Inhibition of Peroxidase-Catalyzed Oxidation of Aromatic Amines by Substituted Phenols

Peroxidase-catalyzed oxidation of o-phenylene diamine (OPD) was competitively inhibited by trimethylhydroquinone (TMHQ), 4-tert-butylpyrocatechol (InH5), and 4,6-di-tert-butyl-3-sulfanyl-1,2-dihydroxybenzene (InH6). InH6 was the most efficient inhibitor (K _i = 11 M at 20°C in 0.015 M phosphate–citrate buffer, pH 6.0). The effects of InH5 and InH6 were not preceded by periods of induction of OPD oxidation products (contrary to TMHQ). Peroxidase-catalyzed oxidation of tetramethylbenzidine (TMB) was noncompetitively inhibited by InH6 and 3-(2-hydroxyethylthio)-4,6-di-tert-butylpyrocatechol (InH4), whereas o-aminophenol acted as a mixed-type inhibitor. The effects of all three inhibitors were preceded by an induction period, during which TMB oxidation products were formed. Again, InH6 was the most efficient inhibitor (K _i = 16 M at 20°C in 0.015 M phosphate–citrate buffer supplemented with 5% ethanol, pH 6.0). Judging by the characteristics of the inhibitors taken in aggregate, it is advisable to use the pairs OPD–InH5 and OPD–InH6 in systems for testing the total antioxidant activity of human biological fluids.     

Inhibition of peroxidase-catalyzed oxidation of chromogenic substrates by propyl gallate and its polydisulfide

Peroxidase-catalyzed oxidation of 2,2'-azino-di-(3-ethyl-2,3-dihydrobenzthiazoline-6-sulfonate) (ABTS) was competitively inhibited by propyl gallate (PG) and its polydisulfide (PGPDS) at 20 degrees C in 0.015 M phosphate-citrate buffer (pH 6.0). Under these conditions, the values of the inhibition constant (Ki) were equal to 62 and 5.6 microM, respectively, for PG and PGPDS. The stoichiometric inhibition factor (f; the number of radicals extinguished per molecule of an inhibitor) equaled 2.0 and 14.7, respectively, for PG and PGPDS. Peroxidase-catalyzed oxidation of o-phenylenediamine was barely affected by PG or PGPDS. PGPDS may be used as a stop-reagent of peroxidase-catalyzed ABTS oxidation, whereas PG may serve as a calibrating inhibitor in test systems for measurement of total antioxidant activity (in human biological fluids, natural preparations, juices, wines, and other objects).     

Propyl gallate inhibition of iodide and tetramethylbenzidine oxidation catalyzed by human thyroid peroxidase

The kinetic characteristics (kcat, Km, and their ratio) for oxidation of iodide (I-) at 25 degrees C in 0.2 M acetate buffer, pH 5.2, and tetramethylbenzidine (TMB) at 20 degrees C in 0.05 M phosphate buffer, pH 6.0, with 10% DMF catalyzed by human thyroid peroxidase (HTP) and horseradish peroxidase (HRP) were determined. The catalytic activity of HRP in I- oxidation was about 20-fold higher than that of HTP. The kcat/Km ratio reflecting HTP efficiency was 35-fold higher in TMB oxidation than that in I- oxidation. Propyl gallate (PG) effectively inhibited all four peroxidase processes and its effects were characterized in terms of inhibition constants Ki and the inhibitor stoichiometric coefficient f. For both peroxidases, inhibition of I- oxidation by PG was characterized by mixed-type inhibition; Ki for HTP was 0.93 microM at 25 degrees C. However, in the case of TMB oxidation the mixed-type inhibition by PG was observed only with HTP (Ki = 3.9 microM at 20 degrees C), whereas for HRP it acted as a competitive inhibitor (Ki = 42 microM at 20 degrees C). A general scheme of inhibition of iodide peroxidation containing both enzymatic and non-enzymatic stages is proposed and discussed.     

