The peroxidase activity of deuterohemin

Activity adaptability of a DhHP-6 peroxidase-mimic in wide pH and temperature ranges and solvent media

Deuterohemin-β-Ala-His-Thr-Val-Glu-Lys (DhHp-6): peroxidase with high activity.

Ligand accessibility to heme cytochrome b5 coordinating sphere and enzymatic activity enhancement upon tyrosine ionization

Recently, we observed that at extreme alkaline pH, cytochrome b₅ (Cb₅) acquires a peroxidase-like activity upon formation of a low spin hemichrome associated with a non-native state. A functional characterization of Cb₅, in a wide pH range, shows that oxygenase/peroxidase activities are stimulated in alkaline media, and a correlation between tyrosine ionization and the attained enzymatic activities was noticed, associated with an altered heme spin state, when compared to acidic pH values at which the heme group is released. In these conditions, a competitive assay between imidazole binding and Cb₅ endogenous heme ligands revealed the appearance of a binding site for this exogenous ligand that promotes a heme group exposure to the solvent upon ligation. Our results shed light on the mechanism behind Cb₅ oxygenase/peroxidase activity stimulation in alkaline media and reveal a role of tyrosinate anion enhancing Cb₅ enzymatic activities on the distorted protein before maximum protein unfolding.

A biomimetic enzyme modified electrode for H2O2 highly sensitive detection

An efficient catalyst based on artificial bionic peroxidase was synthesized for electrocatalysis. A poly(ethyleneimine)/Au nanoparticle composite (PEI-AuNP) was prepared and it was then linked to hemin via a coupling reaction between carboxyl groups in hemin and amino groups in PEI without the activation of a carboxyl group by carbodiimide. Fourier transform infrared (FTIR) spectroscopy verified the formation of amido bonds within the structure. The presence of AuNPs contributed greatly in establishing the amido bonds within the composite. Transmission electron microscopy (TEM) and UV-visible spectroscopy were also used to characterize the PEI-AuNP-hemin catalyst. PEI-AuNP-hemin exhibited intrinsic peroxidase-like catalytic activities. The PEI-AuNP-hemin deposited on a glass carbon electrode had strong sensing for H2O2 with a well-defined linear relationship between the amperometric response and H2O2 concentration in the range from 1 μM to 0.25 mM. The detection limit was 0.247 nM with a high sensitivity of 0.347 mA mM(-1) cm(-2). The peroxidase-like catalytic activity of PEI-AuNP-hemin is discussed in relation to its microstructure. The study suggests that PEI-AuNP-hemin may have promising application prospects in biocatalysis and bioelectronics.

Metal-catalyzed protein tyrosine nitration in biological systems

Protein tyrosine nitration is an oxidative postranslational modification that can affect protein structure and function. It is mediated in vivo by the production of nitric oxide-derived reactive nitrogen species (RNS), including peroxynitrite (ONOO(-)) and nitrogen dioxide ((•)NO₂). Redox-active transition metals such as iron (Fe), copper (Cu), and manganese (Mn) can actively participate in the processes of tyrosine nitration in biological systems, as they catalyze the production of both reactive oxygen species and RNS, enhance nitration yields and provide site-specificity to this process. Early after the discovery that protein tyrosine nitration can occur under biologically relevant conditions, it was shown that some low molecular weight transition-metal centers and metalloproteins could promote peroxynitrite-dependent nitration. Later studies showed that nitration could be achieved by peroxynitrite-independent routes as well, depending on the transition metal-catalyzed oxidation of nitrite (NO₂(-)) to (•)NO₂ in the presence of hydrogen peroxide. Processes like these can be achieved either by hemeperoxidase-dependent reactions or by ferrous and cuprous ions through Fenton-type chemistry. Besides the in vitro evidence, there are now several in vivo studies that support the close relationship between transition metal levels and protein tyrosine nitration. So, the contribution of transition metals to the levels of tyrosine nitrated proteins observed under basal conditions and, specially, in disease states related with high levels of these metal ions, seems to be quite clear. Altogether, current evidence unambiguously supports a central role of transition metals in determining the extent and selectivity of protein tyrosine nitration mediated both by peroxynitrite-dependent and independent mechanisms.

