Effects of Various Protease Inhibitors on the Stability and Permeability of [D-Ala2,D-Leu5]enkephalin in the Rat Intestine: Comparison with Leucine Enkephalin

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Abstract
The effects of various protease inhibitors on the stability of leucine enkephalin (Leu-Enk) and [D-Ala2,D-Leu5] enkephalin (DADLE) were investigated, and the permeability of these peptides was also examined in an in vitro Ussing chamber. Captopril, thiorphan, bacitracin, bestatin, puromycin, amastatin, and sodium glycocholate (Na-GC) were chosen as protease inhibitors. Regional differences in the stability of Leu-Enk and DADLE were observed, and the rank order of the stability of these peptides was colon > duodenum > ileum > jejunum. Na-GC, amastatin, and puromycin were effective protease inhibitors for improving the stability of these peptides, although captopril and thiorphan did not improve the stability of Leu-Enk. In the transport studies, Leu-Enk did not cross the intestinal membrane in the absence of protease inhibitors, but its transport was improved in the presence of Na-GC. In addition, Na-GC, amastatin, and puromycin improved the permeability of DADLE in both jejunum and colon, while the permeability of DADLE was not improved by the addition of captopril, thiorphan, and bestatin. Furthermore, the permeability of 6-carboxyfluorescein, a poorly absorbable and stable compound, was also improved in the presence of Na-GC and bacitracin at a concentration of 10 mM. These findings indicated that amastatin, puromycin, and Na-GC at a concentration of 0.5 mM might increase 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.

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