Characterization of peroxidase-catalyzed oxidation of chromogenic substrates by tetrazole and its 5-substituted derivatives

Peroxidase-catalyzed oxidation of 2,2-azino-di(3-ethyl-benzthiazolydine-6-sulfonic acid) (ABTS) and 3,3',5,5'-tetramethylbenzidine (TMB) is activated by tetrazole and its 5-substituted derivatives--5-amino-(AmT), 5-methyl- (MeT), 5-phenyl- (PhT), and 5-CF3- (CF3-T) tetrazoles. In phosphate-citrate or phosphate buffer (pH 6.4 or 7.2; 20 degrees C), the activating effect of tetrazoles on TMB and ABTS oxidation decreased in the series AmT > MeT > T > PhT > CF3-T and T > AmT > MeT > PhT, respectively. The (coefficient) degree of activation (alpha), expressed in M(-1), determined for both substrates and all activators, depended on substrate type, buffer nature, and pH (it increased as pH increased from 6.4 to 7.2). For TMB oxidation, good correlation between lgalpha and the Hammet constants sigma(meta) for m-substituents in the benzene series NH2, CH3, C6H5, and CF3 was found. It is suggested that AmT, MeT, and T can be used as activators of peroxidase-catalyzed oxidation of TMB and ABTS, as well as in designing peroxidase-based biosensors.     

Inhibition of the peroxidase-catalyzed oxidation of chromogenic substrates by alkyl-substituted diphenols

A comparative kinetic study of the peroxidase oxidation of three chromogenic substrates--2,2'-azino-bis(3-ethyl-2,3-dihydrobenzothiazoline-6-sulfonic acid), o-phenylenediamine (PDA), and 3,3',5,5'-tetramethylbenzidine--inhibited by trimethylhydroquinone and six tert-butylated pyrocatechols (InH) was carried out at 20 degrees C in 0.015 M phosphate-citrate buffer (pH 6.0) containing organic cosolvents (0-10% ethanol or DMF). The inhibitors were quantitatively characterized by the inhibition constants (Ki), the duration of the lag period in the oxidation product formation (delta tau), and the stoichiometric coefficient of inhibition that specifies the number of radicals terminated by one InH molecule (f). The inhibition could be competitive, noncompetitive, mixed, or uncompetitive, which depended on the nature and structure of the chromogenic substrate-diatomic phenol pair. Various substrate-diatomic phenol pairs exhibited Ki values within the range of 11-240 microM and f values from 0.7 to 2.6. The absence of a lag period was characteristic of oxidation of the substituted o-phenylenediamine-substituted pyrocatechol. The total kinetic parameters and properties of the components allowed us to suggest six chromogenic substrate-substituted diatomic phenol pairs for use in test systems for the determination of antioxidant activity in human body fluids, natural biological preparations, and food. The English version of the paper: Russian Journal of Bioorganic Chemistry, 2004, vol. 30, no. 5; see also http: // www.maik.ru.     

Peroxidase-catalyzed co-oxidation of 3,3',5,5'-tetramethylbenzidine in the presence of substituted phenols and their polydisulfides

The steady-state kinetics of the horseradish peroxidase (HRP)-catalyzed oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) has been studied in the presence of 2-amino-4-nitrophenol (ANP), gallic acid (GA) or 4,4'-dihydroxydiphenylsulfone (DDS) and their polydisulfides poly(ADSNP), poly(DSGA), poly(DSDDS) at 20 degrees C in 10 mM phosphate buffer, pH 6.4, supplemented with 5-10% dimethylformamide. The second-order rate constants for the reactions of ANP, GA, poly(DSGA) and poly(DSDDS) with HRP-Compound I (k2) and Compound II (k3) have been determined at 25 degrees C in 10 mM phosphate buffer, pH 6.0 by stopped-flow spectrophotometry. ANP, GA and their polydisulfides strongly inhibited HRP-catalyzed TMB oxidation. Inhibition constants (Ki) and stoichiometric coefficients of inhibition (f) have been determined for these reactions. The most effective inhibitor was poly(DSGA) (Ki=1.3 microM, f=35.6). The oxidation of substrate pairs by HRP, i.e., TMB-DDS and TMB-poly(DSDDS) at pH 7.2 resulted in a approximately 8- and approximately 12-fold stimulation of TMB oxidation rates, respectively. The mechanisms of the HRP-catalyzed co-oxidation of TMB-phenol pairs are discussed.     

