Structure and antioxidant properties of tilapia skin hydrolysate and its selenium chelate

Selenium is one of the essential trace elements in the human body. Lack of selenium can lead to many diseases, such as Keshan disease, large bone disease and so on. Compared with inorganic selenium, organic selenium can cross the intestinal wall actively, so it is necessary to develop organic selenium with high biological activity. Selenium chelate, a collagen protease hydrolysate, has good antioxidant properties, can also supplement selenium and collagen for human body, and has high bioavailability. In order to make efficient use of tilapia by-product resources and provide theoretical basis for the industrialization of selenium chelate, tilapia skin collagen (TSC) was used as raw material for enzymatic hydrolysis by 4 kinds of proteases. The enzymolysis products of acid protease (TSCAc), papain enzymolysis product (TSCP), bromelain enzymolysis product (TSCB) and alkaline protease enzymolysis product (TSCAl) were obtained. Using Na₂SeO₃ as the source of selenium, the hydrolysate with the highest selenium binding amount was prepared and screened. UV, fluorescence, FTIR and surface hydrophobicity of the enzyme hydrolysate and its selenium chelate were determined, and the antioxidant activity and antioxidant stability of the enzyme hydrolysate and its selenium chelate were analyzed. The hydrolysate of alkaline protease (TSCAl) had the highest hydrolysis degree (18.2%), and the proportion of molecular weight < 1 000 u was 53.7%, which was higher than that of other hydrolysates. The chelate of alkaline protease (TSCAl-Se) had the highest selenium binding amount (153.5 mg/g) and the selenium binding rate was 23.7%. After the chelation of TSCAl and selenium, the intensity of the absorption band at 210-238 nm wavelength decreased and showed a blue shift, and the intensity of the absorption band at 238 nm-300 nm wavelength increased. After the chelation of TSCAl and selenium, the fluorescence intensity of the fluorescence spectrum weakened, and the absorption peak red shifted 2 nm. After TSCAl chelated with selenium, in the infrared spectrum, the absorption peak of N-H moved from 3 280 cm⁻¹ to 3 365 cm⁻¹, the absorption peak of O-H shifted from 1 393 cm⁻¹ to 1 400 cm⁻¹, another absorption peak of O-H moved from 1 446 cm⁻¹ to 1 456 cm⁻¹, the absorption peak of C-H shifted from 2 929 cm⁻¹ to 2 930 cm⁻¹, the absorption peak of C=O shifted from 1 635 cm⁻¹ to 1 652 cm⁻¹. These indicate that TSCAl and selenium combine to form a new substance. The IC₅₀ values of TSCAl, TSCAl-Se and Na₂SeO₃ for scavenging OH radicals were 6.6 mg/mL, 0.05 mg/mL and 0.17 mg/mL, respectively. The antioxidant activity of TSCAl and TSCAl-Se fluctuated slightly at different temperature but was relatively stable. The antioxidant stability of them decreased at 100 °C. With the increase of pH, the antioxidant activity slowly increased first and then decreased, and reached the highest value at pH=7. These results indicate that the selenium chelate of alkaline protease hydrolysate of tilapia skin collagen has good antioxidant activity and antioxidant stability, which provides a theoretical basis for the research and development of selenium dietary supplements and antioxidant.