Histidyl-proline diketopiperazine cyclo (His-Pro): identification and characterization in rat pancreatic islets

Measurements of cyclo (His-Pro) in the pancreas were carried out in the rat by a specific radioimmunoassay. Cyclo (His-Pro)-like immunoreactivity was identified in pancreatic islets with a mean concentration of 2023 pg/mg protein, 88-fold higher than that of the whole pancreas. Cyclo (His-Pro) immunoreactivity from pancreatic extracts was indistinguishable immunologically and chromatographically from synthetic cyclo (His-Pro). Insulin-induced hypoglycemia caused a significant, 53% decrease in pancreatic cyclo (His-Pro) concentrations, and FLA-63, a dopamine beta-oxidase inhibitor, also reduced islet cyclo (His-Pro) concentrations 51%. These data indicate that cyclo (His-Pro) is present in rat pancreatic islets and may play a potential role in modulating pancreatic responses to nutrient and pharmacologic stimuli.