Conformational and receptor binding studies of gonadotropin releasing hormone and its analogs with respect to their biological activities

Abstract

The structure of porcine, bovine or ovine hypothalamic gonadotropin releasing hormone (GnRH) as elucidated by Schally and Guillemin and their associates was found to be identical. It is a linear decapeptide with distinctive structural features and blocked N- and C-termini. Chicken has two GnRHs, salmon and mammalian GnRHs are closely related in structure and varied in positions 7 and 8. GnRH of lamprey, the most primitive vertebrate, has 50% of variations in its structure (positions 3,5,6,7, and 8) as compared with others. The synthetic GnRH exhibits potent biological activities among mammalian species via its ability to release the luteinizing hormone (LH) and the follicle-stimulating hormone (FSH) from the anterior pituitary. However&e amount needed to induce carp to spawn was found to be much higher than that required to effect ovulation among mammals (1). It is most probably due to the existence of species variation in the GnRH molecules between mammals and teleosts. We have successfully applied the synthetic analogs, [D-Ala6, des Gly-NH,‘?-GnRH ethylamide (G&H-A), to induce spawning in farm fish in China since 1974 (2). Surprisingly, the synthetic peptide according to structure of salmon GnRH elucidated by Vale et a1.(3) was found to be incapable of inducing carp to spawn more smoothly or in a way better than that obtained with mammalian GnRH. Superactive agonists of GnRH to enhance fertility and exert prolonged pharmacological effects have been synthesized. Most of them are products of GnRH modified in the C-terminal Gly-NH, with ethylamide and in the 6th position Gly, which is involved in the formation of B-bend or U-shaped turn, by D-amino acids having a hydrophobic side chain such as D-Ala, D-Leu, D-Tip, D-Ser(But) and D-Nal. They have much better binding affinity to the GnRH receptor and higher resistance to enzymatic degradation. Conformational studies The conformational study of GnRH and its analogs by fluorescence spectroscopy was described with respect to their biological activities (3). Only the Trp3 fluorescence spectroscopy was observed for GnRH or its analogs (G&H-A) which contains one Trp and one Tyr. The maximal emission wavelength of GnRH in aqueous solution is 352nm, while that of free tryptophane is 360nm (the spectra were not corrected), a slight blue shift of 8nm could be interpreted as being due to the shielding of Trp residue in a certain extent from the aqueous phase by B-bend structure that makes Ser4-Ty?-Gly6-Leu7 to form a reverse turn and the C-terminal region to be in the proximity of the N-terminal region so as to fit well into the recognition site of the receptor molecule.