PROCARBOXYPEPTIDASE IN RAT PANCREAS - OVERALL CHARACTERIZATION AND COMPARISON OF THE ACTIVATION PROCESSES

Three monomeric procarboxypeptidases and a binary complex consisting o f a procarboxypeptidase and a chymotrypsinogen have been isolated from rat pancreas by HPLC. N-terminal sequence determination, substrate-sp ecificity analysis and physico-chemical characterization showed that t he carboxypeptidase precursors were the A1, A2 and B forms. No isomorp hism could be detected for any of these proenzymes and no clear eviden ce was obtained for the presence of procarboxypeptidase-containing qua ternary complexes of the types previously described for other species. Instead, we observed the presence of a binary complex between procarb oxypeptidase A2 and chymotrypsinogen B. Among the major pancreatic end oproteinases, only trypsin was found to be a general activator of rat procarboxypeptidases in vitro. Time-course analysis of the products ge nerated after trypsin addition confirmed that full activation of proca rboxypeptidase A1 requires several cleavages-in the C-terminal region (residues 87-94) of the activation segment, while procarboxypeptidases A2 and B require a single cleavage each. The carboxypeptidases releas ed participate in the trimming of the activation segment in A1 and B, but not in A2, probably because of the high specificity of the latter in the active form.