Dasyatis akajei

Superfluous reactive oxygen species (ROS) are produced under oxidative stress conditions and they can destroy all types of macromolecules; which further lead to many health disorders, such as cancer, diabetes mellitus, and inflammatory diseases. In addition, oxidative decomposition of unsaturated lipids causes food rancidity; shortens the shelf-life of food products.
Antioxidant peptides (APs) from food resources usually contain 2-20 amino acid residues and they can donate hydrogen; trap lipid-derived free radicals, and/or activate antioxidant enzymes in cells. Therefore, APs can convert ROS, including superoxide anion radicals (O-2•) and hydroxyl radicals (HO•) into a stable molecular structure to inhibit the propagation of peroxidizing chain reaction.

The protein hydrolysate

FWKVV and FMPLH isolated from the protein hydrolysate of miiuy croaker muscle and exhibited strong reducing power; lipid peroxidation inhibition ability. The AP fractions of CPF1 (molecular weight (MW) < 1 kDa) and CPF2 (1 < MW < 3 kDa) from corn gluten cytoprotective effect and intracellular ROS scavenging activity in H2O2-damaged HepG2 cells because they could positively affect the activities of the intracellular antioxidant enzymes.
These studies suggested that APs could effectively enhance human health and/or protect food quality by reducing oxidative stress. In addition, APs can serve as ingredients of functional foods due to their nutritional and biological properties. Collagens and peptides from porcine and cow cartilages can provide an adequate nutrient for repairing cartilage; maintain the overall health of subchondral bone and articular cartilage; improve the joint flexibility and mobility in Osteoarthritis (OA) patients.

However, these ingredients used in the food and medical products have caused anxiety among some
consumers due to frequent outbreaks of infectious disease. The structural characteristics of APs provide guides for the evaluation of food-derived proteins as potential precursors of APs and predict the possible release of APs from various proteins while using appropriate proteases.

Metal ion chelating activity

Acidic and basic amino acid residues play critical roles in the metal ion chelating activity; which is related to the carboxyl and amino groups in their side chains.  The carboxyl residues of acidic amino acids are deprotonated for rendering the complex formation with metal ions, and the amino nitrogen loses its proton nitrogen to allow for unshared pairs of electrons of nitrogen to bind with metal ions.
In the experiment, four APs (RSHP-A~D) isolated from water-soluble protein hydrolysate of red stingray cartilages using ultrafiltration and chromatographic methods; their amino acid sequences were identified as VPR, IEPH, LEEEE, and IEEEQ, respectively.  Among the four isolated APs, IEPH showed the strongest scavenging activity reducing power, and lipid peroxidation inhibition activities; VPR showed the strongest O -2 • scavenging activity; and, LEEEE showed the highest Fe2+-chelating ability.
Subsequently, APs from water-soluble protein hydrolysate of red stingray cartilages may possible to serve as antioxidant candidates; for new functional foods. Further studies will done to purify and identify more APs and more detailed studies on the safety, stability, and structure-activity relationship of the purified APs will also be needed.