On the two anionic chymotrypsinogens of porcine pancreas

Abstract

A second anionic chymotrypsinogen (chymotrypsinogen B) was identified in porcine pancreas and purified from pancreatic juice by DEAE-cellulose chromatography and two Sephadex filtrations in buffers of different ionic strength. Its molecular weight (about 26 000) and amino acid composition were found similar to those of already known bovine chymotrypsinogen A, bovine chymotrypsinogen B and porcine chymotrypsinogen A. Like these three zymogens, its molecule is composed of a single peptide chain with an N-terminal half-cystine and a C-terminal asparagine. Activation by trypsin is associated with the cleavage of the 15th bond in the sequence Arg 15-Ile 16-Val 17. The short chain of 15 residues arising from the cleavage has the same amino acid composition and probably the same sequence as porcine chymotrypsinogen A. The other anionic chymotrypsinogen of porcine origin (porcine chymotrypsinogen C) can also be purified from pancreatic juice by passage through DEAE-cellulose, Sephadex G-100 and CM-Sephadex. Sephadex filtration indicated for this protein a molecular weight of about 23 000. However, results obtained by ultracentrifugation and the amino acid composition were consistent with a markedly higher value (about 29 000). It was confirmed that porcine chymotrypsinogen C and Fraction II of bovine procarboxypeptidase A had a similar amino acid composition and that both were activated by the cleavage of an arginyl-valine bond. As for other chymotrypsinogens, this bond was the first basic bond in the N-terminal sequence of the molecule. But a preliminary investigation suggested that it was probably the 13th instead of the 15th, and that the amino acid composition of the short chain arising during activation was different. It appears, therefore, that two sub-groups can be discerned in the chymotrypsinogen group, one composed of bovine and porcine chymotrypsinogens A and B, and the other of bovine Fraction II and porcine chymotrypsinogen C.