PROMOTION OF ACTIVATION OF BOVINE TRYPSINOGEN BY SPECIFIC MODIFICATION OF ASPARTYL RESIDUES

Modification of the carboxylate groups of trypsinogen under mild conditions resulted in the incorporation of about 2.5 glycinamide residues in the protein. The modified zymogen could be activated in the absence of calcium ions. Active trypsin was isolated from the activation mixture and found to be unmodified. Peptides isolated from the activation mixture were found to be a mixture of mono- and diglycinamide derivatives of the activation peptide Val-(Asp)4-Lys. Edman degradation of the substituted peptides indicated that each of the four carboxyls is modified in a rather random manner. The N-terminal undecapeptide of trypsinogen was isolated and it was found that Ca2+ accelerated the tryptic hydrolysis of the Lys6-Ile7 bond. Since modification of the carboxylate groups in the N-terminal region of trypsinogen eliminates the requirement of calcium ions in the activation, it is suggested that the role of calcium ions in the autocatalytic activation of trypsinogen is to bind these carboxylate groups thus enhancing the susceptibility of the Lys6-Ile7 bond for cleavage by trypsin. © 1969, American Chemical Society. All rights reserved.