Met5-enkephalin-Arg6-Gly7-Leu8 immunoreactivity in the human gut.

The presence of the proenkephalin A-derived peptide Met5-enkephalin-Arg6-Gly7-Leu8 was demonstrated throughout the human gastrointestinal tract. Highest concentrations of Met5-enkephalin-Arg6-Gly7-Leu8, as assessed by radioimmunoassay, were measured in the separated muscularis externa, while lower levels were found in the submucosa and only small amounts in the mucosa. The results are consistent with a neuronal location of this peptide in the human gut. Over 65% of total immunoreactivity coeluted with the authentic peptide in both molecular exclusion chromatography and HPLC, while most of the remainder activity eluted earlier on gel filtration. The latter material probably represents N-terminally extended Met5-enkephalin-Arg6-Gly7-Leu8. Taken together with previous studies, our results appear to indicate that there are important species differences in post-translational processing of proenkephalin A in gut nerves.