Opioid and other peptides as inhibitors of leumorphin (dynorphin B-29) converting activity

A thiolprotease from rat brain membranes was shown to convert synthetic dynorphin B-29 (Dyn B-29, "leumorphin") to the tridecapeptide dynorphin B (Dyn B, "rimorphin"). This represents a "single-arginine cleavage" between threonine-13 and arginine-14 of the substrate. The dynorphin converting activity displayed typical Michaelis-Menten kinetics with an apparent K_m for the substrate of 0.58 μM. Surprisingly, a synthetic peptide, Dyn B-29-(9-22), which contains the cleavage site, did not inhibit the activity. Dyn A inhibited the activity competitively with an apparent K_i of 3.7 μM. The converting activity was also inhibited by Dyn A-(6-17) but not by Dyn A-(8-17), suggesting a role of Arg⁶-Arg⁷ in the inhibition of converting activity. Bovine adrenal medulla Peptide E inhibited the converting activity substantially whereas metorphamide did not, suggesting the importance of COOH-terminal residues in recognition. β-Endorphin was an effective inhibitor of converting activity, and [α-N-acetyl]β-endorphin was not, indicating a crucial role of the free NH₂-terminus in recognition by the enzyme. ACTH inhibited the activity competitively with an apparent K_i of 39 nM. The converting activity was also inhibited substantially by ACTH-(1-13) but not by α-MSH, again indicating a requirement of the free NH₂-terminus for recognition. The above results suggest that the converting enzyme recognizes peptides of the three known opioid gene families.