Food Lipids: Chemistry, Nutrition, and Biotechnology

4. Glutathione
While the traditional approach to understanding the role of glutathione in relation to
lipid oxidation has focused on its action as a substrate for detoxification enzymes,
evidence is accumulating that glutathione in and of itself acts to control lipid oxidation
in several ways. In the first case, glutathione may reduce the initiator, ferryl
myoglobin, back to metmyoglobin [235]. In the second case, glutathione may serve
to reduce oxidized sulfhydryl groups nonenzymatically [236]. Based on standard oneelectron
reduction potentials, Buettner [237] contends that it is thermodynamically
feasible for glutathione to be oxidized to glutathione disulfide nonenzymatically, with
the simultaneous reduction of a hydroxy, peroxy, or lipid radical to a hydroperoxide.
Buettner’s pecking order of oxidative activity [237] is supported by the finding that
in frozen minced fish, glutathione declined at a faster rate than did ascorbic acid,
which in turn declined faster than �-tocopherol [238]. Similarly, glutathione and
ascorbate declined faster than �-tocopherol and ubiquinone in both light and dark
muscle of mackerel [239]. However, caution must be taken in viewing glutathione
degradation uniquely as a response to inhibition because in the oxidation of glutathione
to its disulfide, superoxide anions can be produced. If the superoxide anion
is not removed quickly through the action of superoxide dismutase, the net effect of
the reduction of harmful radicals may be minimal owing to the formation of an
active oxygen species that could lead to initiation or promotion of propagation in
lipid oxidation. Alternatively, in the presence of low concentrations of oxygen, thiol
radicals could have a greater tendency to react with each other such that oxidized
glutathione formation without the intermediate production of superoxide anion and
hydrogen peroxide could occur.
The extent to which glutathione is capable of inhibiting cellular lipid oxidation
in muscle foods is questionable. While Murai et al. [240] indicated that reduced
glutathione was effective in limiting the adverse effects of oxidized fish oil in the
diet of yellowtail fish, dietary supplementation of fingerling channel catfish did not
affect fish performance, body composition, or stability of fillet samples [241].
5. Carnosine
Carnosine (�-alanyl-L-histidine) is an endogenously synthesized dipeptide present in
beef, pork, chicken, and fish skeletal muscle at concentrations ranging from 0 to 70
mM [242–244]. Its exogenous addition to salted ground pork and beef muscle inhibited
lipid oxidation [242,246], supporting model system studies demonstrating the
inhibition of oxidation promoted by iron [247], hydrogen peroxide–activated hemoglobin
[247], lipoxygenase [247], singlet oxygen [248], peroxyl radicals
[249,250], and hydroxyl radicals [251]. Inhibitory action by carnosine may be related
to its ability to chelate copper ions [252], scavenge free radicals [253], or trap volatile
aldehydes [254,255]. While carnosine at a 0.09% level in the diet has increased the
oxidative stability of muscle [256], its high prices makes its use as a feed additive
impractical. Supplementation of diets with the carnosine precursors histidine and �alanine
has been attempted; however, it did not prove to be an efficient method for
improving the oxidative stability of pork [257].
6. Flavonoids and Phenolic Acids
Flavonoids are secondary products of plant metabolism and include more than 4,000
individual compounds divided into six subclasses: flavones, flavonones, isoflavones,
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