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PubMed2001

Myocytes isolated from rejecting transplanted rat hearts exhibit a nitric oxide-mediated reduction in the calcium current.

During periods of acute rejection, transplanted hearts have increased nitric oxide (NO) production and depressed contractile function. Myocytes isolated from rejecting hearts exhibit parallel increases in NO production and reduced shortening, indicating that the contractile dysfunction of the transplanted heart is intrinsic to the myocytes. We tested the hypothesis that the contractile dysfunction of the rejecting heart is due to an NO-mediated inhibition of the L-type calcium current. Ventricular myocytes isolated from rejecting rat hearts (allografts) expressed inducible nitric oxide synthase (iNOS) and produced substantially more NO than did myocytes isolated from non-rejecting rat hearts (isografts). Aminoguanidine, an inhibitor of iNOS, reduced NO production by allograft myocytes, but was without effect on NO production by isograft myocytes. In the absence of exogenous l -arginine (the precursor of NO), the calcium current was identical in allograft and isograft myocytes. In the presence of l -arginine, the calcium current was reduced in allograft myocytes compared to isograft myocytes. Superfusion of the myocytes with either aminoguanidine or KT5823 (an inhibitor of the cyclic GMP-dependent protein kinase) reversed the depression of the calcium current in allograft myocytes, but neither inhibitor had an effect on calcium current in isograft myocytes. These results indicate that increased production of NO by myocytes isolated from rejecting hearts leads to a reduction in the calcium current. This mechanism may contribute substantially to the contractile dysfunction of rejecting transplanted hearts.

Alkaloids↗
PubMed1997

[Oxidation and reduction of nitroarene as environmental mutagens].

Newly synthesized dinitrobenzo[a]pyrenes and 6-aza analogues of 1- and 3-nitrobenzo[a]pyrenes showed strong mutagenic activity in Salmonella assays (TA98 and TA98NR). Nitroreduction is essential for metabolic activation of nitroarene. The structure activity relationships of mono-, di- and trinitrophenanthrene were studied. Electrochemical ease of nitroreduction and dihedral angles of nitro substituents to aromatic rings is found to be important factor to determine their mutagenic potency. Nitroarene is oxidatively metabolized to form hydroxylated and nitrosubstituted derivatives and the same oxidative products were obtained by the reaction of nitroarene with chemically generated superoxide anion radical, suggesting the participation of superoxide in metabolic oxidation of nitroarene. Catalytic activity of nitroarene as a mediator in the reductive activation of molecular oxygen was also evaluated by using potential step chronocoulometry method. The potential relevence of toxicity caused by redox cycling could be solved by using this method.

Aza Compounds↗
PubMed1987

Glucagon-stimulated calcium efflux in the isolated perfused rat liver is dependent on cellular redox potential.

In the absence of any exogenous substrates, glucagon (1 X 10(-9) M) stimulated 45Ca2+ efflux from perfused livers derived from fed rats but not in livers of 24-h-fasted animals. In livers of 24-h-fasted animals perfused under conditions which would decrease cellular NAD(P)H/NAD(P)+ ratio (pyruvate (2.0 mM) or acetoacetate (10.0 mM], glucagon (1 X 10(-9) M) did not stimulate 45Ca2+ efflux. Similarly, in livers of 24-h-fasted animals perfused with substrates which increase cellular NAD(P)H content (lactate (2.0 mM) or beta-hydroxybutyrate (10.0 mM], glucagon (1 X 10(-9) M) did not increase 45Ca2+ efflux. Glucagon (1 X 10(-9) M) elicited an increase in 45Ca2+ efflux from livers of 24-h-fasted animals, only when the livers were perfused with [lactate]/[pyruvate] and [beta-hydroxybutyrate]/[acetoacetate] ratios similar to those reported for livers of fed rats. Stimulation of 45Ca2+ efflux elicited by either 8-CPT-cAMP, a cAMP analog, or high glucagon concentrations (1 X 10(-8) M) was not affected whether livers were perfused with pyruvate (2.0 mM) or lactate (2.0 mM). Administration of isobutylmethylxanthine (50 microM) alone, or glucagon (1 X 10(-9) M) in the presence of isobutylmethylxanthine (50 microM) stimulated 45Ca2+ efflux from livers of 24-h-fasted animals perfused with pyruvate (2.0 mM) but not from livers perfused with lactate (2.0 mM). The ability of glucagon (1 X 10(-9) M) to elevate tissue cAMP levels was also regulated by the oxidation-reduction state of the livers. The data indicate that glucagon-stimulated 45Ca2+ efflux from perfused livers is mediated via cAMP and is dependent on the oxidation-reduction state of the livers.

