Degradation of 125I-glucagon, -pancreatic polypeptide and -insulin by acid saline extract of rat submaxillary gland and their protection by proteinase inhibitors

The acid saline extract (ASE) of rat submaxillary gland exerts a powerful degrading effect on 125I-glucagon. In order to study the degradation of other 125I-peptides by ASE and the effects of their inhibitors, 125I-pancreatic polypeptide (PP) and 125I-insulin were used together with 125I-glucagon. The degradation studies were done by the trichloroacetic acid (TCA) method or gel filtration. Besides 125I-glucagon, 125I-PP was found to be destroyed by ASE in the ordinary immunoassay system using the TCA method, but 125I-insulin was intact in the presence of ASE. Leupeptin, and to a lesser extent p-chloromercuriphenyl-sulfonic acid (PCMS) and N-ethylmaleimide, inhibited the destruction of 125I-glucagon or -PP under the TCA method. PCMS was especially protective at high concentrations, for example 16 mM. These findings were confirmed by gel filtration of the assay mixture. In the presence of leupeptin (0.4 mM) and PCMS (16 mM), no shift in the peak of labelled glucagon or PP occurred. Thus ASE degrades not only 125I-glucagon but -PP, and thiol proteinase inhibitors have a strong inhibitory action on them.