Degradation of deltorphins and their analogs by rat brain synaptosomal peptidases

To obtain metabolically stable peptide ligands with high affinity and selectivity for the delta-opioid receptor, the enzymatic stability of deltorphins and their analogs was examined by using a crude rat brain synaptosomal membrane fraction. It was found that deltorphin (DLT: Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2) was easily degraded, producing DLT (1-4) and DLT (1-5) as the major degradation products, whereas [D-Ala2]deltorphin II (DL-II: Tyr-D-Ala-Phe-Glu-Val-Val-Gly-NH2) was very stable. Experiments with some enzyme inhibitors strongly suggested that the degradation of DLT was initiated by cleavage of the Leu5-Met6 bond by a metalloendopeptidase. On the other hand, stability experiments on DL-II analogs demonstrated that the presence of amino acids branched at the beta-carbon atom or with a bulky side chain as residue 5 is of importance for the enzymatic stability. Based on these lines of evidence, some enzyme-resistant DLT analogs were synthesized.