1-Methyl-3-isobutylxanthine↗
PubMed2000

Reduction of stilbene oxide and styrene oxide to the corresponding alkenes by intestinal bacteria.

1. This study provides the first evidence that stilbene oxide and styrene oxide are reductively metabolized to the corresponding alkenes by intestinal bacteria in animals. 2. When trans- or cis-stilbene oxide were incubated with the caecal contents of rat under anaerobic conditions, both trans- and cis-stilbene were isolated from the incubation mixture. Styrene oxide was also reduced to styrene by rat caecal contents. 3. Caecal contents of mouse, hamster and guinea pig also exhibited alkene oxide reductase activities toward cis- and trans-stilbene oxides, and styrene oxide. In contrast, liver microsomes or cytosol exhibited no epoxide reductase activities toward these substrates. 4. Seven pure strains of intestinal bacteria exhibited alkene oxide reductase activities of varying degrees under anaerobic conditions, with the highest activity being observed in Clostridium sporogenes. 5. Cell-free extracts of either the intestinal bacteria in rat caecal contents or C. sporogenes exhibited reductase activity when supplemented with both NAD(P)H and FMN under anaerobic conditions. Reductase activity was also observed on addition of the photochemically reduced form of FMN instead of both NAD(P)H and FMN.

Animals↗
PubMed1977

Anaerobic growth of Escherichia coli on formate by reduction of nitrate, fumarate, and trimethylamine N-oxide.

Anaerobic growth of E. coli, strain K-10, depending on formate oxidation by nitrate, fumarate, and trimethylamine N-oxide was followed in a medium containing peptone. The presence of formate and peptone was indispensable for growth with fumarate and trimethylamine N-oxide reduction. While there was no growth in the absence of acceptor, growth was observed in the absence of formate by nitrate reduction though not as much as under aerobic conditions. Per mole consumed formate equimolar succinate or trimethylamine was formed, but 1.2 mole of nitrate was produced, probably depending partly on peptone oxidation. The molar growth yield on formate was found to be 6.5, 7.6, and 7.0 g cells/mole depending on the reduction of nitrate, fumarate, and trimethylamine N-oxide, respectively, suggesting the formation of one mole ATP coupled to the anaerobic electron transfers from formate.

Aerobiosis↗
PubMed1992

Calcium ions and oxidative cell injury.

Exposure of mammalian cells to oxidative stress induced by oxidation-reduction-active quinones and other prooxidants results in depletion of intracellular glutathione, followed by modification of protein thiols and loss of cell viability. Protein thiol modification during oxidative stress is normally associated with impairment of various cell functions, including inhibition of agonist-stimulated phosphoinositide metabolism, disruption of intracellular Ca2+ homeostasis, and perturbation of normal cytoskeletal organization. The latter effect appears to be responsible for formation of the numerous plasma membrane blebs typically seen in cells exposed to cytotoxic concentrations of prooxidants. Following disruption of thiol homeostasis in prooxidant-treated cells, there is impairment of Ca2+ transport and subsequent perturbation of intracellular Ca2+ homeostasis, resulting in a sustained increase in cytosolic Ca2+ concentration. This increase in Ca2+ can cause activation of various Ca(2+)-dependent degradative enzymes (e.g., phospholipases, proteases, endonucleases), which may contribute to cell death. In contrast to the cytotoxic effects of excessive oxidative damage, low levels of oxidative stress can lead to activation of enzymes involved in cell signaling. In particular, the activity of protein kinase C is markedly increased by oxidation-reduction-cycling quinones through a thiol/disulfide exchange mechanism, which may represent a mechanism by which prooxidants can modulate cell growth and differentiation.

Animals↗
PubMed1980

Effects of piridoxilate, a glyoxylic acid derivative, on the energy metabolism of the heart.

