The biosynthesis of glucagon in perfused rat pancreas

The biosynthesis of glucagon was studied by using the recirculated, isolated perfused rat pancreas. [(3)H]Tryptophan was initially incorporated into acid-ethanol-extractable protein, which on gel filtration was eluted with a molecular weight of about 9000 and contained a small amount of glucagon immunoreactivity. With longer incubation [(3)H]tryptophan incorporation into a second peak was obtained in an identical position with that of the majority of rat glucagon immunoreactivity. This peak of labelled protein exhibited migration characteristics on polyacrylamide-gel electrophoresis identical with those of rat glucagon and was identified as newly synthesized glucagon by demonstration of specific binding and dissociation behaviour with glucagon antibodies. The incorporation of [(3)H]tryptophan into acid-ethanol-extractable protein was inhibited by cycloheximide. High concentrations of glucose increased [(3)H]tryptophan incorporation into high-molecular-weight protein but decreased incorporation into proteins smaller than cytochrome c. The pattern of [(3)H]leucine incorporation into protein was similar to that of [(3)H]tryptophan.