Stimulation by 2-Mercaptoethanol and Alcohols of Protease in Antarctic Krill, Euphausia superba, at Low Temperatures

We fractionated the homogenate of Antarctic krill, Euphausia superba, with a Sephacryl S-400 column and obtained two protease fractions, A (M.W. 50 K) and B (M.W. 30 K). Both fractions digested preferentially the peptide bond adjacent to arginine, and also digested peptide bonds adjacent to lysine and leucine. Their optimum pH was 9.0 and optimum temperature was 37°C in the absence of 2-mercaptoethanol. In the presence of 4.4% 2-mercaptoethanol, the activity of fraction A was stimulated below 30°C, but inhibited at higher temperatures. This led to the shift of optimum temperature from 37°C to 20°C. Some of alcohols also strongly stimulated the krill protease at 20°C.