STRUCTURAL AND BIOCHEMICAL-STUDIES OF HUMAN GALANIN - NMR EVIDENCE FOR NASCENT HELICAL STRUCTURES IN AQUEOUS-SOLUTION

The 30-residue human neuropeptide, galanin, was shown to bind to rat insulinoma RINm5F cells and to inhibit glyceraldehyde-stimulated insulin secretion from these cells in a manner quantitatively similar to that of porcine, galanin. Neither human nor porcine galanin stimulated Ca2+ mobilization in cultured human small cell lung carcinoma cells. Sedimentation equilibrium analysis of human galanin showed that it was strictly monomeric in aqueous solution, indicating that the peptide interacts with its receptor(s) as a monomer. The monomeric nature of the peptide makes it especially suitable for structural studies using NMR. Nuclear Overhauser enhancement spectroscopy experiments performed on galanin dissolved in aqueous solution (150 mM KCl, pH 4) at both 33 and 3 degrees C indicate that certain regions of the peptide are capable of adopting detectable levels of short-range structure in rapid equilibrium with random coil. At 33 degrees C, the short-range structures include a nascent helix spanning residues 3-11 which incorporates a hydrophobic core from residues 6-11. Residues 14-18 and 22-30 display sequential NH-NH and (CH)-H-beta-NH connectivities, indicating that these regions of the peptide adopt nonrandom conformations by significantly populating the ct-region of conformational space. However, no medium-range dipolar connectivities indicative of nascent helix or turn conformations were observed. At 3 degrees C, almost all residues significantly populate the cr-region of conformational space, and the nascent helix between residues 3 and 11, with its hydrophobic core, is retained. As expected, circular dichroism (CD) was insensitive to the presence of short-range structure, and therefore the CD spectrum of human galanin in aqueous solution indicated a completely random coil peptide. However, changes in the CD spectrum resulting from the addition of 30% (v/v) of the helix-promoting organic solvent, trifluoroenthanol, indicated that similar to 6 residues of the peptide were transformed to stable helix.