Isolation and structural characterization of a molecular variant of dog pancreatic secretory trypsin inhibitor

A rapid two-step method has been developed for the purification of pancreatic secretory trypsin inhibitor (PSTI) from pancreatic juice obtained from dogs, stimulated with secretin and the octapeptide of cholecystokinin. The PSTI was isolated in two biologically active molecular forms. Determination of amino acid compositions and NH2-terminal amino acid sequences demonstrated that the major form represented the intact 57-amino-acid residue peptide, and the minor form (comprising 5-10% of the total activity) represented des[Asn1 Asn2 Met3] PSTI. The metabolite arises from cleavage of a Met-Leu bond, and its formation may be a consequence of incomplete inhibition of chymotrypsinogen activation in the juice.