Large Glucagon Immunoreactivity in Extracts of Pancreas

The glucagon immunoreactivity present in canine pancreatic extracts can be separated by gel filtration into two fractions; one fraction, comprising over 90% of the immunoreactivity, is similar in molecular size to the glucagon- 131 I marker, while the other appears to be at least twice as large. Incubation of this large glucagon immunoreactivity (LGI) in solutions such as 1 m acetic acid, 8 m urea, 0.1 m mercaptoethanol, or 6 m guanidine hydrochloride, which favor the dissociation of noncovalent protein complexes, fails to change its molecular size as determined by gel filtration, suggesting that LGI is not glucagon noncovalently bound either to itself or to another protein. However, tryptic hydrolysis of LGI results in the disappearance of most of the immunoreactivity and in a reduction in the molecular size of the remaining immunoreactivity to approximately that of glucagon- 131 I. Neither LGI nor its tryptic product, both of which resemble glucagon immunologically, has glycogenolytic activity in the perfused rat liver. It is possible that LGI includes the immunoreactive sequence of glucagon or of an immunologically similar peptide.