Inhibition of Peroxidase-Catalyzed Oxidation of Chromogenic Substrates by Propyl Gallate and Its Polydisulfide

Peroxidase-catalyzed oxidation of 2,2′-azino-di-(3-ethyl-2,3-dihydrobenzthiazoline-6-sulfonate) (ABTS) was competitively inhibited by propyl gallate (PG) and its polydisulfide (PGPDS) at 20° C in 0.015 M phosphate-citrate buffer (pH 6.0). Under these conditions, the values of the inhibition constant (K _i ) were equal to 62 and 5.6 μM, respectively, for PG and PGPDS. The stoichiometric inhibition factor (f; the number of radicals extinguished per molecule of an inhibitor) equaled 2.0 and 14.7, respectively, for PG and PGPDS. Peroxidase-catalyzed oxidation of o-phenylenediamine was barely affected by PG or PGPDS. PGPDS may be used as a stop-reagent of peroxidase-catalyzed ABTS oxidation, whereas PG may serve as a calibrating inhibitor in test systems for measurement of total antioxidant activity (in human biological fluids, natural preparations, juices, wines, and other objects).__________Translated from Prikladnaya Biokhimiya i Mikrobiologiya, Vol. 41, No. 4, 2005, pp. 376–382.Original Russian Text Copyright © 2005 by Naumchik, Karasyova, Metelitza.     

Inhibition of peroxidase-catalyzed oxidation of 3,3′,5,5′-tetramethylbenzidine by aminophenols

Peroxidase-catalyzed oxidation of 3,3,5,5-tetramethylbenzidine (TMB) was inhibited by o-aminophenol (AP), 2-amino-4-tert-butylphenol (ATBP), 2-amino-4,6-di-tert-butylphenol (ADTBP), and 4-tert-butylpyrocatechol (TBP). Inhibitors were characterized by inhibition constant K _i and stoichiometric coefficient f, the number of radicals terminated by one inhibitor molecule. The most efficient inhibitor is ADTBP characterized by K _i = 36 µM in 0.015 M phosphate citrate buffer, pH 6.0, at 20°C. According to their antiradical efficiency, the studied inhibitors can be arranged as follows: ADTBP > ATBP > AP > TBP. The role of the NH₂ group in the inhibitory capacity of aminophenols is discussed. Using gas-liquid chromatography, kinetics of consumption of the initial components and accumulation of the reaction products on peroxidase-catalyzed oxidation of the TMB-TBP pair was studied; the data clarify the stages of a complex process of co-oxidation of amines and phenols.Translated from Biokhimiya, Vol. 70, No. 3, 2005, pp. 397–405.Original Russian Text Copyright © 2005 by Naumchik, Karasyova, Metelitza, Edimecheva, Sorokin, Shadyro.     

Peroxidase-catalyzed oxidation of 3,3',5,5'-tetramethylbenzidine in the presence of 2,4-dinitrosoresorcinol and polydisulfide derivatives of resorcinol and 2,4-dinitrosoresorcinol

A comparative study of the kinetics of peroxidase-catalyzed oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of 2,4-dinitrosoresorcinol (DNR), its polydisulfide derivative [poly(DNRDS)], and resorcinol polydisulfide [poly(RDS)], substances that competitively inhibit the formation of TMB conversion product, was carried out. The inhibition constants, Ki for DNR, poly(DNRDS), and poly(RSD) were determined at 20 degrees C and pH 6.4 to be 110, 13.5, and 0.78 microM, respectively. The stoichiometric coefficients of inhibition were calculated to be 0.38 and 76 for poly(DNRDS) and poly(RDS), respectively. In the pH range 6.4-7.0, the initial rates of the peroxidative oxidation of TMB, and its mixtures with DNR and poly(DNRDS) and the Ki value for poly(RDS) substantially decreased with increasing pH. The kinetic parameters of poly(RDS) (Ki 0.22-0.78 microM and f76) suggest that it is the most efficient inhibitor of peroxidase oxidation of TMB: in micromolar concentrations, it completely stops this process and can be used in EIA.     

