Studies on the biosynthesis of pancreatic glucagon in the pigeon (Columba livia)

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Abstract
The biosynthesis of glucagon was studied in microdissected pigeon pancreatic islets. [3H]-Tryptophan and [3H]leucine were incorporated into big and little glucagon. No precursor-product relationship was evident between big and little glucagon after radioactive pulse-chase and immunoreactive chase incubations. Radioactive and immunoreactive little glucagon and immunoreactive big glucagon were actively secreted, and the synthesis of both glucagons was inhibited by high concentrations of glucose. [3H]Tryptophan and [3H]leucine were incorporated into an islet protein of about 20,000 MW. Gel filtration of extracts of turkey pancreas revealed the presence of an immunoreactive peak of MW .apprxeq. 20,000. This glucagon-immunoreactive component was also present in dog and ox pancreas and was stable to chaotropic agents and elution at various pH values. A similar-sized glucagon-immunoreactive species was present in the dog circulation. These results are discussed in the light of the presently accepted mechanisms of glucagon biosynthesis.