Identification of a binary complex of procarboxypeptidase A and a precursor of protease E in porcine pancreatic secretion.

In porcine pancreatic secretion procarboxypeptidase A exists in two states: as a monomer and as a binary complex of a type hitherto not observed in the pancreatic secretions of other species. This complex is shown to contain 1 molecule of procarboxypeptidase A and 1 molecule of a proteolytic zymogen we have designated as zymogen E. The two subunits of the complex have been separated by gel filtration in a denaturing solvent and the products used for compositional and NH2-terminal sequence analysis. We also fractionated the mixture obtained on activation of the binary complex and isolated homogenous preparations of porcine carboxypeptidase A and the diisopropylphosphoryl derivative of the enzyme formed from zymogen E. Zymogen E has an Mr of about 26,000 and on activation produces an enzyme of essentially the same Mr with properties very similar to those of human pancreatic protease E. It catalyzes the esterolysis of acetyl-L-alanyl-L-analyl-L-alanine methyl ester, but is inert towards acetyl-L-tyrosine ethyl ester and is readily inactivated by diisopropylophosphorofluoridate. Zymogen E has an amino acid composition different from those of porcine chymotrypsinogen A, B, or C. Its NH2-terminal sequence shows homology with the NH2-terminal sequence of lungfish proelastase A; yet, like human protease E, porcine protease E has relatively very low activity on intact elastin.