A facile one-pot method to synthesize a polypyrrole/hemin nanocomposite and its application in biosensor, dye removal, and photothermal therapy

In this work, we introduced a facile method for the construction of a polypyrrole/hemin (PPy/hemin) nanocomposite via one-pot chemical oxidative polymerization. In this process, a hemin molecule serving as a dopant was entrapped in the PPy nanocomposite during chemical oxidative polymerization. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and UV-visible spectroscopy results demonstrated that the PPy/hemin nanocomposite was successfully synthesized. The as-prepared nanocomposite exhibited intrinsic peroxidase-like catalytic activities, strong adsorption properties, and an excellent near-infrared (NIR) light-induced thermal effect. We utilized the nanomaterials to catalyze the oxidation of a peroxidase substrate 3,3,5,5-tetramethylbenzidine by H2O2 to the oxidized colored product which provided a colorimetric detection of glucose. As low as 50 μM glucose could be detected with a linear range from 0.05 to 8 mM. Moreover, the obtained nanocomposite also showed excellent removal efficiency for methyl orange and rhodamine B and a photothermal effect, which implied a promising application as the pollutant adsorbent and photothermal agent. The unique nature of the PPy/hemin nanocomposite makes it very promising for the fabrication of inexpensive, high-performance bioelectronic devices in the future.

Hemin toxicity: a preventable source of brain damage following hemorrhagic stroke

Hemorrhagic stroke is a common cause of permanent brain damage, with a significant amount of the damage occurring in the weeks following a stroke. This secondary damage is partly due to the toxic effects of hemin, a breakdown product of hemoglobin. The serum proteins hemopexin and albumin can bind hemin, but these natural defenses are insufficient to cope with the extremely high amounts of hemin (10 mM) that can potentially be liberated from hemoglobin in a hematoma. The present review discusses how hemin gets into brain cells, and examines the multiple routes through which hemin can be toxic. These include the release of redox-active iron, the depletion of cellular stores of NADPH and glutathione, the production of superoxide and hydroxyl radicals, and the peroxidation of membrane lipids. Important gaps are revealed in contemporary knowledge about the metabolism of hemin by brain cells, particularly regarding how hemin interacts with hydrogen peroxide. Strategies currently being developed for the reduction of hemin toxicity after hemorrhagic stroke include chelation therapy, antioxidant therapy and the modulation of heme oxygenase activity. Future strategies may be directed at preventing the uptake of hemin into brain cells to limit the opportunity for toxic interactions.

Enhancement of glutathione-dependent haemin degradation by ascorbic acid

In the current work, we investigated the effect of ascorbic acid on GSH-mediated haemin degradation. GSH-mediated haemin degradation in the presence of ascorbic acid in phosphate-buffered saline and in erythrocyte ghosts was determined by recording absorbance at 365 and 399nm, respectively. Generation of intracellular H(2)O(2) was measured indirectly in terms of the inactivation of endogenous catalase in erythrocytes in the presence of 3-amino-1,2,4-triazole. Although ascorbic acid itself did not induce haemin degradation, it enhanced GSH-mediated haemin degradation. Experiments with catalase showed that H(2)O(2) was essential in this process. The oxidation of ascorbic acid in the presence of haemin was stimulated by GSH, suggesting that ascorbic acid can alter the mechanism of H(2)O(2) generation observed with GSH and haemin alone. These results suggest that enhancement of GSH-mediated haemin degradation by ascorbic acid may be due to an increase in the production of H(2)O(2) generated by GSH and haemin in the absence of ascorbic acid.

The peroxidase activity of a hemin--DNA oligonucleotide complex: free radical damage to specific guanine bases of the DNA

A specific DNA oligonucleotide--hemin complex (PS2.M--hemin complex) that exhibits DNA-enhanced peroxidative activity was studied by EPR and UV--visible spectroscopy and by chemical probing analysis. EPR data obtained from low-temperature experiments on the PS2.M--hemin complex showed both a low-field g approximately 6 and a high-field g approximately 2 signal. These EPR signals are typical of high-spin ferric heme with axial symmetry as judged by the EPR spectrum of six-coordinate heme iron in acidic Fe(III)-myoglobin. This similarity is consistent with the presence of two axial ligands to the heme iron within the PS2.M--hemin complex, one of which is a water molecule. Optical analyses of the acid-base transition for the hemin complex yielded a pK(a) value for the water ligand of 8.70 +/- 0.03 (mean +/- SD). Low-temperature EPR analysis coupled with parallel spin-trapping investigations following the reaction of the PS2.M--hemin complex and hydrogen peroxide (H(2)O(2)) indicated the formation of a carbon-centered radical, most likely on the PS2.M oligonucleotide. Chemical probing analysis identified specific guanine bases within the PS2.M sequence that underwent oxidative damage upon reaction with H(2)O(2). These and other experimental findings support the hypothesis that the interaction of specific guanines of PS2.M with the bound hemin cofactor might contribute to the superior peroxidative activity of the PS2.M--hemin complex.

