The reduction of ferryl myoglobin by ergothioneine: a novel function for ergothioneine

One-electron oxidation of ergothioneine and analogues investigated by pulse radiolysis: redox reaction involving ergothioneine and vitamin C

Redox reactions of endogenous and exogenous sulphur-containing compounds are involved in protection against oxidative damage arising from the incidence and/or treatment of many diseases, including cancer. We have investigated, via pulse radiolysis, the one-electron oxidation of ergothioneine, a molecule with antioxidant properties which is detected at millimolar concentrations in certain tissues and fluids subject to oxidative stress, including erythrocytes and plasma. The spectrum of the transient species, assigned to the product of one-electron oxidation, observed after reaction of ergothioneine with the oxidizing radicals OH., N3. and CCl3O2. has a maximum absorption at 520 nm and is very similar to that obtained by oxidation of analogous molecules such as 2-mercaptoimidazole, 1-methyl-2-mercaptoimidazole, S-methyl- and S,N-dimethyl-ergothioneine. In the presence of vitamin C, the oxidized form of ergothioneine is repaired by a rapid reduction (k = 6.3 x 10(8) M(-1).s(-1)) producing ascorbyl radicals. This co-operative interaction between ergothionine and ascorbate, similar to that previously observed between vitamin E and ascorbate, may contribute to essential biological redox protection.

Increased levels of redox-active iron in follicular fluid: a possible cause of free radical-mediated infertility in beta-thalassemia major

OBJECTIVE

Our purpose was to investigate the follicular fluid parameters associated with redox activity and the consequent production of the deleterious hydroxyl radical in beta-thalassemia major.

STUDY DESIGN

The levels of ferritin, total iron, total copper, and redox-active iron were measured in follicular fluid aspirated from three follicles during three consecutive ovum pickups from a patient with beta-thalassemia major and were compared with the levels in nine follicles aspirates from nine healthy control patients. The redox activity in the follicular fluid samples was monitored by the extent of follicular fluid-mediated deoxyribonucleic acid degradation and salicylate hydroxylation.

RESULTS

Total iron and ferritin concentrations were elevated in thalassemic follicular fluid samples compared with control samples (6.7 fold, and 53.3-fold, respectively), whereas the total copper concentration was similar. Thalassemic follicular fluid samples exhibited a marked increase of redox activity, indicating a higher potential of free radical production leading to deoxyribonucleic acid degradation. Likewise, free radical-induced conversion of salicylate to dihydroxybenzoic acid derivatives was enhanced in the thalassemic follicular fluid samples compared with controls (2,3-dihydroxybenzoic acid: 67.7 +/- 22 vs 20.3 +/- 12.9 ng/mg protein; 2,5-dihydroxybenzoic acid: 101.6 +/- 25.9 vs 4.42 +/- 2.7 ng/mg protein).

CONCLUSIONS

The increased level of redox activity found in the follicular fluid from a patient with beta-thalassemia major focuses the attention on the small fraction of redox-active iron ions as mediators of free radical production, inducing tissue injury and possibly contributing to impairment of reproduction in these patients.

Inhibition of metmyoglobin/H2O2-dependent low density lipoprotein lipid peroxidation by naturally occurring phenolic acids

The ferrylmyoglobin <==> metmyoglobin redox transitions promoted by hydrogen peroxide and dietary phenolic acids and their potential role in the oxidation of LDL were studied. The use of parinaric acid incorporated in LDL as a probe for radicals (detected by fluorescence quenching of the probe) revealed an oxidative stress inside LDL shortly ( < 1 min) after addition of hydrogen peroxide to metmyoglobin in the aqueous phase outside the particle, reflecting an efficient access of the oxidant to LDL lipids. However, the propagation step of peroxidation only occurs after a lag phase, as detected by the kinetics of oxygen consumption. Triton X-100 decreases but does not suppress the lag phase of oxidation. Addition of metmyoglobin (without peroxide) to LDL was not followed by significant oxidation during the time of the experiment, unless Triton X-100 was present in the medium. When dietary phenolic acids were present in the medium before peroxide addition, an inhibition of parinaric acid fluorescence quenching and oxygen consumption was recorded as a function of concentration and substitution pattern on the phenol ring of the phenolic acids. This was associated with a conversion of ferrylmyoglobin to metmyoglobin. The results indicate that the naturally occurring phenolic acids prevent ferrylmyoglobin-dependent LDL oxidation in a way strongly dependent on the substitution pattern on the phenol ring. Among the phenolic compounds studied, the o-dihydroxy derivatives of cinnamic and benzoic acids (caffeic, chlorogenic, and protocatechuic acids), in a molar ratio of 1 to metmyoglobin, efficiently blocked LDL oxidation initiated by ferrylmyoglobin. Replacement of one OH group from catecholic structure with an H (p-coumaric acid) or methoxy group (ferulic acid) decreased the antioxidant activity. Also, the catechol structure fused in heterocyclic rings with adjacent carbonyl groups (ellagic acid) resulted in decreased antioxidant activity. These observations correlate with the efficiency of phenolic acids to reduce ferrylmyoglobin to metmyoglobin. Therefore, the protection of LDL against oxidation is assigned to the reduction of the oxoferryl moiety of the hemoprotein to the ferric form. Additionally, it is suggested that an access constraint of oxidants plays a minor role in the ferrylmyoglobin-induced oxidation against LDL.