Piridoxilate is a conjugation product of pyridoxine and glyoxylic acid, which may be a physiological regulator of cell respiration. Effects of this substance on the oxidation-reduction state of the pyridine nucleotides in the heart were studied using the canine heart-lung preparation supported by a donor dog. The oxidation-reduction state of the heart was estimated using the following 2 parameters: 1) myocardial redox potential and 2) NADH fluorescence in the heart muscle. In doses above 0.4 mM, piridoxilate produced a marked shift to more positive values of the myocardial redox potential and a decrease in NADH fluorescence. In contrast, ventilation of the animal with N2 gas and infusion of NaCN into the preparation resulted in a shift to more negative values of myocardial redox potential and an increase in NADH fluoresecence of the heart muscle. After pretreatment of the preparation with piridoxilate, the depression of redox potential and the increase in NADH fluorescence produced by N2 gas inhalation or by NaCN infusion were clearly reduced. These findings suggest a protective action of piridoxilate against hypoxia which may be attributable to rearrangement of the myocardial metabolism.

Animals↗
PubMed1986

Stereoselective effects in the interactions of pterin cofactors with rat-liver phenylalanine 4-monooxygenase.

The (6R) and (6S) epimers of l-erythro-tetrahydrobiopterin (BH4) and some of its structural analogs, were tested as cofactors and non-covalent effectors in the phenylalanine 4-monooxygenase (phenylalanine hydroxylase, EC 1.14.16.1) reaction. The oxidation-reduction potentials (Em,7) of the free (not enzyme-bound) form of the (6R) and (6S) epimers were rather similar (range 174-184 mV) for the oxidation of tetrahydropterins to quinonoid dihydropterins. Rapid-mixing kinetic experiments were performed at 20 degrees C under conditions which allow only a few turnover reactions of the enzyme. Three main oxidation products were identified spectroscopically at pH 6.8 for all three tetrahydropterins tested: the C(4a)-hydroxy derivatives, the quinonoid dihydropterins, and the stable 7,8-dihydropterins (in that sequence). The formation of the C(4a)-hydroxy forms closely paralleled that of tyrosine, and supports the proposal that this covalent adduct is formed as an immediate product on completion of the catalytic cycle. Assay of the initial rate of C(4a)-hydroxy derivative formation represents a new approach in kinetic studies of this enzyme, and the kinetic parameters obtained for the phenylalanine-activated enzyme are presented. The affinity of binding of (6R)-BH4 and (6S)-BH4 to phenylalanine hydroxylase was also estimated on the basis of their quenching of the intrinsic tryptophan fluorescence of the enzyme. The apparent affinities were found to correspond well to the Km values estimated in kinetic studies of the hydroxylation reaction with the phenylalanine activated enzyme, i.e. higher for (6R)-BH4 than for (6S)-BH4. The lower V value observed for the native enzyme with the (6R) epimer in steady-state kinetics is explained by its higher potency as a negative effector, since the oxidation-reduction potentials of the two diastereomers were similar. Dihydrobiopterin (BH2) was found to inhibit the hydroxylation reaction and quenched the intrinsic tryptophan fluorescence of the enzyme with the same concentration dependence as that observed with (6S)-BH4.

Animals↗
PubMed1985

Effect of hypoxia on oxidative and reductive pathways of omeprazole metabolism by the isolated perfused rat liver.

The effect of hypoxia on the elimination of omeprazole, a potent inhibitor of gastric acid secretion, was studied in the isolated perfused rat liver. During normal oxygenation, a 10 mg bolus dose was eliminated rapidly (T 1/2 beta = 8.0 +/- 1.1 min; mean +/- S.E.M., N = 4), while under hypoxic conditions T 1/2 beta was increased to 81.6 +/- 5.4 min (P less than 0.01). Upon reoxygenation, T 1/2 beta returned to 9.6 +/- 1.3 min. During hypoxia, perfusate concentrations of an oxidative metabolite (the sulphone) were reduced by 68%, while those of the reductively-generated sulphide increased 4-fold. With reoxygenation, both formation and elimination of the sulphone were increased, while the sulphide, which had accumulated during the hypoxic period, was eliminated rapidly. These findings were duplicated in steady-state experiments, in which omeprazole clearance during hypoxia fell by at least 70%, and sulphide concentrations in perfusate rose from undetectable levels to 200 ng/ml (at least a 10-fold increase). Sulphone concentrations did not change with hypoxia, consistent with a reduction in both its formation and elimination rates. We conclude that the hepatic elimination of omeprazole is severely retarded by hypoxia, but that this effect is promptly reversed by reoxygenation. The increased formation of reductive metabolite during hypoxia is not of sufficient magnitude to sustain the normal hepatic elimination of omeprazole.

