Enkephalin-processing oligopeptidases in cobra venom: inhibition by thiorphan and bestatin reveals co-operative actions.

The peptidase inhibitors thiorphan and bestatin were tested for their ability to inhibit the actions of the oligopeptidases contained in the venom of the Taiwan cobra (Naja naja atra). With methionine enkephalin (TyrGlyGlyPheMet) as substrate, thiorphan was an effective inhibitor of cleavage of the GlyPhe peptide bond while bestatin inhibited cleavage of the TyrGly peptide bond. Thiorphan and bestatin also inhibited subsequent cleavage of the fragments GlyGlyPheMet and TyrGlyGly respectively. These inhibitors reveal an interplay between the venom oligopeptidases in which the enzymes provide additional substrates for each other following their initial competitive attack on the neuropeptide. A possible explanation is that the system is intended to ensure a steady release of Tyr, GlyGly and PheMet over time. Significantly, Tyr is the favoured substrate of the L-amino acid oxidase present in the venom, which rapidly transforms this aromatic amino acid into phenolic derivatives. The efficacies of these inhibitors also suggest that there are similarities between the venom oligopeptidases and the peptidases associated with the processing of enkephalin in its normal contexts.