enkephalin--leucine-2-alanine and ubenimex

Based on differences in total metabolite accumulation in the presence or absence of selective peptidase inhibitors, rat plasma is found to have its own unique pattern of enkephalin hydrolysis. Approximately 85-90% of the hydrolysis of [leu]enkephalin is attributed to the combined action of aminopeptidase M and angiotensin converting enzyme, whereas "enkephalinase" and aminopeptidase MII activity against [leu]enkephalin are not detectable. Similarly, 80-90% of the hydrolysis of D-ala2-[L-leu] enkephalin (DALLE) is due to the combined action of aminopeptidase M and angiotensin converting enzyme, whereas aminopeptidase MII and enkephalinase activity against this substrate also could not be detected. This is in contrast to the high susceptibility to hydrolysis by enkephalinase, and the low susceptibility to aminopeptidase activity, for DALLE in brain tissue. Among other alternatives, it is suggested that enkephalin hydrolysis in plasma may appear to be unique because of differences in enzyme conformation and/or the availability of a substance(s) that competes with, or alters the binding of, [leu] enkephalin, DALLE or the inhibitors to the enzymes.

Topics: Aminopeptidases; Animals; CD13 Antigens; Enkephalin, Leucine; Enkephalin, Leucine-2-Alanine; Half-Life; Hydrolysis; Leucine; Male; Neprilysin; Peptidyl-Dipeptidase A; Rats; Rats, Inbred Strains; Thiorphan

BAM 18 is a derivative of the opioid precursor proenkephalin A. Although it exists in rat and guinea-pig brain in relatively high concentrations, its physiological function is presently unknown. In the present study we have determined the opioid receptor selectivity of this peptide using radioligand binding and peripheral tissue bioassay. When selective binding conditions were used, BAM 18 bound to the mu opioid receptor with an affinity three times that of the kappa opioid receptor and over 10 times that of the delta opioid receptors (Ki = 0.29, 0.75, and 3.2 nM respectively). BAM 18 also displayed mixed receptor selectivity in in vitro bioassay. Ke values for naloxone antagonism of BAM 18 agonist activity in the electrically stimulated guinea-pig ileum and the mouse vas deferens were 4.3 and 9.9 nM, respectively. These data indicate that BAM 18 binds to all three opioid receptor subtypes with a selectivity profile of mu greater than kappa greater than delta.

Topics: Animals; Biological Assay; Dose-Response Relationship, Drug; Endorphins; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalin, Leucine; Enkephalin, Leucine-2-Alanine; Enkephalin, Methionine; Enkephalins; Guinea Pigs; In Vitro Techniques; Leucine; Male; Naloxone; Protein Precursors; Radioligand Assay; Receptors, Opioid; Structure-Activity Relationship

Transport of [D-Ala2, D-Leu5]enkephalin (DADLE(], Tyr-D-Ala-Gly-Phe (TDAGP) and tyrosine into rat jejunum mucosal cells was investigated in vitro. Active transport of either the pentapeptide (DADLE) or the tetrapeptide (TDAGP) into jejunal villi was not detected. Because substantial degradation of these peptides occurred during incubation, the proteolytic enzyme inhibitors, bestatin (30 microM) or D-phenylalanine (20 mM), were added during uptake studies of DADLE or TDAGP, respectively. The presence of these inhibitors significantly reduced degradation of the oligopeptides; however, no accumulation of peptides occurred in the mucosal tissue. Tyrosine was actively transported by the jejunum mucosal cells demonstrating the viability of the transport mechanisms of this in vitro preparation. The results suggest that there are no active transport systems for enkephalin-like oligopeptides.

Topics: Aminopeptidases; Animals; Biological Transport, Active; Enkephalin, Leucine; Enkephalin, Leucine-2-Alanine; Intestinal Mucosa; Jejunum; Kinetics; Leucine; Male; Oligopeptides; Puromycin; Rats; Rats, Inbred Strains; Tyrosine