Page 45 - Natural Antioxidants, Applications in Foods of Animal Origin

24 Natural Antioxidants: Applications in Foods of Animal Origin
VetBooks.ir (1997b). Hogg et al. (1994) showed that oxymyoglobin can promote oxida-

tive modification of low-density lipoprotein. Galaris et al. (1990) showed
visible absorption spectral change of oxymyoglobin upon incubation with
linoleic acid at physiological pH. This could be attributed to the formation
of the noncatalytic low-spin myoglobin derivative, hemochrome (Akhrem
et al., 1989).

1.6.3 ROLE OF METMYOGLOBIN IN LIPID OXIDATION

High-spin iron (III) myoglobin, commonly known as metmyoglobin, binds
a molecule of water at the sixth coordination site of the heme iron (Pegg
& Shahidi, 1997). Like hemochromes, the low-spin iron (III) myoglobin
species known as hemichromes can be formed by disturbance of the
globin structure. Hemichrome formation is either reversible or irreversible
depending on the type of ligand at the sixth coordination site of the iron and
the extent of globin denaturation. Hemichrome formation from iron (III)
myoglobin is the intermediate step in the heat denaturation of myoglobin
in muscle foods (Baron & Andersen, 2002). Post mortem process, espe-
cially the pH fall, continuously inactivate the reductive enzyme systems and
stimulate acid-catalyzed autoxidation of the iron (II) states to the iron (III)
state of myoglobin, resulting in the accumulation of metmyoglobin in meats
(George & Stratmann, 1954; Gotoh & Shikama, 1976).
Formation of metmyoglobin is highly correlated with lipid oxidation in
muscle foods (Andersen & Skibsted, 1991). Baron et al. (1997) found that
metmyoglobin is an effective prooxidant at acidic pH and in the presence
of hydroperoxides. In contrast, at physiological pH and in the presence of
lipids, metmyoglobin can undergo a rapid neutralization due to formation of
the noncatalytic heme pigment. However, further denaturation of the heme
proteins due to a high lipophilic environment may result in heme release
or further exposure of the heme group to the surrounding lipids, thereby
inducing lipid peroxidation (Baron & Andersen, 2002). Metmyoglobin acts
as a prooxidant in raw fish more effectively than in raw turkey, chicken,
pork, beef, and lamb (Livingston et al., 1981).
In addition, the lipid to heme protein ratio is an important factor
affecting the prooxidative activity of heme proteins (Kendrick & Watts,
1969). At lower linoleate/heme protein ratios, heme proteins become inef-
fective initiators of lipid oxidation (Nakamura & Nishida, 1971). The
mechanism responsible for the inhibition of lipid oxidation at low lino-
leate/heme protein ratios has been proposed. The fatty acid anions bind