Cellular peptide processing after a single arginyl residue. Studies on the common precursor for pancreatic polypeptide and pancreatic icosapeptide.

The processing of the common precursor for pancreatic polypeptide and pancreatic icosapeptide was studied in primary cultures of endocrine cells isolated from the duodenal part of the canine pancreas. Biosynthetically labeled peptides were characterized by enzymatic digestion and radiosequencing and compared to a COOH-terminally extended form of the icosapeptide which was isolated from canine pancreas and also sequenced. It was substantiated that, in these cell cultures, processing can be studied at a classical dibasic site between the pancreatic polypeptide and the icosapeptide, and at a monobasic processing site between the icosapeptide and its COOH-terminal extension. Pulse-chase experiments showed that the monobasic cleavage occurs later than the dibasic one in the biosynthetic process; the monobasic site was apparently not cleaved before the prohormone had been processed at the dibasic site. The monobasic processing could also be distinguished from the dibasic cleavage mechanism as, in time, the cells gradually lost the ability to cleave at the monobasic site while the dibasic processing was unaffected. It is concluded that monobasic conversion, which is important in the activation of a series of hormones, neuropeptides, and growth factors, is a distinct cellular processing mechanism.