N-terminal fragments of intestinal cholecystokinin: evidence for release of CCK-8 by cleavage on the carboxyl side of Arg74 of proCCK.

From porcine duodenal mucosa we have identified three major procholecystokinin (proCCK) fragments: desoctaCCK-33, desnonaCCK-33 and desnonaCCK-39. (DesoctaCCK-33 means CCK-33 devoid of the 8 C-terminal amino acids, etc.). The fragments were purified by immunoaffinity chromatography and three steps of reverse phase HPLC monitored by a radioimmunoassay specific for the N-terminal part of CCK-33. The structures could be deduced from the proCCK sequence by N-terminal sequence determination and mass spectrometry. Whereas desnona-fragments of CCK have been described before, this is the first finding of a desoctaCCK, and it indicates that CCK-8 is released from the longer forms by endogenous cleavage of the Arg-Asp-bond. A carboxypeptidase B-like exopeptidase subsequently must produce the desnona-fragments by removing the arginine residue.