Abstract
Since the discovery of the first endogenous opioid peptides, methionine (Met-) and leucine-enkephalin (Leu-EN K) (Hughes et al. 1975), a large number of biologically active peptides, all possessing the N-terminal sequence Tyr-Gly-Gly-Phe-Met/Leu, have been identified. Early biochemical and immunohistochemical studies suggested that the diversity of opioid peptides results from tissue-specific processing of distinct high molecular weight precursors: their characterization was subsequently achieved by the molecular cloning of three opioid peptide precursors; proopiomelanocortin (POMC) (Nakanishi et al. 1979), proenkephalin (PENK) (Noda et al. 1982), and prodynorphin (PDYN) (Kakidani et al. 1982). POMC and PENK are the best studied of the three opioid peptide precursor genes, both at the structural and functional level. They are expressed in several distinct brain regions and a multitude of peripheral tissues. Their major sites of expression are the pituitary gland and the adrenal gland, respectively.
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