Insulin and four peptides derived from the posttranslational processing of proglucagon have been isolated in pure form from the pancreas of the cane toad, Bufo marinus. Although Bufo insulin contains 9 amino acid substitutions, compared with human insulin, all those residues that are considered to be involved in receptor-binding and in dimer and hexamer formation have been conserved. Bufo insulin was, however, more potent (4- fold) than human insulin in inhibiting the binding of [125I-Tyr-A14] insulin to the soluble full-length recombinant human insulin receptor, which is probably a consequence of the substitution (Thr → His) at position A-8. Bufo glucagon was isolated in two molecular forms: 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 human proglucagon gene contains one copy of glucagon-like peptide (GLP)-1, a potent insulin secretogogue, and one copy of GLP-2 that is devoid of insulin-releasing activity. In contrast, two proglucagon-derived peptides with 32- and 37-amino acid residues (GLP-32 and GLP-37), displaying greater structural similarity to human GLP-1 than to GLP- 2, were isolated from Bufo pancreas. Both peptides produced concentration- dependent increases in insulin release from glucose-responsive rat insulinoma-derived BRIN-BD11 cells. The threshold concentrations producing a significant (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.
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