In vitro administration of ergothioneine failed to protect isolated ischaemic and reperfused rabbit heart

Ergothioneine, a natural thiol-containing molecule, has recently been proposed to protect the heart against damage caused by ischaemia and reperfusion. We investigated the possibility that ergothioneine can have a role in maintaining the myocardial thiol/disulfide balance and consequently also a protective effect against ischaemic and reperfusion injury. We used isolated Langendorff-perfused rabbit hearts subjected to 45 min global and total ischaemia followed by 30 min reperfusion at baseline coronary flow (22 ml/min). Ergothioneine was delivered at 10(-5) M and 10(-4) M 60 min before ischaemia and during reperfusion. Myocardial damage was determined in terms of mechanical function, creatine kinase (CK) and lactate release, energy phosphate stores and the occurrence of oxidative stress. In our experimental conditions the treatment was unable to prevent myocardial damage. Ergothioneine, independently from the dosage used, failed to: (i) increase recovery of developed pressure upon reperfusion (14.4 +/- 2.3 mmHg in control hearts vs. 10.3 +/- 2.9 and 12.5 +/- 2.3 mmHg in 10(-5) M and 10(-4) M ergothioneine treated hearts, respectively); (ii) decrease the rise in diastolic pressure (44.3 +/- 4.4 mmHg in control hearts vs. 49.8 +/- 5.8 and 48.0 +/- 7.7 mmHg in treated hearts); (iii) decrease the release of CK and lactate; (iv) increase the levels of adenosine triphosphate (ATP) and creatine phosphate (CP) in tissue upon reperfusion; (v) maintain ratio between oxidized and reduced forms of adenine nucleotide coenzyme, as index of aerobic metabolism; (vi) prevent the decline of reduced glutathione (GSH), or the accumulation of oxidized glutathione (GSSG) as an index of oxidative stress.

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.

Antioxidant effect of coenzyme Q on hydrogen peroxide-activated myoglobin

In recent years increased attention has been focused on the reduced forms of coenzyme Q as antioxidant compounds inhibiting lipid peroxidation in model systems and in biological membranes, but in spite of extensive experimental evidences the molecular mechanisms responsible for the antioxidant activity of ubiquinones are still debated. Ferrylmyoglobin and/or its free radical form are regarded as powerful oxidizing agents capable of promoting oxidation of essential cellular constituents, particularly cell membranes. Therefore, we investigated the effects of ubiquinol on the formation and survival of ferryl species of myoglobin and on metmyoglobin itself. The addition of a threefold molar excess of hydrogen peroxide to a solution of metmyoglobin induces the rapid formation of a compound with the spectral characteristics of ferrylmyoglobin. The reaction is complete within 4 min, producing up to 76% of ferrylmyoglobin, which remains stable for at least 30 min. The addition of ubiquinol-1 to the same solution provokes a rapid and progressive reduction of ferrylmyoglobin to metmyoglobin and oxymyoglobin. Ubiquinol-1, furthermore, is also capable of protecting metmyoglobin against oxidation when added in the solution before hydrogen peroxide. Ubiquinol-1, indeed, is effective at both limiting the maximal ferrylmyoglobin level attained (59% inhibition) and accomplishing complete removal of the ferryl form (in about 15 min). The results demonstrate that ubiquinol is capable of reducing both ferrylmyoglobin and metmyoglobin to oxymyoglobin, providing a novel antioxidant mechanism for coenzyme Q.

Copper and iron are mobilized following myocardial ischemia: possible predictive criteria for tissue injury

Direct evidence for substantial mobilization of copper in the coronary flow immediately following prolonged, but not short, cardiac ischemia is presented. In the first coronary flow fraction (CFF) of reperfusion (0.15 ml), after 35 min of ischemia, the level of copper (as well as of iron) was 8- to 9-fold higher than the preischemic value. The levels in subsequent CFFs decreased and reached the preischemic value, indicating that both metals appear in a burst at the resumption of coronary flow. When the first CFF was used in a reaction mixture containing ascorbate and salicylate, the latter underwent chemical hydroxylation and was converted to its dihydroxybenzoate derivatives. Likewise, this CFF promoted the ascorbate-driven DNA degradation. Subsequent 150 CFFs were serially collected and demonstrated low activities. Following 18 min of ischemia, the copper level in the first CFF of reperfusion was only 15% over the preischemic value. In contrast, the mobilization of iron into coronary flow was significant but markedly lower than after 35 min. The levels of copper and the redox activity of the first CFF correlated well with the degree of loss of cardiac function, after 18 and 35 min of ischemia, respectively. After 18 min of ischemia, cardiac function was about 50% and the damage is considered reversible, whereas after 35 min the functional loss exceeded 80% and is considered irreversible. These results are in accord with the causative role that copper and iron can play in heart injury following ischemia, by virtue of their capacity to catalyze the production of hydroxyl radicals, and could lead to the development of new modalities for intervention in tissue injury.

