Mouse salivary glands secrete a glucagon-degrading enzyme, not glucagon

Salivary glands have been reported to synthetize glucagon in various animal species. We therefore studied the glucagon-like immunoreactivity (GLI) in mouse saliva. Stimulation with phenylephrine evoked a 15-fold increase of salivary GLI output. On Sephadex G-50 gel filtration, the salivary GLI was significantly larger than glicentin, the hitherto largest known glucagon-related peptide; furthermore, the immunoreactivity was not absorbable on glucagon immunoadsorbent. 125I-Labeled glucagon incubated with high GLI containing saliva, and subjected to gel filtration and immunoadsorption was degraded to low molecular weight, nonimmunoreactive moieties. Among EDTA, phenylmethylsulfonylfluoride, Pepstatin-A, and N-ethylmaleimide, only N-ethylmaleimide inhibited the degradation. Renin, also found in mouse saliva, degraded the tracer but did not cochromatograph with salivary GLI. In conclusion, GLI of mouse saliva is not a peptide containing a glucagon-immunoreactive sequence but represents tracer-degrading activity, probably composed of sulfhydryl enzymes.