Which enzymes degrade dynorphins and what drugs inhibit these enzymes?
This paper was looking at inhibition of dynorphin converting enzymes and their importance. Although they don't seem to know if it is a single enzyme, or if it is multiple enzymes.

This paper looked at bikunin as an endogenous inhibitor of dynorphin convertase in human cerebrospinal fluid.

This paper looked at several inhibitors (some of which are known to irreversibly inhibit other enzymes), like N-peptidyl-O-acyl hydroxylamines and their effects on hCSF-DCE, the enzyme that cleaves dynorphin A, dynorphin B and alpha-neoendorphin to release Leu-enkephalin-Arg6. However, it is known to inhibit serine and cysteine proteinases.

However, I don't see any actual medications that are meant to inhibit Dynorphin enzymes. Instead I found Al-Fayoumi et. al were actually looking to identify stabilized dynorphin A derivatives instead of just inhibition of enzymes.