Trypsin is widely distributed in animals and in some bacteria and is classed as a serine protease. Trypsin is present in the pancreas and pancreatic juice in vertebrates. Trypsinogen, secreted into the duodenum as a pro-enzyme, is activated by Enterokinase secreted by the duodenal mucosa. Trypsin is obtained by activation of its inactive precursor (Trypsinogen) by Enterokinase and by Trypsin itself. Calcium stimulates the autoactivation of Trypsinogen. Trypsin has a high degree of substrate specificity as it catalyses the hydrolysis of peptide bonds on the carboxyl terminus of positively charged amino acids such as Lysine and Arginine. Trypsin has a weak action on native proteins and will not hydrolyze serum globulins, haemoglobin, ovalbumin or collagen.