Horseradish peroxidase compound oxidation

The form of the enzyme with the greatest oxidation potential is known as horseradish peroxidase, compound 1 (HRP-I), which consists of a radical cation stabilized throughout the highly conjugated protoporphyrin IX ring system. In the presence of vindoline, HRP-I is reduced to HRP-H, an Fe(IV) form of the enzyme. The vindoline cation radical 55 thus formed eliminates a second elec-... 

Job D, Dunford HB (1976) Substituent effect on the oxidation of phenols and aromatic amines by horseradish peroxidase Compound I. Eur J Biochem 66 607-614... 

Sakurada J, Sckiguchi R, Sato Ket al (1990) Kinetic and molecular orbital studies on the rate of oxidation of monosubstituted phenols and anilines by horseradish peroxidase Compound II. Biochemistry 29 4093 1098... 

Both oxidizing equivalents of the hydroperoxide are incorporated into compound I, through an oxygen-atom transfer process ". A free radical is generated elsewhere in the molecule on amino acid residue(s) in the case of yeast cytochrome c peroxidase" and at a site strongly coupled to the iron in horseradish peroxidase . (Compound I of yeast cytochrome c peroxidase is called complex ES in earlier literature.) EPR results on horseradish peroxidase are interpreted in terms of a porphyrin rr-cation radical for compound I . Thus, EPR data prove that one oxidizing equivalent obtained from the hydroperoxide is a free radical species"" ... 

Although the precise mechanism for the enhancement remains unknown, Thorpe and Kricka (T8) have proposed that enhancers render the sequence of events for unenhanced luminol oxidation (see Section 3.1.3) more efficient. This would be consistent with kinetic studies on the reactivity of phenol enhancers with the horseradish peroxidase intermediates Compounds I and II (Hll, VIO). Specifically, the hypothesis is that (a) horseradish peroxidase Compounds I and II... 

MODELS FOR HORSERADISH PEROXIDASE COMPOUND H PHENOL OXIDATION WITH OXOIRON(IV) PORPHYRINS... 

The first application of EXAFS spectroscopy to the ferryl states of heme systems was reported by Penner-Hahn et al. in 1983 [143]. This work included a comparative study of the Groves and Balch model complexes, and of horseradish peroxidase Compounds I and II. The EXAFS spectra and corresponding Fourier transforms of the four high-valent systems (two proteins and two models), taken from a subsequent, more complete, analysis of the data [107], are displayed in Figs. 19 and 20. Table 2 contains a summary which shows the Fe-O(oxo) and Fe-Np bond lengths for a variety of oxidized heme proteins and their models. 

Chance et al. concluded that the valence states for the central iron of horseradish peroxidase Compounds I and II were both more- highly oxidized than Fe [144]. The identical conclusion had been reached in the earlier study of high-valent horseradish peroxidase complexes [143]. Furthermore, an Fe=0 bond distance of 1.64 + 0.02 A was determined by Chance et al. for horseradish peroxidase Compound I [144]. This finding was essentially identical to that of Penner-Hahn et al. [143],... 

Addition of organic peroxides to deuterioferrihaem produces an oxidant which contains iron in a formal oxidation state greater than three. The species formed with hydrogen peroxide and different organic peroxides exhibit identical kinetic behaviour in the oxidation of I . A rate constant of 1.6 x 10 1 moh s was evaluated at pH 8.05 (25 °C) and no dependence on pH was detected above pH 7.5. Comparison with the corresponding reactions of horseradish peroxidase Compounds I [iron(v)] and II [iron(iv)] does not provide an unambiguous assignment of the oxidation state of the adduct. 

According to Reichl et al. (2000), the oxidation of pholasin by compound I or II of horseradish peroxidase induces an intense light emission, whereas native horseradish peroxidase shows only a small effect. The luminescence of pholasin by native myeloperoxidase (verdoperoxidase) is diminished by preincubation with catalase, which is interpreted as the result of the removal of a trace amount of naturally occurring H2O2 in the buffer (about 10-8 M) that forms compound I... 

Regeneration of superoxide during the oxidation of thiols hints at the possible prooxidant effect of these antioxidants. This suggestion was recently confirmed by Mottley and Mason [212] who have showed that superoxide was formed in the oxidation of DHLA by horseradish peroxidase in the presence of phenol. However, DHLA is dithiolic compound and the other mechanisms such as the concerted mechanism, which has been proposed earlier for flavonoids may be realized (Figure 29.6). 

Nitric oxide and nitrite react with other peroxidase enzymes such as horseradish peroxidase (HRP) (138a,139), lactoperoxidase (138a) and eosinophil peroxidase (140) similarly. The rate constants for reaction of NO with compounds I and II in HRP were found to be 7.0 x 105M 1s 1 and 1.3 x 106M 1s 1, respectively (139). Catalytic consumption of NO as measured by an NO sensitive electrode in the presence of HRP compounds I and II is shown in Fig. 5 where accelerated consumption of NO is achieved even in deoxygenated solutions (140). 

Acyl nitroso compounds (3, Scheme 7.2) contain a nitroso group (-N=0) directly attached to a carbonyl carbon. Oxidation of an N-acyl hydroxylamine derivative provides the most direct method for the preparation of acyl C-nitroso compounds [10]. Treatment of hydroxamic acids, N-hydroxy carbamates or N-hydroxyureas with sodium periodate or tetra-alkyl ammonium periodate salts results in the formation of the corresponding acyl nitroso species (Scheme 7.2) [11-14]. Other oxidants including the Dess-Martin periodinane and both ruthenium (II) and iridium (I) based species efficiently convert N-acyl hydroxylamines to the corresponding acyl nitroso compounds [15-18]. The Swern oxidation also provides a useful alternative procedure for the oxidative preparation of acyl nitroso species [19]. Horseradish peroxidase (HRP) catalyzed oxidation of N-hydroxyurea with hydrogen peroxide forms an acyl nitroso species, which can be trapped with 1, 3-cyclohexanone, giving evidence of the formation of these species with enzymatic oxidants [20]. 

Indicine IV-oxide (169) (Scheme 36) is a clinically important pyrrolizidine alkaloid being used in the treatment of neoplasms. The compound is an attractive drug candidate because it does not have the acute toxicity observed in other pyrrolizidine alkaloids. Indicine IV-oxide apparently demonstrates increased biological activity and toxicity after reduction to the tertiary amine. Duffel and Gillespie (90) demonstrated that horseradish peroxidase catalyzes the reduction of indicine IV-oxide to indicine in an anaerobic reaction requiring a reduced pyridine nucleotide (either NADH or NADPH) and a flavin coenzyme (FMN or FAD). Rat liver microsomes and the 100,000 x g supernatant fraction also catalyze the reduction of the IV-oxide, and cofactor requirements and inhibition characteristics with these enzyme systems are similar to those exhibited by horseradish peroxidase. Sodium azide inhibited the TV-oxide reduction reaction, while aminotriazole did not. With rat liver microsomes, IV-octylamine decreased... 

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