Characterizing Endogenous Pro-oxidants and Potent Antioxidants in Muscle Food Systems

Abstract

There are many possible pro-oxidants in muscle foods that include low molecular metal complexes, lipoxygenases, myoglobin, and hemoglobin. An inhibitor selective for hemoglobin was shown to strongly inhibit lipid oxidation in comminuted trout whole muscle, turkey thigh muscle, and salted pork semimembranosus. This suggested residual hemoglobin was the major pro-oxidant in these muscle foods in a highly comminuted state. The ability of hypervalent hemoglobin species and heme released from the globin represent two possible pathways by which hemoglobin can promote lipid oxidation. The heme release mechanism was investigated by using site-directed mutagenesis of sperm whale myoglobin to produce variants with low and high heme affinity. The low heme affinity variant readily facilitated lipid oxidation in washed muscle while the high heme affinity variant was weakly pro-oxidative. These finding implicated heme release as a likely mechanism by which hemoglobin promotes lipid oxidation. Regarding antioxidants, we have found that quercetin was more effective at inhibiting lipid oxidation in muscle systems compared to glycosylated quercetin indicating increasing water solubility of the antioxidant decreased efficacy. Further, propyl gallate most effectively inhibited lipid oxidation compared to gallic acid, octyl gallate, and lauryl gallate, indicating intermediate polarity most effectively inhibited lipid oxidation. We have also shown that phospholipase A2 (PLA2) can be highly effective at inhibiting hemoglobin-mediated lipid oxidation. We observed a lipid hydroperoxide depletion mechanism by PLA2 without formation of secondary lipid oxidation products. It is postulated that cysteine or methionine residues of proteins in muscle reduce fatty acid hydroperoxides released from phospholipids by PLA2 to fatty acid hydroxides that are weakly reactive. Other antioxidant mechanisms of PLA2 may involve rearrangement of the lipid bilayer or antioxidant effects of the hydrolysis products, namely lysophospholipids and free fatty acids.