Page 167 - Natural Antioxidants, Applications in Foods of Animal Origin

146 Natural Antioxidants: Applications in Foods of Animal Origin
VetBooks.ir 4.2.2 PROTEIN OXIDATION

Like lipid oxidation, ROS also take part in protein oxidation. Their mecha-
nisms are very similar to that of lipid oxidation and caused by metal catal-
ysis, irradiation, light exposure, and peroxidation. The negative effect of
protein oxidation is seen in the oxidation of sulfhydryl groups, reactions
with aldehydes, establishment of cross-links between proteins, formation of
oxidative adducts on amino acids, and protein fragmentations (Stadtman &

Oliver, 1991). The outcome of oxidation of the majority of amino acids leads
to the generation of several products and formation of protein carbonyls
viz., glutamic semialdehyde from alanine, hydroperoxides and carbonyl
compounds from arginine, sulfenyle chloride from cysteine, hydroperox-
ides from glutamic acids, asparatate and asparagines form histidine, alco-
hols and carbonyl compounds from isoleucine, hydroxyleucine and alcohols
carbonyls from leucine, etc. (Stadtman and Oliver, 1991).
According to Stadtman(2006), the mechanisms that can lead to protein–

protein cross-linking by reactive species are described as:

• Formation of disulfide cross-links (RSSR) as a result of oxidation of
cysteine sulfhydryl groups on the proteins.
• Interaction between active groups on different proteins such as the
reaction of an aldehyde group of a 4-Hydroxynonenal (HNE) protein
adduct and the –NH group of a lysine residue in two separate proteins.
2
• Reaction of MDA with the reactive –NH groups of lysine residues in
2
two different protein molecules.
• The formation of covalent linkages between carbon-centered radicals
in two different protein molecules.
• The interaction between a carbonyl group of a glycated protein and
an –NH group of lysine residue in another protein.
2
Oxidation of amino acids as reported by Orrenius et al. (2007) can
be used as a marker for protein oxidation and modification. In protein
oxidation, ROS abstract a hydrogen atom which leads to the creation of
a protein with a free radical (P•). The protein carbon-centered radical,
in the presence of oxygen, reacts with a peroxyl radical (POO•) and as a
result of its reactivity the peroxyl radical abstracts a hydrogen atom from
another molecule leading to the formation of alkyl peroxide (POOH•)
(Lund et al., 2011).
Protein oxidation leads to changes in protein hydrophobicity in processed
fish thereby reducing the water holding capacity (WHC) and altering the