FIGURE 11-15 Low-resolution model of enkephalin precursor showing the distribution of Met-enkephalin sequences (M:, M2, M3, M4, M5, and M6) and Leu-enkephalin (L). The position of potential carbohydrate (CHO) attachment sites and the sequences corresponding to peptides F, E, and B of bovine adrenal medulla are shown. Reproduced from Comb, M., Seeburg, P. H., Adelman, J., Eiden, L., and Herbert, E. (1982). Primary structure of human Met- and Leu-enkephalin precursor and its mRNA. Nature (London) 295, 663-666. © 1982 Macmillan Journals Limited.

figure should be compared with Figure 5-10, which shows the proopiomelanocortin (POMC) of the corticotropin The structure of proenkephalin was obtained from a complete cDNA copy of the specific mRNA from a human pheochromocytoma. The precursor is 267 amino acids long. Five of the seven enkephalins are flanked on both sides by pairs of basic amino acid residues, which probably serve as cutting sites for proteases. The precursor does not contain sequences of the opioid peptides, dynorphin, a-neoendorphin, or (3-endorphin. It is apparent that, in addition to the different enkephalin precursors in adrenal medulla and in hypothalamus, it is likely that the proteolytic processing to active enkephalins is also different.

As yet it is not clear what the role is of these adrenal medulla enkephalins (see Table 11-2). Ultimately, it can be considered that since the release of these hormones is triggered by stress, they likely play a role in stress adaptation.

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