Protein antioxidants for the stabilization of lipid foods: current and potential applications - Oxidation in Foods and Beverages and Antioxidant Applications - page 255

reactions. In biological tissues, copper is bound by proteins such as serum
albumin (one cupric ion per protein) and ceruloplasmin (six cupric ions per
protein). These proteins can inhibit lipid oxidation as can been seen in the
copper-catalyzed oxidation of low-density lipoprotein (LDL), where oxidation
rates are inversely related to the concentration of BSA (Bourdon et al., 1999;
Schnitzer et al., 1997).

11.4 Non-specific protein antioxidant mechanisms
11.4.1 Scavenging of free radicals and reactive oxygen species
Proteins derived from both animal and plant sources have been shown to inhibit
lipid oxidation reactions in food lipid systems (Taylor and Richardson, 1980;
Tong et al., 2000; Faraji et al., 2004; Diaz et al., 2003; Elias et al., 2006). It has
been reported that a major contributor to the antioxidant activity of these
proteins and polypeptides is free radical scavenging by certain amino acid side
groups, thus allowing them to behave act as radical trapping devices (Neuzil et
al., 1993; éstdal et al., 2002). In reality, all 20 amino acids can be oxidized
assuming the radical insult is sufficiently powerful, as is the case with the highly
oxidizing

·

OH radical. However, as is the case with all chain-breaking
antioxidants in food lipid systems, protein antioxidants are only effective if they
do not initiate or propagate further oxidation reactions. Protein- and peptide-
based radical scavengers disrupt peroxidation chain reactions by quenching lipid
radicals, usually by hydrogen donation. This reaction yields a protein radical
which, ideally, is of insufficient strength to propagate lipid oxidation (i.e., it is
incapable of abstracting a hydrogen atom from a methylene interrupted double
bond). For example, tyrosine radicals formed on bovine serum albumin are
significantly longer lived and are therefore less reactive than free tyrosine
radicals (éstdal et al., 1999). The long half-lives of tyrosinyl radicals in bovine
serum albumin is likely due to the ability of the protein to transfer radicals on
surface exposed amino acid residues to tyrosine residues buried in the protein's
hydrophobic core. It is conceivable that a protein's antioxidant activity is at least
partially attributable to the lower reactivity of protein radicals if those radicals
are transferred to the interior of the protein where they are unable physically
interact with lipids. However, it should be noted that it is possible that not all
protein radicals are transferred to within a protein's interior, as bovine serum
albumin radicals have been found to promote the oxidation of linoleic acid
emulsions (éstdal et al., 2002).
Methionine has been proposed to be an important free radical scavenger in
proteins in biological systems (Levine et al., 1996, 1999, 2000; Stadtman et al.,
2003). This is because methionine residues are very labile to oxidation and can
potentially scavenge radicals before they are able to attack other amino acid
residues that are critical to protein structure or function. For example, it has been
proposed that the high concentration of methionine residues at the active site of
bacterial glutamine synthetase serves as a `last chance' antioxidant defense