Optimisation of hydrolysis of purple sea urchin (Strongylocentrotus nudus) gonad by response surface methodology and evaluation of in vitro antioxidant activity of the hydrolysate

Background: Hydrolysates prepared from sea urchin (Strongylocentrotus nudus) gonad by enzymatic treatment showed strong 1,1-diphenyl-2-picrylhydrazyl radical scavenging activity and reducing power. Results: Hydrolysis of S. nudus gonad by the commercial protease papain was optimised for maximum degree of hydrolysis (DH) and trichloroacetic acid-soluble peptide index (TCA-SPI) using response surface methodology. Results showed that the optimal conditions were the following: temperature of 48.83 °C, pH of 6.92, enzyme-to-substrate ratio of 3143 U g ^-1, and substrate concentration of 83.5 g L ^-1. Under these conditions, a DH of 27.96 ± 0.54% and a TCA-SPI of 57.32 ± 0.63% were obtained. The hydrolysate prepared in the optimal conditions was fractionated by an ultra-filtration system and the resultant fraction below 10 kDa was found to effectively scavenge hydroxyl radical (EC ₅₀ = 13.29 ± 0.33 mg mL ^-1) and hydrogen peroxide (EC ₅₀ = 16.40 ± 0.37 mg mL ^-1), inhibit lipid peroxidation (EC ₅₀ = 11.05 ± 0.62 mg mL ^-1), chelate Fe ²⁺ (EC ₅₀ = 7.26 ± 0.44 mg mL ^-1), and protect mice macrophages against death induced by tert-butyl hydroperoxide. Conclusion: Hydrolysates prepared from S. nudus gonad have the potential to be applied as natural antioxidant agents.