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

Abstract

Somatostatin is a hypothalamic tetradecapeptide 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 a-structure, but not a-helix. Sedimentation equilibrium results demonstrate that somatostatin exists in its monomeric form (i.e., a molecular weight of 1610 4 36 was obtained) and, thus, the structure must arise from intramolecular interactions. The diffusion constant of sometostatin 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 a-pleated sheet, is somewhat elongated, and contains a hydrophobic domain at one end and a hydrophilic domain at the other end.

Fingerprint Analysis of Bovine Hypothalamic Preprosomatostatin

“…Both antisera are able to recognize a n N-terminally extended SRIF-14 (Table I ) , a n essential prerequisite for the identification of any precursor to a peptide hormone, and are sensitive to changes in the central region of the tetradecapeptide, particularly residues 5 and 8-10. Interestingly, this is also the region of most importance for biological activity [22] and probably represents an exposed site at the hydrophobic end of the proposed hairpin-cyclic conformation [23]. Additionally, CysI4 appears to be critical for the antigenicity of the tett-adecapeptide; this is not only due to a possibly altered cyclic conformation, since the competitiveness of both SRIF-14 as well a s [~C y s '~] S R 1 F -1 4 is unchanged on performing the radioimmunoassays in a reducing buffer ( 2 m M dithiothreitol.…”

“…3) as SRIF-28( 1 -8) (the methionine in position 8 becoming homoserine lactone) and spot B (Fig.3) as SRIF-28 (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28) ninhydrin-stained spot B, SRIF-28 (9 -28). Since this peptide, as well as including the whole SRIF-14 sequence, also includes part of the N-terminal tryptic peptide (spot 4) and thus also the intervening sequence, the 3sS-labelled cyanogen bromide cleavage product must be entirely located at the C terminus of the 15 500-M, precursor.…”

Conformationally restricted bicyclic analogs of somatostatin.

“…The synthesis of the novel bicyclic analogs of somatostatin and the effects of these on the inhibition of secretion of insulin, glucagon, growth hormone, and gastric acid are described. A proposed solution conformation for the peptide hormone somatostatin has been presented by Holladay and Puett (1,2). We wish to propose an alternate conformation at the receptors involving a type II' p-turn from Phe7 to Thr10 and an antiparallel (-pleated sheet structure involving Phe6-Phe7 and Thrl0-Phell.…”

Properties of endogenous somatostatin-like immunoreactivity and synthetic somatostatin in dog plasma.

“…This conclusion is supported by the lack of binding to plasma components of the NH,-terminally modified somatostatin analogue, 1251I-labeled Tyr'-somatostatin, in contrast to COOH-terminally modified analogue Tyr"'-somatostatin. It has been proposed (35) that the somatostatin molecule consists of an elongated "hairpin" loop containing a hydrophobic domain at one end of the molecule, and a hydrophilic domain, associated with the disulfide link and the NH2-terminal residues, at the other end. The present study suggests that the NH-terminal region of somatostatin is involved in the binding to plasma components.…”