Protein antioxidants for the stabilization of lipid foods: current and potential applications - Oxidation in Foods and Beverages and Antioxidant Applications - page 257

11.4.2 Two-electron reduction of lipid hydroperoxides
Proteins have been shown to reduce lipid hydroperoxides to non-reactive lipid
hydroxides by two-electron (non-radical) processes (Garner et al., 1998a, 1998b;
Pryor et al., 1994; Pryor and Squadrito, 1995), which may contribute signifi-
cantly to their overall antioxidant activity. Methionine residues are thought to be
central to this process, as it has been observed that canine high density
lipoprotein (HDL) showed weaker lipid hydroperoxide reducing activity than
human HDL, which has two additional methionine residues (Met112, Met148)
(Garner et al., 1998a). The proposed mechanism for the reduction of lipid
hydroperoxides to lipid hydroxides (Fig. 11.2) involves a direct two-electron
transfer from the sulfide of thioethers of oxidatively labile methionine residues,
resulting in the oxidation of methionine to methionine sulfoxide (MetO)
(Panzenbock and Stocker, 2005; Garner et al., 1998b).
Under this scenario, methionine residues exert their activity by competing for
lipid hydroperoxides with transition metal catalysts, thereby inhibiting lipid
peroxidation. In processed foods, hydroperoxide scavenging would not be a
significant antioxidant mechanism once all available methionine residues have
been oxidized. However, in some foods that are physiologically active under
postharvest or postslaughter conditions (e.g., fruits, vegetables, minimally
processed muscle foods), methionine residues could be vastly more important
antioxidants if methionine sulfoxide reductases ± enzymes capable of reducing
oxidized methionine ± remain active.
11.4.3 Quenching of volatile aldehydes
Oxidized lipids are not perceived as rancid until lipid alkoxyl radicals decom-
pose via -scission reactions into lower molecular weight, volatile compounds
that are perceived as off-flavors and off-aromas. These compounds are aldehydic
species that have considerably longer half-lives than the hydroperoxides or lipid
alkoxyl radicals from which they derive (Bruenner et al., 1995; Davies and
Dean, 1997) and have a deleterious effect on the sensory, functional, and
nutritive quality of lipid-containing foods (Stangelo et al., 1987, Zhou and
Decker, 1999b). However, aldehydes are known to react with the side chains of
certain amino acid residues by either Schiff base reactions or 1,4- (Michael
Fig. 11.2 Two-electron reduction of lipid hydroperoxide (LOOH) by thioether-
containing side chain of methionine (Garner et al., 1998a, 1998b; Panzenbock and
Stocker, 2005).