Proteases, as the most important group of industrial enzymes, are used in a wide variety of industries such as the laundry detergent, food processing, agro-chemistry and pharmacology. They are estimating for about 60 % of the total sale of industrial enzymes in the world market. Fish viscera, one of the most important by-products of the fishing industry that usually discarded, could be applied as a new source of bioactive molecules (e.g. proteases) for biotechnological aspects. Trypsin (EC 3.4.21.4), as an important member of serine proteinases, acts as a digestive enzyme in the intestine and it is also responsible for activating all of the pancreatic enzymes, including itself. Common kilka is one of the most important species of family clupeid harvested from temperate water of Caspian Sea in Iran and is used mostly for fish meal. Common kilka, like other small pelagic fish, autolyzes very quickly by protease enzymes leaking from the digestive organs especially pyloric caeca and intestine. Thus, an alternative is to isolate and purify these enzymes from viscera such as intestine. On the other hand, the fish intestine is also an alternative important source for isolation of trypsin. Our aim was to purify common kilka trypsin and assay the surfactants and oxidising agents effects on purified trypsin. Trypsin from common kilka intestine was purified by ammonium sulfate precipitation, Sephadex G-75 and DEAE-cellulose chromatography, with a 30-fold increase in specific activity and 12% recovery. The molecular weight of the purified trypsin was estimated to be 23.2 kDa based on SDS-PAGE (Figure 1). The trypsin had optimal activity at pH 8.0 and 60°C for the hydrolysis of BAPNA substrate. The trypsin had high stability in the pH range of 7.0-10.0 and highly stable up to 50 °C. Trypsin showed a significant increased proteolytic activity in presence of surfactants (P < 0.05), While it had a decreased proteolytic activity in presence of oxidising agents significantly (P < 0.0
