ENHANCEMENT BY STREPTOZOTOCIN-INDUCED DIABETES OF PANCREATIC PROLYL ENDOPEPTIDASE ACTIVITY IN NEONATAL RATS

The effect of streptozotocin (STZ)-induced diabetes on the concentrations of deamidated TRH (TRH-OH), a metabolite of thyrotropin-releasing hormone (TRH) and prolyl endopeptidase (PE) activity were studied in the pancreas of neonatal rats to determine the contribution of beta-cells to PE activity and TRH-OH levels that we have previously found in this tissue. STZ treatment caused a significant reduction of immunoreactive TRH-OH levels on day-3 and -5 compared to untreated control rats. Reverse-phase high performance liquid chromatography of pooled extracts of 3-day-old normal rat pancreas revealed that about 50% of immunoreactive TRH-OH was found in the fractions representing authentic TRH-OH, whereas the remaining 50% eluted earlier. In STZ treated rats, all of the TRH-OH immunoreactive was associated with this early peak, no authentic TRH-OH could be detected. The specific activity of PE, on the other hand, rose 2.5-fold in diabetic 3-day-old pups (4.06 +/- 0.13 compared with 1.59 +/- 0.83 nmol/min/mg protein, P < 0.01, in controls). This increase declined with age (1.6- and 1.3-fold in 5 day- and 7-day-old pups, respectively). STZ treatment did not change pancreatic PE levels in 20-day-old rats control. Normalization of STZ induced hyperglycemia by sodium metavanadate treatment or by replacement of exogenous insulin did not restore pancreatic PE activity. The enhancement of PE activity following STZ treatment was specific for pancreas tissue. Furthermore, beta cytoxin drugs other than STZ that cause permanent diabetes such as alloxan enhanced PE activity to the same extend. Kinetic studies for PE activity show that V(max) is 3-fold higher in 3-day-old STZ-treated than in rat controls. In contrast, values for K(m) were comparable in rats of both groups (25 to 34 muM). We then tested whether the decrease of V(max) might have been caused by the presence of an PE inhibiting factor in these preparations. Gel-filtration experiments of pancreatic extracts revealed that the total apparent activity of eluted PE in 3-day-old control rats was 2-fold higher than in the original extract. In contrast, the recovery of eluted PE activity was not increased in the case of STZ-treated 3-day-old and untreated 20-day-old rats. These findings demonstrate that TRH-OH is identified in beta-cells and that an inhibiting factor(s) present in beta-cells appear(s) to be responsible for the unexpected enhancement of PE activity observed in STZ-treated neonatal rats. This endogenous factor(s) may play an important role in the regulation of PE enzyme and peptides degradation in neonatal rat pancreas.