Activation of peroxidase-catalyzed oxidation of 3,3',5,5'-tetramethylbenzidine with poly(salicylic acid 5-aminodisulfide)

5-Aminosalicylic acid (5-ASA) inhibited by a mixed mechanism the peroxidase catalyzed oxidation of tetramethylbenzidine (TMB) in 0.015 M phosphate-citrate buffer (pH 6.4) supplemented with 5% DMSO and 5% DMF. Poly(salicylic acid 5-aminodisulfide) (poly(SAADS)) in 0.01 M phosphate buffer (pH 6.2-7.4) supplemented with 5% DMSO and 5% DMF effectively activated the peroxidase-catalyzed oxidation of TMB. The activation was quantitatively characterized by coefficients (M^–1) determined at different pH values: increased linearly with increase in pH up to the maximal value of 2.44·10⁵ M^–1 at pH 7.0. The activating effect of poly(SAADS) on the peroxidase-catalyzed oxidation of TMB is explained by the activator properties of polyelectrolyte, with its anionic form interacting with peroxidase sites responsible for the acid-base catalysis.     

Human thyroid peroxidase: inhibition of iodide ion and 3,3',5,5'-tetramethylbenzidine hydrolysis by phenol antioxidants]

A comparative kinetic study on the poly(gallic acid disulfide) (poly(DSGA)) inhibition of the iodide ion oxidation and on the 2-hydroxy-3,5-di-tert-butyl-N-phenylaniline (butaminophene) inhibition of 3,3',5,5'-tetramethylbenzidine (TMB) oxidation involving human thyroid peroxidase (hTPO) and horseradish peroxidase (HRP) was performed. The inhibition processes were characterized with the inhibition constants Ki and stoichiometric inhibition coefficients f, indicating the number of radical particles perishing on one inhibitor molecule. In the case of poly(DSGA), the Ki values for the I- oxidation were 0.60 and 0.04 microM, and the coefficients f were 13.6 and 16.5 for hTPO and HRP, respectively, which evidences the regeneration and high effectiveness of the polymeric inhibitor. In the case of butaminophene, the Ki values for TMB oxidation were 38 and 46 microM for hTPO and HRP, respectively. The coefficients f were 1.33 and 1.47, respectively, to reveal that butaminophene does not regenerate. The inhibition mechanisms for I- and TMB oxidation involving the two peroxidases are discussed.     

Noncompetitive activation of peroxidase oxidation of o-phenylenediamine by melamine

Peroxidase-catalyzed oxidation of o-phenylenediamine (PDA) is greatly activated with melamine (MA) in 15 mM phosphate-citrate buffer at pH 6.0-7.4 in a noncompetitive manner: kcat and Km increase in direct proportion to the MA concentration. An extent of the activation is quantitatively characterized with a coefficient alpha (in M-1), which essentially increases along with the rise in pH from 6.0 to 7.4. MA acts as a nucleophilic catalyst in the oxidation process: it most likely affects the peroxidase active site from the distal position of heme. MA non-competitively inhibits the peroxidase oxidation of PDA at pH 4.3, since it completely loses its nucleophilic properties in acidic medium. A rapid, highly accurate, and simple analytical test system based on the kinetics of melamine-activated oxidation of PDA is proposed for the quantitative determination of melamine within the concentration range of 10(-4)-10(-3) M. This test system uses the spectrophotometric determination of the PDA oxidation product at 455 nm.     

Inhibition of peroxidase oxidation of 3,3',5,5'-tetramethylbenzidine and o-phenylenediamine by 1-amino-2-naphthol-4-sulfonic acid and its polysulfide]

The kinetics of coupled peroxidation of 3,3',5,5'-tetramethylbenzidine and 1-amino-2-naphtol-4-sulfonic acid (ANSA) or its polydisulfide (poly(ADSNSA)) was studied in 0.01 M phosphate buffer (pH 6.4) at 20 degrees C. Both ANSA and poly(ADSNSA) strongly inhibited the TMB oxidation resulting in a marked delay in the product formation. Stoichiometric inhibition coefficients f, i.e., the average numbers of free-radical particles terminated by one inhibitor molecule, were estimated. The free-radical trapping effect of poly(ADSNSA) was 7.5 times greater than that of ANSA. Kinetics of coupled o-phenylenediamine (PhDA) and ANSA or poly(ADSNSA) oxidation was studied in phosphate-citrate buffers at pH 3 to 7. No lag periods in oxidation product accumulation were observed under any of the reaction conditions. A weak activation of PhDA conversion depending on pH and PhDA/ANSA ratios was observed at low ANSA concentrations, whereas increased ANSA or poly(ADSNSA) concentrations were inhibitory. The degree of PhDA inhibition was maximal in acid media, reached minimum at pH 5 to 6, and than again increased at pH above 6. Tentative mechanism of coupled aromatic amine phenol bi-substrate system peroxidation is discussed.     