Surfactant-histidine-heme ternary complex as a simple artificial heme enzyme in organic media

A surfactant-heme complex which shows peroxidase activity in organic media has been prepared by a method utilizing water-in-oil (W/O) emulsions. Both the aqueous phase pH and the type of surfactant appeared to have prominent effect on the catalytic activity of the heme complex in benzene. The catalytic efficiency of the heme complex was enhanced more than ten times by adding histidine to the aqueous phase of W/O emulsions in the preparation process. The enhancement of peroxidase activity was observed only in a nonaqueous medium due to the increase of the effective concentration of histidine as an activator. In the present study, we propose a simple preparation method for an artificial heme enzyme which works in nonaqueous media. Copyright 1999 John Wiley & Sons, Inc.

DNA-enhanced peroxidase activity of a DNA-aptamer-hemin complex

BACKGROUND

In vitro selection (SELEX) previously identified short single-stranded DNAs that specifically bound N-methylmesoporphyrin IX (NMM), a stable transition-state analogue for porphyrin-metallation reactions. Interestingly, iron(III)-protoporphyrin (hemin) was a good competitive inhibitor for the DNA-catalyzed metallation reaction, and appeared to bind strongly to the NMM-binding DNA aptamers. We investigated the peroxidase activity of the aptamer-hemin complexes to see if the DNA component of the complex, like the apoenzymes in protein peroxidases, could enhance the low intrinsic peroxidatic activity of hemin.

RESULTS

Two porphyrin-binding DNA aptamers bound hemin with submicromolar affinity. The aptamer-hemin complexes had significantly higher peroxidase activity than hemin alone, under physiological conditions. The Vobs of the PS2.M-hemin complex was 250 times greater than that of hemin alone, and significantly superior to a previously reported hemin-catalytic-antibody complex. Preliminary spectroscopic evidence suggests the coordination of the hemin iron in the complex changes, such that the complex more closely resembles horseradish peroxidase and other heme proteins rather than hemin.

CONCLUSIONS

A new class of catalytic activity for nucleic acids is reported. The aptamer-hemin complexes described are novel DNA enzymes and their study will help elucidate the structural and functional requirements of peroxidase enzymes in general and the ways that a nucleic acid 'apoenzyme' might work to enhance the intrinsic peroxidatic ability of hemin. These aptamer-hemin complexes could be regarded as prototypes for redox-catalyzing ribozymes in a primordial 'RNA world'.

Porphyrin-linked oligonucleotides. Synthesis and sequence-specific modification of ssDNA

Oligonucleotide derivatives bearing hemin and deuterohemin groups were synthesized. The derivatives efficiently react with the complementary nucleotide sequence in ssDNA forming covalent adducts and piperidine-labile sites. In the case of the deuterohemin derivative, some direct cleavage of the target DNA occurs.

The reaction of hemin with H2O2

The kinetics of formation of the dominant intermediate (CII) formed between hemin and H2O2 has been studied by the stopped-flow method. CII is preceded by a precursor (CI) for which a steady state is established at an early stage of the reaction. The formation of CI from hemin and H2O2 causes only a marginal change in the optical absorbance (A). The transition CI----CII is accompanied by a substantial decrease of A in the Soret region. Relevant rate constants (or combinations of them) and the molar absorption coefficients of the intermediates at 400 nm have been determined. The absorption spectrum of CII in the Soret region has been evaluated. Aspects of the catalysis of decomposition of H2O2 by hemin in relation to the Fe3+ ion and catalase are discussed.

The pH dependence of the mechanism of reaction of hydrogen peroxide with a nonaggregating, non-mu-oxo dimer-forming iron (III) porphyrin in water

The reaction of hydrogen peroxide with 5, 10,15,20-tetrakis(2,6-dimethyl-3-sulfonatophenyl)porphinato- iron(III) hydrate [(P)FeIII(H2O)] has been investigated in water between pH 1 and pH 12. The water-soluble (P)FeIII(H2O) neither aggregates nor forms a mu-oxo dimer. The pH dependence and rate-limiting second-order rate constants (kly) for oxygen transfer from H2O2 and HO2- to the iron(III) porphyrin were determined by trapping of the resultant higher-valent iron-oxo porphyrin species with 2,2'-azinodi(3-ethylbenzthiazoline)-6-sulfonate (ABTS). Reactions were monitored spectrophometrically by following the appearance of the radical ABTS.+. From a plot of the logarithm of the determined second-order rate constants for reaction of hydrogen peroxide with iron(III) porphyrin vs. pH, the composition of the transition states can be assigned for the three reactions that result in oxygen transfer to yield a higher-valent iron-oxo porphyrin species. The latter not only reacts with ABTS to provide ABTS.+ in a peroxidase-type reaction but also reacts with hydrogen peroxide to provide O2 in a catalase-type reaction. The nitrogen base 2,4,6-collidine serves as a catalyst for oxygen transfer from hydrogen peroxide to the (P)FeIII-(H2O) and (P)FeIII(HO) species. The preferred mechanism involves a 1,2-proton shift concerted with heterolytic cleavage of the peroxide O-O bond. An analogous mechanism is believed to occur in the peroxidase enzymes.

