An in vitro evaluation of metabolism and poor membrane permeation impeding intestinal absorption of leucine enkephalin, and methods to increase absorption.

The goals of this study were to evaluate how metabolism and poor membrane permeability act as barriers to absorption of a model peptide, leucine enkephalin (YGGFL), and how those barriers can be overcome. The in vitro everted rat intestine method was used. YGGFL was rapidly metabolized when exposed to the mucosa of the jejunum, and destyrosyl leucine enkephalin was formed. Metabolism was slower for ileal intestinal segments, however, and was almost absent when colonic segments were used. The aminopeptidase inhibitor boroleucine, an aminoboronic acid derivative, reduced the YGGFL metabolism rate 2-fold at 1/100th the concentration of substrate when the intestine was simultaneously exposed to substrate and inhibitor. Pretreatment of the intestine with boroleucine further reduced the metabolism rate. Thiorphan, an inhibitor of endopeptidase 24.11, had additive inhibitory effects with boroleucine. Inhibition of metabolism alone did not enable YGGFL to permeate the membrane. The permeability enhancer, sodium glycocholate, also did not alone enable membrane permeation. Substantial YGGFL permeated the membrane only when metabolism was inhibited and permeability was enhanced. EDTA had both these effects.