Initiation and inhibition of free-radical processes in biochemical peroxide systems: A review

The role of complexes containing oxygen or peroxide in monooxygenase systems and models thereof, as well as in peroxidase-and quasi-peroxidase-catalyzed processes, has been reviewed. Pathways of conversion of these intermediate complexes involving single-electron (radical) and two-electron (heterolytic) mechanisms are dealt with. Peroxidase-catalyzed co-oxidation of aromatic amines and phenols is analyzed; inhibition and activation of peroxidase-catalyzed reactions are characterized quantitatively. Oxidation of chromogenic substrates (ABTS, OPD, and TMB) in the presence of phenolic inhibitors or polydisulfides of substituted phenols is characterized by inhibition constants (K _i, μmol). Activation of peroxidase-catalyzed oxidation of the same substrates is characterized by the degree (coefficient) of activation (α, M^?1), which was determined for 2-aminothiazole, melamine, tetrazole, and its 5-substituted derivatives. Examples of applied use of peroxidase-catalyzed enzyme and model systems are given (oxidation of organic compounds, chemical analysis, enzyme immunoassay, tests for antioxidant activity of biological fluids).     

Activation of peroxidase-catalyzed oxidation of chromogenic substrates by tetrazole and its 5-substituted derivatives

Peroxidase-catalyzed oxidation of 2,2-azino-di(3-ethyl-benzthiazolydine-6-sulfonic acid) (ABTS) and 3,3,5,5-tetramethylbenzidine (TMB) is activated by tetrazole and its 5-substituted derivatives—5-amino-(AmT), 5-methyl-(MeT), 5-phenyl-(PhT), and 5-CF₃-(CF₃-T) tetrazoles. In phosphate-citrate or phosphate buffer (pH 6.4 or 7.2; 20°C), the activating effect of tetrazoles on TMB and ABTS oxidation decreased in the series AmT > MeT > T > PhT > CF₃-T and T > AmT > MeT > PhT, respectively. The coefficient (degree) of activation (), expressed in M^–1, determined for both substrates and all activators, depended on substrate type, buffer nature, and pH (it increased as pH increased from 6.4 to 7.2). For TMB oxidation, good correlation between log and the Hammet constants _meta for m-substituents in the benzene series NH₂, CH₃, C₆H₅, and CF₃ was found. It is suggested that AmT, MeT, and T can be used as activators of peroxidase-catalyzed oxidation of TMB and ABTS in enzyme immunoassay and designing peroxidase-based biosensors.Translated from Prikladnaya Biokhimiya i Mikrobiologiya, Vol. 41, No. 2, 2005, pp. 148–157.Original Russian Text Copyright © 2005 by Karasyova, Gaponik, Metelitza.     

Inhibition of thymidine kinase by P1-(adenosine-5')-P5-(thymidine-5')-pentaphosphate

Potential bisubstrate analogs, with adenosine and thymidine joined at their 5' positions by polyphosphoryl linkages of varying lengths (ApndT, where n = the number of phosphoryl groups), were examined as inhibitors of cytosolic thymidine kinase from blast cells of patients with acute myelocytic leukemia. Ki values were 1.2 microM for Ap3dT, 0.31 microM for Ap4dT, 0.12 microM for Ap5dT, and 0.19 microM for Ap6dT. The best inhibitor of the cytosolic enzyme, Ap5dT, was somewhat less effective as an inhibitor of the mitochondrial enzyme (Ki = 0.50 microM). In addition to their inhibitory modes of binding by the cytosolic enzyme, these compounds were bound at considerably lower concentrations (Kd = 0.029 microM for Ap4dT, 0.0025 microM for Ap5dT, and 0.0027 microM for Ap4dT), in such a way as to protect the cytosolic enzyme from thermal inactivation at 37 degrees C in the absence of substrates.     

