Abstract
We have isolated and sequenced a C-terminally amidated peptide from bovine striatum. The peptide was purified to homogeneity by adsorption to XAD-2 resins and four different HPLC steps. Amino acid composition analysis and gas-phase sequence analysis revealed identity of this peptide with residues 8-26 of the proenkephalin-derived opioid peptide amidorphin, which we have recently isolated from bovine adrenal medulla. C-terminal amidation of amidorphin-(8-26) from bovine striatum was demonstrated by its stability to carboxypeptidase A digestion and full crossreactivity in a radioimmunoassay that required the C-terminal amide group as part of the recognition site. The nonopioid peptide amidorphin-(8-26), which lacks the N-terminal [Met]enkephalin sequence of amidorphin, is a major product of the opioid peptide precursor proenkephalin in the brain. In the adrenal medulla, however, where amidorphin occurs in remarkably high concentrations, amidorphin-(8-26) could not be detected. This is indicative of differential post-translational processing of proenkephalin in different tissues. In the brain, as opposed to the adrenal medulla, amidorphin is further processed at the typical cleavage signals of two basic residues, giving rise to the nonopioid peptide amidorphin-(8-26) and, possibly, to the opioid peptide [Met]enkephalin. Thus, proenkephalin in the brain might be considered as a precursor in which an opioid peptide is linked with a nonopioid peptide of possibly different biological function.