---

Aspartic proteases (APs) (EC 3.4.23.X) catalyze the hydrolysis of peptide bonds, a reaction that is fundamental to many biological processes. All of the vertebrate and most of the fungal APs are synthesized as zymogens. Porcine pepsin (EC 3.4.23.1) belongs to the aspartic protease family. Pepsin is a gastric aspartic protease and one of the three principal protein degrading enzymes in the digestive system. Pepsin is an industrial enzyme in the food industry. In this study, thermal stability of pepsin investigated in the different concentrations of aluminium in presence and absence of organic solvents ) butanol, ethanol, 1,4-Butanediol and glycerol). Thermal stability of pepsin increased in the presence of aluminium and decreased in presence of organic solvents ) butanol, ethanol, 1,4-butanediol ) and unchanged in presence of glycerol .Thermal stability of pepsin increased in presence organic solvents with adding of aluminium to its absence. possibly aluminum ions through electrostatic and dative interactions with carboxylate groups of Aspartic acid and Glutamic acid residues are bonded to pepsin structure, and causing to condense enzyme structure which leading to increasing thermal stability of pepsin. Mechanism of increasing thermal stability of pepsin is unknown in presence of aluminium. Therefore, we can reduce the instability of pepsin in presence of organic solvents by
Aluminium.

---

---

In fish processing industries, 50-70% of primary fish are produced as waste, while they are rich sources of protein and essential amino acids. The optimal use of these wastes and the production of compounds with high added value that have significant health-giving properties is one of the important challenges of fish processing industries. In this research, the effect of hydrolysis conditions (time: 30-300 minutes and enzyme concentration 0.5-3%) and type of protease (pepsin and trypsin) on the degree of hydrolysis and antioxidant properties (DPPH radical scavenging activity, Fe chelating activity, No radical scavenging activity, total antioxidant capacity and Fe reducing power) of hydrolyzed protein obtained from Skipjack viscera were evaluated using response surface methodology. The results showed that the optimal conditions for the production of hydrolyzed protein with the maximum antioxidant properties with pepsin and trypsin enzymes were respectively: hydrolysis time of 179.09 and 143.62 minutes and enzyme concentration of 2.63 and 1.94 %; In this condition, the degree of hydrolysis of the hydrolyzed proteins resulting from the activity of trypsin was calculated to be higher than that of pepsin. Comparing the antioxidant properties of the hydrolysates obtained from the two enzymes used showed that the hydrolyzed protein obtained from trypsin had a stronger antioxidant potential than pepsin. Therefore, it can be stated that the hydrolyzed protein of the Skipjack viscera using trypsin enzyme as a health-giving supplement and with high added value can be used in the production of functional food products and health supplements for athletes and elderly people.