Receptor interactions with peptide hormones

When loaded with GDP, Ns must also interact with occupied hormone receptors. Such Ns-receptor complexes can be detected by an increase in apparent receptor size and by the presence of ADP-ribosylated peptides (i.e., Nso, see below) in the enlarged complex [7]. In the presence of GTP this complex dissociates. The net result is that Nsa acts as a shuttle, interacting alternately with hormone receptors and with the catalytic unit of cyclase and in effect informing the latter of the presence of the former. 

Early investigations of peptides in membrane model systems included studies of mel-letin 124,125 220 221 spectra and polarization properties. This water-soluble peptide is found to be structureless in solution at neutral pH but was sensitive to environmental change. The undecapeptide hormone, substance P, a member of the tackykinin family, was also found by Choo et a].1222 to be unstructured in solution at physiological pH and to aggregate at high pH or on interaction with charged lipids. These data were used as counter-evidence to a hypothesis that the membrane surface structured the peptide to facilitate interaction with the receptor. 

Hormones Some lipophilic hormones (e.g. the steroid hormones, thyroxine, retinoic acid and vitamin D) diffuse across the plasma membrane and interact with intracellular receptors in the cytosol or nucleus. Other lipophilic hormones (e.g. the prostaglandins) and hydrophilic hormones (e.g. the peptide hormones insulin and glucagon and the biogenic amines epinephrine and histamine) bind to receptor proteins in the plasma membrane. 

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