Animals↗
PubMed2005

Biochemical properties of purified human retinol dehydrogenase 12 (RDH12): catalytic efficiency toward retinoids and C9 aldehydes and effects of cellular retinol-binding protein type I (CRBPI) and cellular retinaldehyde-binding protein (CRALBP) on the oxidation and reduction of retinoids.

Retinol dehydrogenase 12 (RDH12) is a novel member of the short-chain dehydrogenase/reductase superfamily of proteins that was recently linked to Leber's congenital amaurosis 3 (LCA). We report the first biochemical characterization of purified human RDH12 and analysis of its expression in human tissues. RDH12 exhibits approximately 2000-fold lower K(m) values for NADP(+) and NADPH than for NAD(+) and NADH and recognizes both retinoids and lipid peroxidation products (C(9) aldehydes) as substrates. The k(cat) values of RDH12 for retinaldehydes and C(9) aldehydes are similar, but the K(m) values are, in general, lower for retinoids. The enzyme exhibits the highest catalytic efficiency for all-trans-retinal (k(cat)/K(m) approximately 900 min(-)(1) microM(-)(1)), followed by 11-cis-retinal (450 min(-)(1) mM(-)(1)) and 9-cis-retinal (100 min(-)(1) mM(-)(1)). Analysis of RDH12 activity toward retinoids in the presence of cellular retinol-binding protein (CRBP) type I or cellular retinaldehyde-binding protein (CRALBP) suggests that RDH12 utilizes the unbound forms of all-trans- and 11-cis-retinoids. As a result, the widely expressed CRBPI, which binds all-trans-retinol with much higher affinity than all-trans-retinaldehyde, restricts the oxidation of all-trans-retinol by RDH12, but has little effect on the reduction of all-trans-retinaldehyde, and CRALBP inhibits the reduction of 11-cis-retinal stronger than the oxidation of 11-cis-retinol, in accord with its higher affinity for 11-cis-retinal. Together, the tissue distribution of RDH12 and its catalytic properties suggest that, in most tissues, RDH12 primarily contributes to the reduction of all-trans-retinaldehyde; however, at saturating concentrations of peroxidic aldehydes in the cells undergoing oxidative stress, for example, photoreceptors, RDH12 might also play a role in detoxification of lipid peroxidation products.

Alcohol Oxidoreductases↗
PubMed1985

Circular dichroism and redox properties of high redox potential ferredoxins.

The circular dichroism (CD) spectra of 13 examples of high-potential iron-sulfur proteins (HiPIPs), a class of [4Fe-4S] ferredoxins, have been determined. In contrast to the proposal of Carter [Carter, C. W., Jr. (1977) J. Biol. Chem. 252, 7802-7811], no strict correlation between visible CD features and utilization of the [4Fe-4S]2+/[4Fe-4S]3+ oxidation levels was found. Although most HiPIPs have these features, the model requires their presence in all species. There is also no simple relationship between CD spectral features and the presence of conserved tyrosine-19. In addition, no apparent correlation between CD properties and oxidation-reduction potential could be detected. However, amino acid side chains in close contact to the iron-sulfur cluster appear to be important in modulating spectral and oxidation-reduction properties. In particular, the negative shoulder at 290 nm and negative maximum at 230 nm correlate with the presence of Trp-80 (Chromatium vinosum numbering). Two HiPIPs that do not have Trp at this position have positive bands at 290 and 230 nm. These bands in the Ectothiorhodospira halophila HiPIPs are apparently associated with Trp-49, which is located on the opposite side of the effective mirror plane of the cluster from Trp-80. The effect of pH on circular dichroism and redox potential in Thiocapsa roseopersicina HiPIP, which has a histidine at position 49, is consistent with the interaction of the side chain with the cluster. Despite specific differences in their CD spectra, the various HiPIPs studied show general similarity consistent with structural homology within this class of iron-sulfur proteins.

Amino Acid Sequence↗
PubMed2006

Colloid mobilization during soil iron redox oscillations.

