Food Lipids: Chemistry, Nutrition, and Biotechnology

Table 6 Potential Catalysts of Lipid Oxidation in Muscle Foods
Nonenzymic
Transition low molecular weight metal ions
Metmyoglobin–H 2O 2
Porphyrin compounds (sensitizers for the generation of singlet oxygen)
Enzymic
Lipoxygenase
Myeloperoxidase
Membrane enzymic systems that reduce iron
and pork generally contain more myoglobin than hemoglobin, whereas chicken contains
a greater amount of hemoglobin [109]. The ability of heme proteins to promote
lipid peroxidation has been demonstrated by many researchers [110–112]. One
mechanism of this activation involves decomposition of preformed fatty acid hydroperoxides
to peroxyl radicals, which in the presence of oxygen propagate lipid peroxidation
[113]. Results of experiments with inhibitors suggest that the major pathway
of peroxyl radical production involves high valence state iron complexes in a
reaction analogous to the classical peroxidase pathway [114]:
3� 4� �•
Fe (porphyrin) � ROOH → Fe = O(porphyrin) � ROH
4� �• 4� • �
Fe = O(porphyrin) � ROOH → Fe = O(porphyrin) � ROO � H
Heme compounds, particularly metmyoglobin and oxy-/deoxymyoglobin, also
activate lipid oxidation through an intermediate species following interaction of the
heme moiety with hydrogen peroxide [115–118]. An ongoing debate attempts to
ascertain whether this intermediate species activates oxidation of PUFAs through
ferryl oxygen [119,120], through a tyrosine peroxyl radical on the heme compound
[121,122], or through radical transfer to other proteins that generate long-lived radicals
[123,124]. Enhanced formation of this activated heme species has been observed
when the heme compound is preincubated in the presence of the secondary lipid
oxidation product 4-hydroxynonenal [125]. Decreased formation of the activated species,
on the other hand, occurred in the presence of the free fatty acid linoleate [126].
In the latter case, it was proposed that metmyoglobin bound to the fatty acid anion
forming a hemichrome species that could not be activated by hydrogen peroxide.
However, exposure to hydrogen peroxide has also been shown to lead to release of
nonheme iron from myoglobin and hemoglobin [127–129]. Alternatively, heme may
donate its reducing equivalents to low molecular weight iron and copper complexes
[130,131] and thereby contribute to catalysis of lipid oxidation.
In the nonheme form, iron participates in the production of the reactive oxygen
species, the hydroxyl radical, via the chemical Fenton reaction: Fe 2� � H 2O 2 → Fe 3�
� OH • � OH � . This reaction is effective when Fe 3� can be recycled to Fe 2� by
various reducing agents.
Levels of low molecular weight nonheme iron are initially low, being only
2.4–3.9% of total muscle iron in beef, lamb, pork, and chicken [109] and 6.7–13.9%
of total iron in flounder and mackerel muscle [132]. However, in muscles that have
been processed and stored, increases in the catalytic low molecular weight iron fraction
have been found [132–134]. Potential sources of nonheme iron are dislodgment
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