Isolation and characterization of a new form of the porcine pancreatic secretory trypsin inhibitor. Biochemical studies and high-resolution 1H-NMR.

A new active form of porcine PSTI (pancreatic secretory trypsin inhibitor) was isolated during the fractionation by ion-exchange chromatography of the already known forms PSTI I and II. Biochemical and 1H-NMR techniques were used to characterize the new inhibitor, which is referred to as PSTI III. The amino acid composition, the nature of the N-terminal residue and data obtained from the tryptic peptides and indicate that PSTI III lacks the N-terminal octapeptide of PSTI I; hence, it starts and ends with disulfide bridges. The conclusion is supported by the 1H-NMR spectrum of the protein at 270 MHz. The biological activity and the most prominent conformational and dynamic features of forms I and II are retained in inhibitor III. However, PSTI III appears to be less compact than its parent forms I and II, suggesting that in the latter inhibitors an interaction between the N-terminal tail and the bulk of the protein may contribute to the overall stability. The genetic origin of PSTI III is discussed.