EFFECTS OF VARIOUS PROTEASE INHIBITORS ON THE STABILITY AND PERMEABILITY OF [D-ALA(2),D-LEU(5)]ENKEPHALIN IN THE RAT INTESTINE - COMPARISONWITH LEUCINE-ENKEPHALIN

The effects of various protease inhibitors on the stability of leucine enkephalin (Leu-Enk) and [D-Ala(2),D-Leu(5)] enkephalin (DADLE) were investigated, and the permeability of these peptides was also examined in an in vitro Ussing chamber. Captopril, thiorphan, bacitracin, best atin, puromycin, amastatin, and sodium glycocholate (Na-GC) were chose n as protease inhibitors. Regional differences in the stability of Leu -Enk and DADLE were observed, and the rank order of the stability of t hese peptides was colon > duodenum > ileum > jejunum. Na-GC, amastatin , and puromycin were effective protease inhibitors for improving the s tability of these peptides, although captopril and thiorphan did not i mprove the stability of Leu-Enk. In the transport studies, Leu-Enk did not cross the intestinal membrane in the absence of protease inhibito rs, but its transport was improved in the presence of Na-GC. In additi on, Ha-GC, amastatin, and puromycin improved the permeability of DADLE in both jejunum and colon, while the permeability of DADLE was not im proved by the addition of captopril, thiorphan, and bestatin. Furtherm ore, the permeability of 6-carboxy-fluorescein, a poorly absorbable an d stable compound, was also improved in the presence of Na-GC and baci tracin at a concentration of 10 mM. These findings indicated that amas tatin, puromycin, and Na-GC at a concentration of 0.5 mM might increas e the permeability of DADLE due to the improved stability of DADLE in the donor site, However, Na-GC and bacitracin at a concentration of 10 mM had absorption-enhancing activities which might be also related to the enhanced permeability of DADLE across the intestinal membrane.