The purification and biological properties of pancreatic big glucagon

Abstract

Big glucagon was present in extracts of ox, dog, rat and turkey pancreas, representing 10-15% of the glucagon immunoreactivity, and was shown to be of islet origin by its presence in extracts of isolated pigeon islets. Big glucagon was homogeneous by immunoassay after polyacrylamide-gel electrophoresis and was more electronegative than little glucagon. Big glucagon was purified from bovine pancreas and its apparent MW was estimated by gel filtration as 8200 .+-. 9%. Limited tryptic proteolysis of the molecule produced an immunoreactive component slightly smaller than little glucagon. Linear dilution curves were obtained with mammalian big glucagons by using both enteroglucagon cross-reacting and little-glucagon-carboxyl-end-specific antisera. The half-times for the disappearance of the immunoreactivity of big and little glucagon that had been injected into the rat circulation were 6.9 and 3.2 min, respectively. Big glucagon was approximately 1/6 as effective as little glucagon in displacing radioactive little glucagon from its liver membrane receptor. Big glucagon was equipotent on a molar basis with little glucagon in the stimulation of the mouse islet adenylate cyclase, an indicator of insulinogenic activity. On a molar basis big glucagon inhibited basal liver adenylate cyclase activity to the same extent that little glucagon stimulated the enzyme. Big glucagon was without effect on blood glucose concentration in the rat in doses up to 5 .mu.g/kg. Big glucagon was equipotent, on a molar basis, with little glucagon in stimulating lipolysis in isolated chicken fat-cells.