Antioxidant Activity of Cysteine, Tryptophan, and Methionine Residues in Continuous Phase β-Lactoglobulin in Oil-in-Water Emulsions

Abstract

Proteins dispersed in the continuous phase of oil-in-water emulsions are capable of inhibiting lipid oxidation reactions. The antioxidant activity of these proteins is thought to encompass both free radical scavenging by amino acid residues and chelation of prooxidative transition metals; however, the precise mechanism by which this occurs remains unclear. In this study, the oxidative stability of cysteine, tryptophan, and methionine residues in continuous phase beta-lactoglobulin (beta-Lg) in a Brij-stabilized menhaden oil-in-water emulsion was determined. The presence of low concentrations of continuous phase beta-Lg (250 and 750 microg/mL) significantly inhibited lipid oxidation as determined by lipid hydroperoxides and thiobarbituric acid reactive substances analysis. It was observed that cysteine oxidized before tryptophan in beta-Lg, and both residues oxidized before lipid oxidation could be detected. No oxidation of the methionine residues of beta-Lg was observed despite its reported high oxidative susceptibility. It is conceivable that surface exposure of amino acid residues greatly affects their oxidation kinetics, which may explain why some residues are preferentially oxidized relative to others. Further elucidation of the mechanisms governing free radical scavenging of amino acids could lead to more effective applications of proteins as antioxidants within oil-in-water food emulsions.

Byssal proteins of the freshwater zebra mussel,Dreissena polymorpha

“…Such specific cysteine localization could possibly indicate a role for disulfide bridge interactions by the middle region of the protein. Cysteine residues are also known for their potential roles as antioxidants owing to the hydrogen atom in the thiol group available for donation (Elias et al 2005). Yu et al (2011) demonstrated that cysteine containing byssal proteins in marine mussels can reduce oxidized DOPA (dopaquinone) back to DOPA, restoring adhesive function, and can later transform into a cross-linker forming S-cysteinyldopa adducts.…”

Effects of xanthan–locust bean gum mixtures on the physicochemical properties and oxidative stability of whey protein stabilised oil-in-water emulsions

“…Several studies have demonstrated that proteins dispersed in the continuous phase of oilin-water emulsions can inhibit deleterious oxidation reactions by a combination of metal chelation and free radical scavenging (Elias, McClements, & Decker, 2005;Elias, McClements, & Decker, 2007). Whey protein isolate (WPI) is a surface-active globular protein and can be adsorbed to oil droplet surfaces in the form of a monolayer.…”

Global Metabolic Response of Enterococcus faecalis to Oxygen

“…Similarly, depleted levels of cysteine also indicated a higher demand for this amino acid intracellularly in response to oxygen. Cysteine itself has strong antioxidant activity (32) and is a key precursor for glutathione and a key metabolite in the synthesis of other sulfur-containing compounds, including thiamine, coenzyme A (CoA), and biotin, which have all been implicated in the oxidative stress response in other organisms such as Escherichia coli and Bacillus subtilis (33,34).…”