Initiation and inhibition of free-radical processes in biochemical peroxidase systems: a review

The role of complexes containing oxygen or peroxide in monooxygenase systems and models thereof, as well as in peroxidase- and quasi-peroxidase-catalyzed processes, has been reviewed. Pathways of conversion of these intermediate complexes involving single-electron (radical) and two-electron (heterolytic) mechanisms are dealt with. Coupled peroxidase-catalyzed oxidation of aromatic amines and phenols is analyzed; inhibition and activation of peroxidase-catalyzed reactions are characterized quantitatively. Oxidation of chromogenic substrates (ABTS, OPD, and TMB) in the presence of phenolic inhibitors or polydisulfides of substituted phenols is characterized by inhibition constants (Ki, micromol). Activation of peroxidase-catalyzed oxidation of the same substrates is characterized by the degree (coefficient) of activation (alpha, M(-1)), which was determined for 2-aminothiazole, melamine, tetrazole, and its 5-substituted derivatives. Examples of applied use of peroxidase-catalyzed enzyme and model systems are given (oxidation of organic compounds, chemical analysis, enzyme immunoassay, tests for antioxidant activity of biological fluids).     

Potent inhibition of membrane-bound rat intestinal alkaline phosphatase by a new series of phosphate analogues.

The inhibition by phosphonates and phosphate analogues of the alkaline phosphatase activity of rat intestinal brush-border membrane vesicles was studied at pH 7.5 and 30 degrees C. Phenylene-1,3-diphosphonate, 2,6-dinitrophenylphosphonate and phosphonoacetaldehyde were found to be competitive inhibitors, with Ki values in the range 16-80 microM. Adenosine 5'-[beta-thio]diphosphate and adenosine 5'[gamma-thio]triphosphate are also very potent inhibitors, with Ki values of approx. 10 microM. The inhibition produced by these thiophosphates was mainly competitive but with a slight non-competitive element. Adenosine 5'-[beta gamma-imido]triphosphate is also a competitive inhibitor of the alkaline phosphatase, but oxidation of the ribose moiety of this compound with NaIO4 results in an active-site-directed irreversible inhibitor that could be of general use in studies of the mechanism of action of this enzyme.     

Sea urchin sperm peroxidase is competitively inhibited by benzohydroxamic acid and phenylhydrazine

Sea urchin sperm contain a phenylhydrazine-sensitive peroxidase that is believed to use hydrogen peroxide produced by the fertilized egg to reduce sperm fertility and thereby assist in the prevention of polyspermy. Strongylocentrotus purpuratus sperm were treated initially with hypotonic phosphate buffer (pH 7.0) to remove catalase and then extracted with 0.5% Triton X-100 in 0.5 M acetate buffer (pH 5.0). Peroxidase activity in this detergent extract was assayed using 3,3',5,5'-tetramethyl benzidine (TMB) as oxidizable substrate. Kinetic studies showed that the Km for TMB is 250 microM. Benzohydroxamic acid and phenylhydrazine are known to be competitive inhibitors of a variety of plant and animal peroxidases. These substances were found to competitively inhibit the sea urchin sperm peroxidase: for benzohydroxamic acid, Ki = 51.2 microM, mean inhibitory dose (ID50) = 146.7 microM; for phenylhydrazine, Ki = 201 nM, ID50 = 303 nM. These findings indicate that the biochemical properties of the sea urchin sperm peroxidase resembles those of peroxidases found in somatic tissues where oxygen radicals are produced by phagocytes to kill bacteria and support our hypothesis that the sperm peroxidase has a functional role in the prevention of polyspermy during fertilization.     

Characterization of maize polyamine oxidase.

Some structural and biochemical characteristics of polyamine oxidase (PAO) purified from maize shoots have been examined. The enzyme has only alanine as N-terminal amino acid and its N-terminal sequence shows a significant degree of homology with tryptophan 2-monooxygenase from Pseudomonas syringae pv. savastanoi. The pH optimum for the stability of the native enzyme is 5, similar to that of the barley leaf enzyme. Calorimetric analysis shows a single two-state transition at pH 6 with Tm 49.8 degrees. At pH 5 the thermal stability is increased by more than 14 degrees. Amine oxidation products, delta 1-pyrroline and diazabicyclononane, are competitive inhibitors of PAO activity (apparent Ki = 400 and 100 microM respectively). Moreover these compounds improve the thermal stability of the enzyme. N1-Acetylspermine, which is a good substrate for mammalian PAO, acts as a non-competitive inhibitor for the plant enzyme.