Metmyoglobin/hydrogen peroxide

Peroxidases and prostaglandin H synthase, in the presence of hydrogen peroxide or a hydroperoxide, oxidize GSH to thiyl radicals [21, 58-61]. Active mammalian peroxidases include lactoperoxidase, myeloperoxidase, and eosinophil peroxidase [28]. Superoxide generated for example by xanthine/xanthine oxidase oxidizes thiols [56, 62]. Metmyoglobin/hydrogen peroxide also oxidizes GSH to form thiyl radicals [63]. 

To date, a number of procedures have been proposed for the production of ABTS + from ABTS in solutions by using chemical reactions with a variety of oxidants. The original assay developed by Miller and Rice-Evans (sometimes referred to as TEAC I) utilized the metmyoglobin—hydrogen peroxide system to produce a highly reactive intermediate (HO ), to which ABTS donates an electron, generating ABTS + [12,22]. In this case. 

The Trolox equivalent antioxidant capacity (TEAC) assay was reported first by Miller and others (1993) and Rice-Evans and Miller (1994). They used the peroxidase activity of metmyoglobin to oxidize 2,2 -azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) in the presence of hydrogen peroxide. The TEAC assay is based on the... 

The reaction of peroxidase (metmyoglobin) with hydrogen peroxide leads to the generation of a green-blue radical from a colorless compound 2,2 -azino-(u s(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS). It is slowed down in the presence of an antioxidant, an effect that is used for its quantitation in the Total Antioxidant Status Kit (manufacturer Randox, UK) [10]. Problems associated with this method are due to potential interference of the reaction compound H202 with components of the sample to be investigated. No investigation of fat-soluble compounds is possible. 

Other proteins such as methemoglobin and metmyoglobin have been observed to produce molecular oxygen in the presence of hydrogen peroxide, but at a very low rate (5). This may simply be a property of the heme, which can promote a low-level catalatic reaction in the absence of protein. Consequently, it is possible that all heme-containing proteins may exhibit catalatic reactions if assayed carefully, but such minor, largely nonquantifiable activities are not considered here. 

The reaction of other minor or type D catalases such as methemoglo-bin and metmyoglobin is not treated in detail here, because they are minor activities, significantly lower than even that of chloroperoxidase. The orientation of residues on the distal side of the heme is not optimized for the catalatic reaction to the extent that there is even a sixth ligand of the heme, a histidine, that would preclude a close association of the heme with hydrogen peroxide without a significant side-chain movement. It is only after an extended treatment with H2O2 and oxidation of the Fe that a low level of catalatic activity becomes evident. 

Nicolis S, Monzani E, Roncone R, Gianelli L, Casella L (2004) Metmyoglobin catalyzed exogenous and endogenous tyrosine nitration by nitrite and hydrogen peroxide. Chem Eur J 10 2281-2290... 

Y9. Yusa, K., and Shikama, K., Oxidation of oxymyoglobin to metmyoglobin with hydrogen peroxide Involvement of ferryl intermediate. Biochemistry 26,6684-6688 (1987). 

Fig. 4.5. ESR spectrum of the ferryl myoglobin radical (Turner et al., 1990). ESR spectrum observed on reaction of (a) metmyoglobin (225 //M) and (b) oxymyoglobin with 250 / M hydrogen peroxide in the presence of 25 mM DMPO at pH 7.4 under normoxic...

Fig. 4.6. Spectroscopic identification of ferryl myoglobin. The activation of metmyoglo-bin to ferryl myoglobin by hydrogen peroxide (x 1.25 molar excess) the development of the characteristic peaks of ferryl myoglobin as a function of time (minutes) (a) 0, (b) 0.5, (c) 2.5, (d) 4.5, (e) 6.5 and (0 8-5. The development of the characteristic peaks for ferryl myoglobin around 550 and 580 nm is shown, with a decrease in the shoulder at 630 nm, characteristic of metmyoglobin, and the peak at 515 nm.

Fig. 4.7. Formation of ferryl, met- and oxymyoglobin on activation of metmyoglobin (20 /

George, P. and D. H. Irvine. 1952. The reaction between metmyoglobin and hydrogen peroxide. Biochemical Journal 52 511-517. 

In a more recent series of experiments, George (97) has shown that the secondary hydrogen peroxide complexes of horseradish peroxidase and cytochrome-c peroxidase can be titrated with ferroevanide or ferrous ions and also appear to take part in a one oxidizing equivalent reduction to the ferric form of the enzyme. In this important chemical property they thus resemble the metmyoglobin complex in spite of marked spectroscopic differences in the visible region of the spectrum (Keilin and Hartree, 48). If the peroxide molecule is not a component part of the structure it... 

Whether lipid oxidation in muscle foods is catalysed by the iron redox cycle or by formation of the ferryl ions is not clear. However, ferrous ions react with lipid hydroperoxides much faster than with hydrogen peroxide. As shown above, if the reaction of metmyoglobin with hydroperoxides produces ferryl radicals capable of initiating lipid oxidation, it is necessary to prevent the formation of metmyoglobin or methemoglobin. At acidic pH, ferric myoglobin can initiate lipid oxidation in the presence of lipid hydroperoxides. 

Jensen and Urbain, 1936a Jensen, 1945 Niven et al., 1949 Niven, 1951). Methemoglobin and metmyoglobin, as well as hemoglobin and myoglobin, form unstable addition compounds with hydrogen peroxide, which then decompose with destruction of the heme nucleus (Keilin and Hartree, 1950). 

Compound III, the red compound found by Keilin and Mann (1937) to be produced in the presence of excess hydrogen peroxide, was not implicated in this reaction mechanism. Although these workers state that compound III is also reduced by added donors, according to Theorell (1947) it reacts more slowly than compound II. Alkyl peroxides cannot replace hydrogen peroxide in this reaction, and there is an analogy with metmyoglobin, which reacts as showm in Eq. (5) (Keilin and Hartree, 1954). 

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