The proenkephalin-A-derivative Met-enkephalin-Arg-Gly-Leu is not present in the feline species.

No opioid octapeptide Met-enkephalin-Arg-Gly-Leu was detected either in the brain or in the adrenal gland of the cat using a specific radioimmunoassay. Whereas it was possible to determine the Met-enkephalin and Leu-enkephalin contents. The Met-enkephalin versus Leu-enkephalin concentration ratio was around five in each area of the brain assayed. The presence of authentic Met-enkephalin and Leu-enkephalin was confirmed by high performance liquid chromatography analysis. All in all, these data seem to indicate that the cat proenkephalin is partly different from that previously analysed in mammalian species including humans, rats and cows.