Metmyoglobin

As mentioned earlier, physiological concentrations of carotenoids in vivo are in the micromolar range, mainly because of limited bioavailabiUty. Also, the antioxidant efficiencies of carotenoids after absorption are probably limited. Concentrations before absorption are much higher and can justify possible antioxidant actions in vivo. To test this hypothesis, Vulcain et al. developed an in vitro system of lipid peroxidation in which the oxidative stress is of dietary origin (metmyoglobin from meat) and different types of antioxidants (carotenoids, phenols) are tested. 

Vulcain, E. et al.. Inhibition of the metmyoglobin-induced peroxidation of hnoleic acid by dietary antioxidants action in the aqueous versus lipid phase. Free Rad. Res., 39, 547, 2005. 

Kanner, J. and Harel, S. (1985). Initiation of membranal lipid peroxidation by activated metmyoglobin and methemoglobin. Arch. Biochem. Biophys. 237, 314-321. 

For CO-myoglobin a Fe-CO stretch at 502 cm and a Fe-C-O bend at 572 cm has been observed [112]. The drastic increase of the out-of-plane stretch compared to deoxy- and metmyoglobin is due to the strong covalent Fe-CO bond. Raising the temperature from 50 to 110 K led to a broad resonance at around 25 cm which has been assigned to the translational motion of the whole heme moiety. 

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. 

Fig. 44. Single crystal-like I4N-ENDOR spectrum of metmyoglobin with B0 along gli temperature 2.1 K (Mb chromatog-raphically purified, 6 nM in 50% (v/v) glycerol, 0.1 M potassium phosphate buffer, pH = 6.0). (Adapted from Ref. 241)...

Hayashi A, Suzuki T, Shin M. An enzymic reduction system for metmyoglobin and methemoglobin, and its application to functional studies of oxygen carriers. Biochim Biophys Acta 1973 310 309. 

Denaturation of hemoproteins in cooked meats leads to liberation of the heme and oxidation of the porphyrin ring. Nonheme iron is less available nutritionally than heme iron and affects lipid oxidation more. In methemoglo-bin and metmyoglobin solutions heated for one hour at 78°C and 100°C the degradation of heme was about 22 to 26%, while after two hours at 120°C it increased to about 85 to 95% (Oellingrath, 1988). In meat cookery, however, such severe conditions do not apply. 

Evans, S.V. Brayer, G.D. High-resolution study of the three-dimensional structure of horse heart metmyoglobin./. Mol. Biol. 1990, 213, 885-897. 

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. 

In vivo heme is released into the plasma by erythrocyte lysis in the form of hemoglobin and by tissue trauma in the form of myoglobin, and both heme proteins are quickly oxidized to their ferric heme forms (methemoglobin and metmyoglobin) at the oxygen tension found in tissue capillary beds. 

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