observed with the apoprotein or oxyhaemoglobin, confirming that the reaction requires the presence of an iron(III) haem. The concentration of the phenoxyl radical is not affected by hydroxyl-radical scavengers but is affected by certain reducing agents and antioxidants, demonstrating that the protein radical is accessible to reagents in bulk solution. Analysis of the protein structure suggests that this radical may be centered on the tyrosine at alpha-42 as this residue is in close proximity to the haem groups and partially exposed on the surface. Addition of the spin trap DMPO to the reaction system results in the observation of a broad, anisotropic, spectrum from a protein-derived spin adduct; this signal is assigned to a peroxyl radical adduct on the basis of the hyperfine coupling constants (aN 2.03, aH 1.4 mT), its short life-time, the detection of oxygen uptake, and the decrease in the intensity of this signal under anoxic conditions. Experiments with modified haemoproteins and inhibitors suggest that this species arises via the tyrosine phenoxyl radical. These observations suggest that the tyrosine residues act as a 'sink' for oxidising equivalents generated by electron-transfer within the protein after initial oxidation at the haem centre.

Identification of the site of the globin-derived radical in leghaemoglobins

Reaction of the Fe3+ form of the oxygen-carrying protein leghaemoglobin (MetLb), derived from the root nodules of lupins, with H2O2 is shown to generate, in addition to an iron (IV)-oxo (ferryl) species, a globin radical. This radical has been detected by EPR spectroscopy and is analogous to the species previously observed with the soybean protein. Analysis of the hyperfine coupling constants and g value of the EPR signal, together with computer simulations and the similarity of the observed spectra of that detected with the soybean form suggest that this species is also a tyrosine-derived phenoxyl radical; this species is believed to arise via an electron-transfer process within the protein with an electron being transferred from the tyrosine residue to an initially-generated Compound-1-type species. Comparison of the protein sequences and structures of the two proteins show that there is only one conserved tyrosine residue (at position 133 in the soybean and 138 in the lupin); this is believed to be the site of the phenoxyl radical. The lupin phenoxyl radical reacts with added water-soluble antioxidants and reducing agents which result in repair of the radical; this may be an important protective mechanism in vivo. Analysis of molecular models of the protein structures is in accord with both the assignment of the radical to this conserved tyrosine residue and the observed radical reactivity.

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.

Evidence for the structure of the active site heme P460 in hydroxylamine oxidoreductase of Nitrosomonas

Hydroxylamine oxidoreductase (HAO) is responsible for the oxidation of hydroxylamine to nitrite in nitrification by Nitrosomonas europaea. It has an alpha n subunit structure and eight covalently bound hemes per subunit. Seven of these have visible spectra indistinguishable from heme c. The eighth, designated as P460, has unusual visible spectroscopic features in the enzyme and in a heme-containing proteolytic fragment. Its structure has not been previously determined. Enzymatic digestions of HAO were performed, and various proteolytic fragments were purified. Mass spectrometry confirmed the presence of authentic heme c in some fragments, that is, iron protoporphyrin IX cross-linked by two thioether bonds to cysteine residues. It was possible to detect the presence of the P460 pigment in some fragments, based upon the sensitivity of this pigment to treatment of the holoenzyme with hydrogen peroxide. A proteolytic fragment produced by sequential digestion with trypsin and pronase was shown to contain heme c and a hydrogen peroxide-sensitive heme with an unusual visible spectrum. This fragment contained two covalently cross-linked peptides. Mass spectrometry and NMR indicated that the P460 heme was iron protoporphyrin IX covalently bonded by two thioether bridges to peptide, but in addition there was a new, third covalent bond between a meso heme carbon and an aromatic ring carbon on a tyrosyl residue. The new covalent bond has been tentatively assigned to the C2 carbon of the tyrosyl ring and the 5-meso heme carbon (IUPAC-IUB tetrapyrrole nomenclature), although this location requires further proof.

Decomposition of hydrogen peroxide by metmyoglobin: a cyclic formation of the ferryl intermediate

1. Metmyoglobin reacted with H2O2 to form ferryl-myoglobin, which reverted back spontaneously to the met-form. 2. Through this cyclic reaction of myoglobin between metMb(III) and ferryl-Mb(IV), we proposed that H2O2, one of the potent oxidants in vivo, can be decomposed continuously in cardiac and skeletal muscle tissues in the absence of catalase and peroxidase.

