Antioxidant and antihypertensive protein hydrolysates produced from tuna liver by enzymatic hydrolysis

In order to better utilize a fish by-product, the enzymatic hydrolysis of tuna liver was performed using commercially available proteases such as Flavourzyme, Alcalase, Protamex, and Neutrase. The hydrolysates were prepared as both first step hydrolysates and second step hydrolysates. The molecular weight distribution of the hydrolysates was determined by size exclusion chromatography and matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry, which analyzed a representative hydrolysate type with a weight range of 1000-3000 Da. The antioxidant activities of the tuna liver hydrolysates against 2,2-diphenyl-1-picrylhydrazyl (DPPH), hydroxyl radical, and hydrogen peroxide, along with a reducing power assay, showed that the second step hydrolysates had more potent antioxidant activity than the first step hydrolysates. However, the first step hydrolysates exhibited more effective chelating activity. Furthermore, the protection ability of the hydrolysates toward hydroxyl radical-induced oxidative DNA damage was evaluated by measuring the conversion of supercoiled pBR322 plasmid DNA to the open circular form. In addition, an angiotensin I-converting enzyme (ACE) inhibition assay revealed that all hydrolysates had similar ACE-inhibitory properties. These findings suggest that tuna liver hydrolysates may be a beneficial ingredient to use in functional foods.