- In biological systems, antioxidants are bioactive compounds that protect against oxidation of lipids, proteins, and DNA. Natural antioxidant peptides are often purified and isolated from proteins as a less toxic alternative to synthetic peptides, but the mechanisms behind their activity are not well understood. The purpose of this study is to understand the mechanism behind the antioxidant activity of a tuna-backbone derived peptide (APTBP). 33 synthetic analogs of APTBP were made by adding, removing, and substituting amino acids into the original 14-residue peptide. APTBP and the analogs were screened from their antioxidant activity with the linoleic acid assay, the DPPH free radical assay, and the ABTS free radical assay. Insolubility of APTBP in the ethanol-based linoleic acid and DPPH free radical assays made them unsuitable to measure antioxidant activity of APTBP or its analogs. APTBP and the analogs displayed antioxidant activity in the water-based ABTS assay.
The results of the screens showed the importance of solubility and basic residues in antioxidant activity. Most notably, tryptophan residues greatly affected antioxidant activity. The removal of the lone tryptophan residue significantly decreased activity while the addition of a single tryptophan residue significantly increased activity. The position of tryptophan within the peptide also affected antioxidant activity as analogs which decreased steric hindrance around the tryptophan side chain had higher activity. Analogs with multiple tryptophans added in did not display significant increases in activity which indicates that protein folding and secondary structure is important for activity. The results suggest that tryptophan plays a significant role in antioxidant activity and screening peptides for similar sequences identified in the study may allow for discovery of other antioxidant peptides. Several analogs of APTBP displayed improved antioxidant activity, which may be valuable for drug optimization.
Key Words: antioxidant, peptide, protein, amino acid