Evidence for the presence of prolyl oligopeptidase and its endogenous inhibitor in neonatal rat pancreatic beta-cells.

Prolyl oligopeptidase (PE), an enzyme that may be involved in the maturation and degradation of hormones and neuropeptides has been detected in neonatal rat pancreatic islet cell monolayer cultures. PE activity was not observed in islet cell homogenates but when cellular extracts were subjected to gel-filtration, a such activity with a molecular mass about 70 kDa can be detected. Gel-filtration experiment has led to the finding of a PE inhibitor in these extracts with an estimated molecular mass of 6.5 kDa. After separation of the endogenous inhibitor from PE enzyme by gel-filtration, PE inhibitor was partially purified in a single activity peak by reverse-phase high-performance liquid chromatography (HPLC). It inhibited the fluorogenic substrate Z-Gly-Pro-beta Na degradation by partially purified PE in a competitive manner. Inhibitor is shown to be specific for PE enzyme and it is not released by potassium depolarization of islet cell membrane. These findings indicated that inhibitor is localized in the cytosolic compartment as prolyl oligopeptidase. The specific activity of the inhibitor in beta-cell cultures derived from donor rats varying from 3-20 days of age was unchanged. In contrast, PE inhibitor can only be detected in pancreatic tissue from 3-day-old rats compared with tissue from 20-day-old and adult rats after gel filtration. This discrepancy can be relevant to the different endocrine/exocrine tissue ratios in the pancreas during developing rats. Furthermore, pancreatic tissue from streptozotocin-treated 3-day-old rats did not show PE inhibitory activity indicating that PE inhibitor was principally contained in beta-cells. Based on the biochemical characteristics of the beta-cell PE inhibitor, the enhancement of PE activity observed in neonatal pancreas of STZ-treated rat as previously described (P. Salers, Regul. Pept., 50 (1994) 101-111), appears to be due to the presence of the endogenous PE inhibitor in neonatal rat pancreatic beta-cells that disappears following STZ-treatment.