A peptide from the eel pancreas with structural similarity to human pancreatic secretory trypsin inhibitor

(Formula Presented.) . There was no indication of microheterogeneity. This peptide shows structural similarity to pancreatic secretory trypsin inhibitors from several mammalian species and to a cholecystokinin‐releasing peptide isolated from rat pancreatic juice. A comparison of the amino acid sequences of the peptides has identified a domain in the central region of the molecules that has been strongly conserved during evolution. In contrast, the amino acid sequence in the region corresponding to the reactive centre of the mammalian trypsin inhibitors is very poorly conserved in the eel peptide. The P₁‐P₁′ reactive site lysine‐isoleucine (or arginine‐isoleucine) bond in the mammalian trypsin inhibitors is replaced by a methionine‐asparagine bond. This region does, however, show limited homology to the reactive centre of human α₁‐protease inhibitor suggesting that the eel peptide may function as an inhibitor of other proteolytic enzymes in the pancreas.