Biochemistry of the enkephalins and enkephalin-containing peptides

The enkephalins are present in many tissues not only as the free pentapeptides, but also as internal sequences in larger polypeptides of varying size. Fourteen enkephalin-containing peptides (EC peptides) from beef adrenal medulla were isolated and sequenced, and the presence of a protein that contained several [Met]enkephalin sequences and one of [Leu]enkephalin was demonstrated. Because the latter was assumed to represent the gene product, it was named proenkephalin. Sequence data from the EC peptides made possible the synthesis of a polynucleotide probe with essentially no degeneracy and permitted the cloning of a partial proenkephalin cDNA. The complete structure of proenkephalin was deduced from both peptide and cDNA sequencing data. Proenkephalin is now known to be one of three enkephalin-containing gene products, each of which gives rise to many physiologically active peptides.