Immunoreactive Met‐Enkephalin Arg6 in Rat Brain, and Bovine Brain, Gut, and Adrenal

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Abstract
Antibodies directed against the Met-enkephalin-related hexapeptide, Met-enk Arg6, were used in radioimmunoassays in the characterization of material in rat brain, and bovine striatum, colon and adrenal medulla. Met-enk Lys6 reacted 0.27 relative to Met-enk Arg6, but Leu-enk Arg6 and C-terminal extensions or deletions of Met-enk Arg6 showed less than 0.02 immunoreactivity. In rat brain, the concentration of Met-enk Arg6-like immunoreactivity was < 20 pmol/g in all regions, but after trypsinization of tissue extracts there were up to 80-fold increase in immunoreactivity as a result of cleavage of C-terminally extended forms. The tryptic product eluted as Met-enk Arg6 on gel filtration. In control extracts of rat brain there were at least 3 immunoreactive forms of Met-enk Arg6. One eluted in the position of the hexapeptide standard on gel filtration and HPLC [high performance liquid chromatography] while the others had properties of N-terminally extended forms. In bovine striatum and colon the hexapeptide-like material predominated. In bovine adrenal extracts, there were relatively low concentrations of the hexapeptide and, instead, the dominant immunoreactive forms corresponded to 2 components that were probably N-terminally extended variants. Trypsin again produced marked increases in immunoreactivity. HPLC studies indicated that Met-enk Arg6Phe7- and Met-enk Arg6Gly7Leu8-like immunoreactive peptides were important substrates in bovine brain for the production of hexapeptide immunoreactivity after trypsin. The differences in the patterns of immunoreactive forms in bovine adrenal, colon and brain are consistent with tissue variations in the pathways of posttranslational processing of the precursor molecules.