METABOLISM AND TRANSPORT OF THE PENTAPEPTIDE METKEPHAMID BY BRUSH-BORDER MEMBRANE-VESICLES OF RAT INTESTINE

Intestinal metabolism and transport of the pentapeptide metkephamid (T yr-D-Ala-Gly-Phe-N-Me-Met-NH2) were studied using isolated brush-borde r membranes from the rat. Analysis of the metabolic fragments of enzym atic hydrolysis revealed that cleavage of the N-terminal peptide bond leads to the formation of tyrosine and a tetrapeptide D-Ala-Cly-Phe-N- Me-Met-NH2. The inactivation was due to aminopeptidase N activity and could be inhibited by peptidase inhibitors puromycin, bacitracin and c ertain dipeptides. Transport studies demonstrated uptake of the intact pentapeptide into the intravesicular space of the vesicles. The trans port was a first-order process; no participation of known intestinal p eptide carrier systems in the transport of metkephamid could be shown. Modelling of simultaneous metabolism and transport kinetics suggests strategies to improve the fraction absorbed of a peptide by either dec reasing its affinity to the metabolizing enzymes (increase K-m) or dec reasing the concentration of the metabolizing enzymes e.g. by deliveri ng the peptide to an absorption site with reduced enzymatic activity ( decrease V-max) or increasing its absorption velocity.