Somatostatin conformation: evidence for a stable intramolecular structure from circular dichroism, diffusion, and...

Somatostatin is a hypothalamic tetradeca peptide that inhibits the release of growth hormone insulin, and glucagon. The circular dichroism spectrum is characterized by negative extrema at 238 nm and 270 nm, and a positive extremum at 225 nm. The far ultraviolet circular dichroism spectrum is consistent with the presence of ordered secondary structure such as beta-structure, but not alpha-helix. Sedimentation equilibrium results demonstrate that somatostatin exists in its monomeric form (i.e., a molecular weight of 1610 +/- 36 was obtained) and, thus, the structure must arise from intramolecular interactions. The diffusion constant of somatostatin was estimated to be 1.66 X 10(-6) cm2/sec. These data are consistent with an ellipsoidal rather than a spherical shape. The magnitude of the ellipticity at both 225 nm and 238 nm is quite dependent on guanidinium hydrochloride concentration; the midpoint occurs at about 3 M and the transition is cooperative-like. These data strongly suggest that somatostatin has a stable conformation in aqueous solution. A model, consistent with the results of the physicochemical studies and with semi-empirical rules for secondary structure formation, is proposed for somatostatin. The proposed structure consists of a hairpin loop with several residues in an antiparallel beta-pleated sheet, is somewhat elongated, and contains a hydrophobic domain at one end and a hydrophilic domain at the other end.