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Purpose: To evaluate protein hydrolysates and membrane ultrafiltration fractions of blue-spotted stingray for metal chelating and radical scavenging activities, as well as protection against oxidative protein damage. Methods: Stingray protein isolates were hydrolysed with alcalase, papain and trypsin for 3 h. Alcalase hydrolysate was fractionated by membrane ultrafiltration to yield < 3, 3 - 10 and > 10 kDa fractions. Peptide contents, iron and copper chelating activity, 2, 2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) and hydroxyl radical scavenging activities, and protection against oxidative protein damage were evaluated. Results: Three-hour alcalase hydrolysate (3AH) had the highest peptide content and the lowest half maximal effective concentration (EC50) for ABTS radical scavenging (793.9 µg/mL), hydroxyl radical scavenging (6.93 mg/mL), iron chelating (116.4 µg/mL) and copper chelating activity (2136.9 µg/mL) among the hydrolysates. Among the fractions of 3AH, < 3 kDa fraction had the best iron chelating activity, 3 - 10 kDa fraction exhibited the highest ABTS radical scavenging activity, while > 10 kDa fraction showed the best copper chelating activity. The < 3 kDa and 3 - 10 kDa fractions had similar levels of hydroxyl radical scavenging activity to reduced glutathione. The protective effects of 3AH and < 3 kDa fraction against oxidative protein damage were comparable to that of reduced glutathione. Conclusion: Alcalase is the best protease for producing hydrolysates with metal chelating and antioxidant activities from stingray proteins. Alcalase hydrolysate, specifically its < 3 kDa fraction, has potential for future applications in antioxidant therapy and health food formulation

Anti-oxidative, metal chelating and radical scavenging effects of protein hydrolysates from blue-spotted stingray