In redox-dynamic soils, iron reduction-oxidation events may initiate wide shifts in the concentration of colloidal and dissolved material because of either Fe mineral dissolution or pH shifts associated with Fe oxidation state changes. This can have profound effects on the mobilization of organic and metal constituents. We conducted laboratory studies of colloid dynamics in a Hawaiian soil subjected to four consecutive 14-day reduction-oxidation cycles across the "soil-Fe" (Fe(OH)3)/(Fe2+(aq)) equilibrium. Size fractionated samples were isolated by differential centrifugation and characterized based on analysis of the framework and trace elements (Si, C, Fe, Ti, Al, Zr, Nb, La, and U). Intracycle oscillations in all colloidal (3 kDa to 160 nm) elements peaked during the reduction half-cycles, mobilizing 10% of total soil Ti and from 1-5% of total soil Zr, Nb, La, and U at peak dispersion. Colloid dynamics were dependent on pH shifts accompanying the redox oscillations rather than the fluctuating solubility of Fe oxides. TEM/EDS and mass-balance calculations suggest a carbon-based colloid matrix with zones of metal enrichment. The cumulative effects of four redox cycles included an apparent increase in colloid stability. Proton production/consumption associated with Fe-redox cycling has important implications for mobilization colloid-borne trace elements and sorbed contaminants.

Colloids↗
PubMed1997

Redox dependence for photoligation of manganese to the apo-water-oxidizing complex in chloroplasts and photosystem II membranes.

Effects of reducing reagents and redox potentials on photoactivation were studied in Mn-depleted chloroplasts and PSII membranes. Exogenous reducing reagents abolished photoactivation in PSII membranes, while they stimulated photoactivation in chloroplasts. To determine how reducing reagents can have such opposing effects in these preparations, we studied how redox potentials affect photoactivation in the range from 0 mV to +500 mV. In chloroplasts, a modest yield of photoactivation was obtained in the redox potential range of +100 and +330 mV at pH 7.5. The yield of photoactivation decreased at redox potentials above +330 mV, and drastically increased below potentials of +100 mV. Nernst plots of the data show that an n = 1 redox component with an Em7.5 of +374 mV, as well as an n = 2 redox component with an Em7.5 of +61 mV, is involved in photoactivation of chloroplasts isolated from dark-grown spruce seedlings. In the case of PSII membranes, photoactivation decreased sharply on either side of +335 mV at pH 5.5. The n = 1 redox components with Em5.5 of +375 and +319 mV may be involved, both of which showed pH dependences of -60 mV/pH unit. DCMU abolished photoactivation in chloroplasts, but did not affect the dependence of photoactivation on oxidation-reduction potentials in PSII membranes. The component with an Em5.5 of +319 mV involved in photoactivation of PSII membranes was also observed in the dependence of Mn solubilization on oxidation-reduction potentials with PSII membranes lacking extrinsic proteins, suggesting that the reduction of Mn with higher valences to Mn(II) by exogenous reducing reagents reversibly occurs in the intermediates or an active center during photoactivation in PSII membranes. Involvement of such redox components in photoactivation in chloroplasts and PSII membranes is discussed.

Chloroplasts↗
PubMed1979

Comparative solvent perturbation of horse heart cytochrome c and Rhodospirillum rubrum cytochrome c2.

The extent of exposure of heme to solvent in horse heart cytochrome c and Rhodospirillum rubrum c2 was investigated to determine whether a correlation exists between the properties of these oxidation-reduction proteins and their heme environments. Solvent perturbation absorption difference spectra were measured using ethylene glycol, glycerol, and sucrose at concentrations between 0 and 30%. Cytochrome c appears to exhibit a somewhat greater extent of heme exposure than cytochrome c2 for both the oxidized and reduced states. These results suggest that the lower oxidation-reduction potential of cytochrome c may in part be due to a greater extent of exposure of the heme. The oxidized state of both proteins appears to exhibit a greater exposure than that of the reduced state which is consistent with a more favorable environment for the charge on the ferric heme coordination center.

Animals↗
PubMed2004

In situ reduction of oxidative damage, increased cell turnover, and delay of mitochondrial injury by overexpression of manganese superoxide dismutase in a multistage skin carcinogenesis model.

To study early subcellular pathologic changes of tumorigenesis in mouse skin and possible modulation by overexpression of the mitochondrial antioxidant enzyme manganese superoxide dismutase (MnSOD), skin keratinocytes from nontransgenic (Ntg) and transgenic (TgH) mice overexpressing MnSOD topically treated with one dose of 7,12-dimethylbenz(a)anthracene (DMBA) and a subsequent dose of 12-O-tetradecanoylphorbol 13-acetate (TPA) were analyzed in situ for levels of MnSOD and the oxidative damage product 4-hydroxy-2-nonenal (4HNE)-modified proteins using specific antibodies and immunogold electron microscopy. At all selected time points analyzed after TPA treatment, there was more MnSOD immunoreactive protein in mitochondria of keratinocytes of TgH mice than Ntg mice. Compared with untreated groups, there was a large increase in 4HNE-modified proteins at 6-24 h after TPA treatment, and this increase was larger in Ntg than TgH mice. Indices of mitosis and apoptosis of keratinocytes were greater in DMBA/TPA-treated TgH than Ntg mouse skin. Mitochondrial injury detected by transmission electron microscopy was delayed in keratinocytes of TgH compared with Ntg mice. The present study demonstrated that overexpression of MnSOD not only protected cells from oxidative damage, but also affected cell turnover kinetics. Thus, previously identified reduction in papilloma formation observed in TgH mice is correlated with mitochondrial events.

