Molecular Forms of Glucagon-Like Immunoreactivity in Porcine Intestine and Pancreas*

Abstract

Glucagon-related polypeptides in porcine pancreas and intestine were analysed by gel-permeation chromatography and RIA. Three assays were employed: a nonspecific glucagon assay (R59) of 94% cross-reactivity with glicentin; a pancreatic glucagon assay (RCS5) directed against the C-terminal region of glucagon and of less than 0.01% cross-reactivity with glicentin; and a glicentin assay (R64) of less than 0.01% cross-reactivity with glucagon. For extracts of porcine pancreas all three assays gave similar molar concentrations of immunoreactivity. In porcine intestinal extracts immunoreactivity was detected in significant amounts only by the nonspecific glucagon (R59) and the glicentin (R64) assays, again in similar molar concentrations. The immunoreactivities present in pancreas and intestine were chromatographically and immunologically separable into six main peaks, peaks I, II, III, V, and VI being present in the pancreas, and peaks I, II, and IV in the intestine. The different immunoreactivities of the peaks allowed probable identities to be assigned to their main components. Apart from peak I, which consists of void-volume material that may interfere nonspecifically with the assays, the main components of the peaks can be interpreted as glicentin (in peak II) or fragments derived from glicentin. Peak III contains the N-terminal portion of glicentin (glicentin-related pancreatic peptide), peak IV probably contains glucagon with its 8 aminoacid C-terminal extension, peak V is pancreatic glucagon and peak VI contains smaller N-terminal glicentin fragments. These findings fit with the proposition that glicentin fulfills the role of proglucagon in the pancreas, and is the major component of enteroglucagon in the intestine.