The interaction of Trolox C, a water-soluble vitamin E analog, with ferrylmyoglobin: reduction of the oxoferryl moiety

The oxidation of the heme iron of metmyoglobin by H2O2 yields an oxo ferryl complex (FeIV = O), similar to Compound II of peroxidases, as well as a protein radical; this high oxidation state of myoglobin is known as ferrylmyoglobin. The interaction of Trolox, a water-soluble vitamin E analog, with ferrylmyoglobin entailed two sequential one-electron oxidations of the phenolic antioxidant with intermediate formation of a phenoxyl radical and accumulation of a quinone end product. These oxidation reactions were linked to individual reductions of ferrylmyoglobin to metmyoglobin, as indicated by the value of the relationship [metmyoglobin]formed/[Trolox]consumed: 1.92 +/- 0.28. The Trolox-mediated reduction of ferrylmyoglobin to metmyoglobin could proceed directly, i.e., electron transfer from the phenolic-OH group in Trolox to the oxoferryl moiety, or indirectly, i.e., sequential electron transfer from Trolox to a protein radical to the oxoferryl moiety. The former mechanism is supported by the finding that the high oxidation heme iron is reduced under conditions where the tyrosyl residues are blocked by o-acetylation and when hemin is substituted for myoglobin. The latter mechanism is consistent with the following observations: (a) the EPR signal ascribed to the protein radical is suppressed by Trolox, with the concomitant appearance of the EPR spectrum of the Trolox phenoxyl radical and (b) the rate of ferrylmyoglobin reduction by Trolox is decreased with increasing number of tyrosyl residues in the proteins of horse myoglobin (titrated by o-acetylation) and sperm whale myoglobin. The apparent discrepancy between these observations can be reconciled by considering that both electrophilic centers in ferrylmyoglobin--the oxoferryl heme moiety and the protein radical--function independently of each other and that recovery of ferrylmyoglobin by Trolox could be effected through the tyrosyl residues, albeit at slower rates. The mechanistic aspects of these results are discussed in terms of the two main redox transitions in the myoglobin molecule encompassing valence changes of the heme iron and electron transfer of the tyrosyl residue in the protein and linked to the two sequential one-electron oxidations of Trolox.

Coupling of dihydroriboflavin oxidation to the formation of the higher valence states of hemeproteins

The reactions between hydrogen peroxide and hemeproteins have been coupled to the oxidation of dihydroriboflavin so as to provide a simple method for measuring the rate constant of hemeprotein peroxidation. Dihydroriboflavin rapidly reduces the higher oxidation states of iron and the hydroxy radicals which are the products of the hemeprotein/hydrogen peroxide reaction. The rapid reduction of these highly reactive compounds prevents the hemeproteins from undergoing irreversible chemical modifications and thus allows the kinetics of peroxidation to be studied. The rate constants at pH 7.2 and 23 degrees C for the peroxidation of horseradish peroxidase, myoglobin, and ferrocytochrome c are found to be 6.2 x 10(6), 7.5 x 10(4), and 8 x 10(3)M-1s-1, respectively. These studies suggest that reduced riboflavin might efficiently protect cells from oxidative damage such as that occurring in inflammation and reperfusion injury.

Mechanistic aspects of the oxidation of phenothiazine derivatives by methemoglobin in the presence of hydrogen peroxide

Mechanistic aspects of the reaction of hydrogen peroxide with methemoglobin with respect to phenothiazine oxidation have been studied. Three phenothiazines, methoxy- (MoPZ), chlor- (CPZ) and methoxycarbonylpromazine (MaPZ), have been used. These phenothiazines differ only in substitution at the 2-position, which contributes substantially to the electron-donating properties of these compounds. Reaction with hydrogen peroxide oxidizes methemoglobin to ferrylhemoglobin, which contains iron(IV)-oxo porphyrin moiety and a protein radical. The phenothiazines are oxidized by ferrylhemoglobin in the presence of H2O2 mainly to their sulfoxides, with a radical cation as intermediate. The conversion rates (MoPZ greater than CPZ greater than MaPZ) decrease with the electron-withdrawing ability of the 2-substituent, as indicated by Hammett sigma para values. Hydrogen peroxide consumption during the reaction is similar for the three phenothiazines. Denaturation reactions that occur upon exposure of methemoglobin to hydrogen peroxide have been investigated. For this heme-protein cross-linking was studied by means of heme retention by the protein after methyl ethyl ketone extraction. Furthermore, oxygen consumption during the reaction was assayed, which indicates formation of protein-peroxy radicals. The extent of both heme-protein cross-linking and oxygen consumption is decreased by phenothiazines in the same order as the phenothiazine conversion rate. CPZ sulfoxide is not converted by methemoglobin in the presence of hydrogen peroxide, and CPZ sulfoxide shows no effect on heme-protein cross-linking and oxygen consumption. The results are explained by electron transfer from phenothiazine to the protein radical. Stronger electron donors (MoPZ greater than CPZ greater than MaPZ) are converted faster and by reducing the protein radical they better protect hemoglobin against denaturation. A catalytic cycle, that takes into account our observation and the existing knowledge of hemoglobin oxidation states, is presented.

