Identification of zymogen E in a complex with bovine procarboxypeptidase A

The presence of zymogen E in bovine pancreatic secretion was demonstrated by activation with trypsin and isolation of protease E. The enzyme, pI = 4.85, has k3 and Km values of 48 s-1 and 4.4 mM, respectively, for acetyl-tri-L-alanine methyl ester and an amino acid composition similar to that of porcine protease E. Gel filtration of the proteins in the secretion provided evidence that bovine zymogen E forms complexes with procarboxypeptidase A, including a ternary complex of these two proteins with chymotrypsinogen C. Taken together with previous observations on porcine zymogen E (Kobayashi, R., Kobayashi, Y., and Hirs, C. H. W. (1978) J. Biol. Chem. 253, 5526-5530) the data suggest that complex formation between procarboxypeptidase A and zymogen E may be a more widespread property of these proteins. The fact that zymogen E occurs as a major constituent in bovine pancreatic secretion in a physiological context n which no requirement for elastolysis exists suggests further that the functional significance of protease E in the digestion of proteins is general and not specific for elastin.