Chemistry Reference
In-Depth Information
consequence of the protein's tertiary structure, which is made especially rigid
by the presence of four disulfide bonds (Davies and Dean, 1997). When these
disulfide bridges are reduced, however, radical exchange between Trp25 and
Tyr29 was observed, presumably due to the protein's more flexible
conformation (Prutz et al., 1982).
There are various ways to measure protein oxidation in foods. A common
method for determining general protein oxidation is the spectrophotometric
analysis of protein carbonyls (Requena et al., 2003), which can be derivatized to
hydrazones with 2,4-dinitrophenylhydrazine. The resulting hydrazones are
stable (compared to the carbonyls from which they derive) and have high
extinction coefficients. Proteins carbonyls are known to increase with oxidative
stress and age in vivo, but these compounds can also be readily detected in meat
and dairy products, indicating that protein oxidation occurs during food
processing and storage (Fedele and Bergamo, 2001; Stagsted et al., 2004;
Salminen et al., 2006). EPR can also be used for the direct detection of stable
protein radicals, as well as spin adducts resulting from the reaction between a
spin trap (e.g., 5,5-dimethylpyrroline-N-oxide, DMPO) and a protein radical
(Gomez-Mejiba et al., 2009). General, or non-specific, oxidation can also be
assessed by measuring protein fragmentation and polymerization rates and
changes in proteolytic susceptibility (Stadtman et al., 2003). Amino acid R-
group oxidation is commonly measured by reverse phase HPLC following acid
hydrolysis of the protein, although this approach cannot determine which
particular amino acid residue has been modified. Site-specific oxidation of food
proteins can be determined by MS/MS (Elias et al., 2006), however, which can
offer important mechanistic insight.
11.7 Future trends
Proteins are unique antioxidants in that their mode of action often encompasses
multiple pathways, including inactivation of reactive oxygen species, free
radical scavenging, decreasing the catalytic capacity of prooxidative transition
metals, reduction of lipid hydroperoxides to lipid hydroxides, and alteration of
the physical properties of food systems. These attributes make peptide and
proteins potentially attractive replacements for synthetic antioxidants in foods.
Studies have shown that the antioxidant activity of native food proteins can
be augmented by manipulating tertiary structure by heating or enzymatic
hydrolysis, or through Maillard chemistry. In some cases, peptides give greater
activity than the native proteins from which they derive; however, more work
needs to be done in this area, particularly with respect to correlating peptide
activity with amino acid composition and sequence. Progress in this area will
inevitably lead to the development of a new class of multifunctional, `natural'
antioxidants that can be used to produce oxidatively stable foods and clean
product labels.
Search WWH ::
Custom Search