9,10-Dimethyl-1,2-benzanthracene↗
PubMed1991

Inhibitory effects of catechol derivatives on arachidonic acid-induced aggregation of rabbit platelets.

Pyrocatechol and other non-substituted dihydric phenols, which have strong redox power, inhibited arachidonic acid-induced aggregation of rabbit platelets at much lower concentrations than those at which these phenols inhibited stable prostaglandin endoperoxide, U46619-induced aggregation. Among non-substituted dihydric phenols, pyrocatechol was most potent. In order to clarify the physicochemical properties of the phenolic compounds which control the inhibitory potencies of dihydric phenols, we observed the inhibitory effects of 3- and 4-substituents of pyrocatechol on arachidonic acid-induced platelet aggregation. Among seven derivatives tested, the inhibitory effect of 4-C6H5-substituent was strongest and 4-COOH-substituent was weakest. Inhibitory effects of the catechol derivatives were well correlated with the quotients of their hydrophobicities and oxidation-reduction potentials. Inhibitory effects of hydroquinone and resorcinol were also on the same correlation line. These results suggest that the inhibitory effects of catechol derivatives and other dihydric phenols are controlled by two physicochemical properties: oxidation-reduction potential and hydrophobicity.

Animals↗
PubMed1992

Oxidative and reductive metabolism by cytochrome P450 2E1.

We are constantly exposed to many potentially toxic chemicals. Most require metabolic activation to species responsible for cell injury. Although cytochrome P450 2E1 is only one of many different forms of cytochrome P450 that catalyze these reactions, it has an important role in human health as a result of being readily induced by acute and chronic alcohol ingestion. The enzyme efficiently catalyzes the low Km metabolism of compounds commonly used as solvents in industry and at home as well as components found in cigarette smoke, many of which are established carcinogens and hepatotoxins. As a result, there is the potential for increased risk to low level exposure to such chemicals while cytochrome P450 2E1 is induced. Many substrates have been identified for cytochrome P450 2E1. Of the 52 substrates for the enzyme identified in this review, the demethylation of N,N-dimethylnitrosamine and the hydroxylation of p-nitrophenol and chlorzoxazone are the most effective for monitoring the level of this enzyme. In addition to oxidative reactions, cytochrome P450 2E1 is also an efficient catalyst of reductive reactions. CCl4-induced hepatotoxicity is one of the best-documented cases for the participation of cytochrome P450 2E1 in a toxicologically important reductive reaction. The reduction of oxygen to superoxide and peroxide are also important reductive reactions of the enzyme and could be important in lipid peroxidation. However, the role of this reaction in vivo remains controversial.

Animals↗
PubMed1975

Characterization of the reaction between ferrocytochrome c and cytochrome c oxidase.

The reaction between cytochrome c oxidase and ferrocytochrome c has been investigated by the stopped-flow method. It has been found that only one electron acceptor, a heme group, in the oxidase is rapidly reduced by cytochrome c. The presence of N3- does not affect the reduction of the acceptor, which supports the hypothesis that this is identical with cytochrome a. The results are consistent with the existence of a simple equilibrium between cytochrome a and cytochrome c: c-2 + a-3+ in equilibrium c-3+ + a-2+ with an equilibrium constant corresponding to an oxidation-reduction potential of cytochrome a 30 mV higher than that for cytochrome c at pH 7.4. The oxidation-reduction potential of the a-3+ /a-2+ couple, 285 mV (based on a potential of 255 mV for cytochrome c), and the optical properties of the reduced form indicate that it is identical with neither of the reduced hemes seen in potentiometric titrations. The oxidase species resulting from the rapid reduction of cytochrome a by cytochrome c is proposed to represent a metastable intermediate state which, under anaerobic conditions, eventually is transformed into a more stable state characterized by a reduced high-potential heme.

Anaerobiosis↗