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.

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

The reactivity of the endogenous antioxidants ascorbate, ergothioneine, and urate toward the high oxidation state of sperm whale myoglobin, ferrylmyoglobin-formed upon oxidation of metmyoglobin by H2O2--was evaluated by optical spectroscopy and SDS-PAGE analysis. Depending on whether these antioxidants were present in the reaction mixture before or after the addition of H2O2 to a metmyoglobin suspension, two different effects were observed: (a) In the former instances, ascorbate, ergothioneine, and urate reduced efficiently the oxoferryl moiety in ferrylmyoglobin to metmyoglobin and prevented dimer formation, a process which requires intermolecular cross-link involving specific tyrosyl residues. In addition, all the reducing compounds inhibited--albeit with different efficiencies--dityorosine-dependent fluorescence build up produced via dimerization of photogenerated tyrosyl radicals. (b) In the latter instances, the antioxidants reduced the preformed sperm whale ferrylmyoglobin to a modified metmyoglobin, the spectral profile of which was characterized by a blue shift of the typical 633 nm absorbance of native metmyoglobin. In addition, under these experimental conditions, the antioxidants did not affect dimer formation, thus indicating the irreversible character of the process. The dimeric form of sperm whale myoglobin--separated from the monomeric form by gel electrophoresis of a solution in which ergothioneine was added to preformed ferrylmyoglobin--revealed optical spectral properties in the visible region identical to that of the modified myoglobin. This suggests that the dimeric form of the hemoprotein is redox active, inasmuch as the oxoferryl complex can be reduced to its ferric form. These results are discussed in terms of the potential reactivity of these endogenous antioxidants toward the reducible loci of ferrylmyoglobin, the oxoferryl moiety, and the apoprotein radical.

Possible mechanism of inhibition of nitrite-induced oxidation of oxyhemoglobin by ergothioneine and uric acid

The time course of oxyhemoglobin oxidation by nitrite consisted of a kinetic lag followed by a transition phase which progressed into a rapid autocatalytic phase. The imidazolthione and imidazolone derivatives, ergothioneine and uric acid, respectively, caused an increase in the duration of the lag phase in a concentration-dependent manner, without affecting the onset and rate of the autocatalytic phase. Neither compound reacted with H2O2 or nitrite, oxidizing species required in the initiation steps of oxyhemoglobin oxidation. On the other hand, both compounds reduced effectively and at comparable rates the high oxidation state of hemoglobin, i.e., ferrylhemoglobin, which is an intermediate species occurring in the autocatalytic phase. In addition, the rate of ergothioneine oxidation, upon its reaction with ferrylmyoglobin, was accelerated by nitrite, thus suggesting a reaction between the thione and nitrogen dioxide. Nitrogen oxide and ferrylhemoglobin are key species in the free radical chain propagation leading to oxyhemoglobin oxidation by nitrite. These data support the view that ergothioneine and urate delay oxyhemoglobin oxidation by nitrite upon the temporary removal of the propagating species, i.e., nitrogen dioxide and, secondarily, ferrylhemoglobin, and within a mechanism encompassing alterations of the nitrite in equilibrium with nitrogen dioxide and ferrylhemoglobin in equilibrium with methemoglobin redox transitions.

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.

The antioxidant action of ergothioneine

Ergothioneine is a product of plant origin that accumulates in animal tissues. Its suggested ability to act as an antioxidant has been evaluated. Ergothioneine is a powerful scavenger of hydroxyl radicals (.OH) and an inhibitor of iron or copper ion-dependent generation of .OH from hydrogen peroxide (H2O2). It is also an inhibitor of copper ion-dependent oxidation of oxyhaemoglobin, and of arachidonic acid peroxidation promoted by mixtures of myoglobin (or haemoglobin) and H2O2. Ergothioneine is a powerful scavenger of hypochlorous acid, being able to protect alpha 1-antiproteinase against inactivation by this molecule. By contrast, it does not react rapidly with superoxide (O2-) or hydrogen peroxide (H2O2) and it does not inhibit microsomal lipid peroxidation in the presence of iron ions. Overall, our results show that ergothioneine at the concentrations present in vivo could act as an antioxidant.

Detection of ferryl myoglobin in the isolated ischemic rat heart

Reflectance spectroscopy was utilized to monitor the oxidation states of myoglobin (Mb) in isolated, buffer-perfused rat hearts. Hearts were subjected to 30 min global, no-flow ischemia, followed by reperfusion under anoxic conditions. The addition of Na2S to the buffer at reperfusion permitted the detection of ferryl myoglobin (MbIV) as its sulfmyoglobin derivative. The accumulation of MbIV was prevented by addition of ascorbic acid (1 mM), ergothioneine (2 mM), or desferal (1 mM) to the buffer prior to ischemia. Ascorbate and other agents have been previously shown to serve as one-electron reductants of MbIV. We propose that during the early phases of ischemia, deoxymyoglobin is oxidized to MbIV by residual H2O2. It also seems reasonable that the peroxidative activity of Mb(IV), during oxygenated reperfusion, might lead to cellular damage if this hypervalent form of Mb is not reduced.