Does cathepsin B play a role in intracellular protein degradation?

Some properties of cathepsin B from a hitherto unstudied source (goat spleen) have been reported. The activity of the enzyme was optimal at pH 6.8 and it was fairly stable between pH 4.0-7.0. The maximum activity was observed at 37 degrees C and the enzyme could withstand temperature shocks up to 40 degrees C for 20 min without any significant loss of activity. Gamma-irradiation of the enzyme led to an increase in its activity in the beginning (up to 10 Gy) followed by a gradual decrease to about half of its activity at 1200 Gy. The enzyme was most active at an ionic strength of 0.022 and lost its activity substantially as the ionic strength was raised above the optimum value. The preferred protein substrate for the enzyme was found to be casein. The enzyme also hydrolyzed hemoglobin and serum albumin, but to lesser extents. In contrast to prevailing opinion, it was concluded that cathepsin B can act for a limited period even at physiological temperature, pH, etc. before it is inactivated.