Structure and function in neuropeptides

This report reviews some aspects of the organization of molecular information, structure-activity relations and receptor interactions of three members of a family of neuroactive and hormonal peptides, adrenocorticotrophin (ACTH), alpha-melanotrophin (alpha-MSH) and the enkephalins, as they are presently being investigated in the author's laboratory (in collaboration with H. W. Kosterlitz, D. Schulster and P. W. Schiller). It has been established that ACTH acts on two different steroidogenically responsive receptors in rat adrenocortical cells: one that stimulates steroidogenesis without cyclic AMP synthesis and one that induces cyclic AMP production. This finding is a further example of the pleiotropic action of the opiocortin gene through different mechanisms. Another recent discovery, also related to the organization of ACTH information, reveals an unexpected interaction of ACTH(1-24) with lipid bilayer membranes: the molecule binds to the membranes in a reversible fashion and penetrates them. Parts of the hormonal message are exposed on the side of the membrane opposite to that to which the hormone was added. New 'fat' amino acids like carboranylalanine and adamantylalanine, as well as t-butylglycine, have been used to establish structure-activity relations and the influence of strongly enhanced lipophilicity on the biological activity of the enkephalins. An address/potentiator function of the two C-terminal amino acids is suggested. Tobacco mosaic virus (TMV) has been used as a carrier for covalently attached peptide hormones. Attachment of large numbers of hormone molecules endows these conjugates with the properties of superpotency, superaffinity and prolonged action on the target cell. Some of these properties are illustrated by TMV-enkephalin conjugates.