Oxidation of chlorpromazine by methemoglobin in the presence of hydrogen peroxide. Formation of chlorpromazine radical cation and its covalent binding to methemoglobin

The oxidation of chlorpromazine by methemoglobin plus H2O2 has been studied. The transient formation of the chlorpromazine radical cation in this reaction has been demonstrated by light absorption measurements. Under the experimental conditions complete conversion of chlorpromazine yields approximately 60% chlorpromazine sulfoxide. From studies with 3H-labeled chlorpromazine it appears that the remaining 40% is covalently bound to apohemoglobin. Upon reaction of methemoglobin with H2O2 a stable ferrylhemoglobin is formed. This ferrylhemoglobin is not the reactive species, which accepts the chlorpromazine electron, as its presence is not sufficient to induce chlorpromazine oxidation. For this the presence of H2O2 is a prerequisite. This indicates that a transient species in the formation of the stable ferrylhemoglobin is involved, whether this is a compound I analogue or a ferrylhemoglobin with a free radical on one of the apoprotein residues. Exposition of methemoglobin to H2O2 denatures hemoglobin and induces protein-heme crosslinks, as appears from changes in the visible absorption spectrum and heme retention by the protein after methyl ethyl ketone extraction. Reaction with CPZ partly protects against denaturation and crosslinking.

Irreversible binding of heme to microsomal protein during inactivation of cytochrome P450 by 4-alkyl analogues of 3,5-diethoxycarbonyl-1,4-dihydro-2,4,6-trimethylpyridine

The porphyrinogenicity of 4-alkyl analogues of 3,5-diethoxycarbonyl-1,4-dihydro-2,4,6-trimethylpyridine (DDC) is related to the process of mechanism-based destruction of cytochrome P450 (P450) heme, accompanied by conversion of heme to N-alkylprotoporphyrins (N-alkylPPs). Certain DDC analogues (4-isopropyl, 4-isobutyl, 4-hexyl) are weakly porphyrinogenic in comparison to the potent porphyrinogen, 4-ethyl DDC. We have examined the abilities of these DDC analogues to promote irreversible binding of radiolabeled heme to protein in rat liver microsomal preparations. The goals of this study were to determine whether DDC analogues with different porphyrinogenicities differ in the extents to which they cause heme adduct formation, and whether P450 isozymes differ in their capacities to catalyze heme covalent binding. Incubation of microsomes with NADPH alone promoted heme covalent binding, while loss of spectral P450 heme was minimal or absent. In microsomal incubations containing NADPH, the 4-ethyl, 4-isopropyl, and 4-isobutyl analogues caused heme covalent binding to extents which paralleled their P450 destructive activities. In contrast, 4-hexyl DDC caused less heme covalent binding as a function of P450 loss than the other analogues in microsomes from untreated and beta-naphthoflavone (beta NF)-treated rats. Thus, the weakly porphyrinogenic DDC analogues do not cause greater heme covalent binding than 4-ethyl DDC. Weak porphyrinogenicity, therefore, cannot be explained by diversion of the heme moiety of P450 from conversion to N-alkylPPs towards utilization for formation of heme-derived protein adducts. Treatment of rats with P450 inducing agents altered the degree to which DDC analogues caused heme covalent binding. The greatest heme adduct formation occurred in microsomes from untreated and dexamethasone (DEX)-treated rats, whereas treatment with phenobarbital and especially beta NF reduced heme covalent binding as a function of P450 loss. Thus, these microsomal studies suggest that constitutive P450 isozymes and members of the DEX-inducible P450IIIA subfamily appear to catalyze heme covalent binding, while beta NF-inducible forms such as P450IA1 (P450c) seem to be relatively inactive in this regard.

Free radical modification of prosthetic heme groups

Hemoproteins catalyze reductive and oxidative one-electron transformations. Not infrequently, the radicals produced by these one-electron reactions add to the prosthetic heme group of the enzyme and modify or terminate its catalytic function. Reactions of the radicals with the heme group include additions to the iron atom, pyrrole nitrogens, pyrrole carbons, vinyl groups, and meso carbons. The radicals involved in these reactions derive from the oxidizing agent, the substrate, or the amino acid residues of the catalytic site. The mechanism by which the radicals are generated, their steric and electronic properties, and the extent to which they have access to the heme group determine the nature and regiospecificity of the reaction. The reaction of heme prosthetic groups with radicals is relevant to the inhibition of hemoprotein enzymes, the normal and pathological degradation of heme, and our understanding of hemoprotein function.