Heme models of peroxidase enzymes: deuteroferriheme-catalyzed chlorination of monochlorodimedone by sodium chlorite

The iron(III) complex of deuteroporphyrin(IX), deuteroferriheme, catalyzes the chlorination, by sodium chlorite, of the active methylene compound monochlorodimedone (MCD) to dichlorodimedone. Rate studies, carried out on a stopped-flow spectrophotometric time scale, show the chlorination to be zero-order in MCD, first-order in ClO2- and to display a complex dependence on heme. The active chlorinating agent is believed to be hypochlorite, OCl-, formed as a result of the initial two-electron oxidation of heme to peroxidatic intermediate by chlorite ion. This scheme is supported by the fact that the normal (4:1) heme:ClO2- molar stoichiometry is reduced in the presence of MCD to values approaching 2:1. This suggests that MCD is an effective scavenger of OCl-, which, in the absence of active methylene compound, serves as a two-electron oxidant of heme. The zero-order dependence of rate on MCD is attributed to the slow formation of OCl-, consequent to a mechanism in which the rate-limiting step is viewed to be the regeneration of free heme from peroxidatic intermediate, probably via a catalatic pathway. Support for such a mechanism is provided by the fact that addition of ascorbate greatly enhances the rate of MCD chlorination, presumably by accelerating the rate of heme regeneration via perioxidation reduction of the heme intermediate.

Hemin-catalyzed generation of triplet acetone

Hemin can substitute for horseradish peroxidase as a catalyst for the aerobic oxidation of isobutanal to acetone and formate. Previous studies have shown that the chemiphosphorescent emission observed in the enzyme-catalyzed reaction is due to the production of acetone in its triplet state. Although no chemiphosphorescence is observed with the model system (hemin), generation of triplet acetone in this system is indicated by an analysis of data for energy transfer to the 9,10-dibromoanthracene-2-sulfonate ion and for interception of the excited species by the sorbate ion, a known triplet quencher. These data are compared to those obtained with triplet acetone generated by thermal cleavage of tetramethyldioxetane in aqueous solution. The results are in agreement with the hypothesis that the quenching of triplet acetone by oxygen is less efficient in the enzyme catalyzed reaction, pointing to a protective role for the apoenzyme in that system.

Oxidation of deuteroferrihaem by hydrogen peroxide

1. The oxidation of deuteroferrihaem by H(2)O(2) to bile pigment and CO was studied both by stopped-flow kinetic spectrophotometry and mass spectrometry, at 25 degrees C, I=0.1m. 2. Spectrophotometric studies imply that, at constant pH, the rate of bile pigment formation is first-order with respect to [H(2)O(2)] and also proportional to [deuteroferrihaem monomer]. The effect of pH on the apparent second-order rate constant suggests that acid-ionization of deuteroferrihaem monomer is important in the reaction mechanism. 3. The relative rates of formation of O(2) (from catalytic decomposition of H(2)O(2)) and CO (from oxidation of ferrihaem) have been measured by mass spectrometry. The results are in excellent agreement with those obtained by combining kinetic data for catalytic decomposition (Jones et al., 1973, preceding paper) with the spectrophotometric results for deuteroferrihaem oxidation.

Formation of compound I by the reaction of catalase with peroxoacetic acid

1. The formation of Compound I by the reactions of bacterial and ox liver catalases with peroxoacetic acid was examined. In both cases the process occurs almost entirely by reaction of catalase with un-ionized peroxoacetic acid molecules. The result suggests an important role for the bound peroxidic proton in the enzyme-substrate interaction. 2. The peroxidatic properties of the Compounds I formed when peroxoacetic acid was used were examined by studying the oxidations of ethanol and formate; the results closely resemble those previously reported when H(2)O(2) and alkyl hydroperoxides were used. 3. Compound I formed with bacterial catalase and peroxoacetic acid is remarkably stable in the absence of added donor and the preparation has considerable potential for detailed studies of the nature of this intermediate.