PURIFICATION AND CHARACTERIZATION OF INSULIN, GLUCAGON, AND 2 GLUCAGON-LIKE PEPTIDES WITH INSULIN-RELEASING ACTIVITY FROM THE PANCREAS OF THE TOAD, BUFO-MARINUS

Insulin and four peptides derived from the posttranslational processin g of proglucagon have been isolated in pure form from the pancreas of the cane toad, Bufo marinus. Although Bufo insulin contains 9 amino ac id substitutions, compared with human insulin, all those residues that are considered to be involved in receptor-binding and in dimer and he xamer formation have been conserved. Bufo insulin was, however, more p otent (4-fold) than human insulin in inhibiting the binding of [I-125- Tyr-A14] insulin to the soluble full-length recombinant human insulin receptor, which is probably a consequence of the substitution (Thr --> Kis) at position A-8. Bufo glucagon was isolated in two molecular for ms: glucagon-29 shows only one amino acid substitution (Thr29 --> Ser) , compared with human glucagon; and glucagon-36 comprises glucagon-29, extended from its C-terminus by Lys-Arg-Ser-Gly-Gly-Met-Ser. The huma n proglucagon gene contains one copy of glucagon-like peptide (GLP)-1, a potent insulin secretogogue, and one copy of GLP-2 that is devoid o f insulin-releasing activity. In contrast, two proglucagon-derived pep tides with 32- and 37-amino acid residues (GLP-32 and GLP-37), display ing greater structural similarity to human GLP-1 than to GLP-2, were i solated from Bufo pancreas. Both peptides produced concentration-depen dent increases in insulin release from glucose-responsive rat insulino ma-derived BRIN-BD11 cells. The threshold concentrations producing a s ignificant (P < 0.001) effect were 10(-8) M (GLP-32) and 10(-9) M (GLP -37), and the maximum increase in the rate of insulin release produced by 10(-6) M concentrations of both peptides was approximately 5-fold.