STRUCTURE OF A HYPOTHALAMIC PEPTIDE POSSESSING GONADOTROPIN‐RELEASING ACTIVITY

A second batch of porcine gonadotropin‐releasing hormone, isolated by a new procedure, gave an amino acid composition: Trp 1, His 1, Arg 1, Ser 1, Glu 1, Pro 1, Gly 2, Leu 1, and Tyr 1, with 67% amino acid recovery. Edman‐dansyl degradation of the intact hormone did not give a positive result, suggesting a blocked N‐terminus. Esterification of the peptide with [¹⁴C]diazomethane failed to yield any [¹⁴C]methylester, indicating the absence of any free carboxyl‐group in the molecule. This suggested that the C‐terminus of the hormone is also blocked. High and low resolution mass spectral fragmentation of the hormonal peptide gave intense bands due to (pyro)‐glutamyl residue which occupies the N‐terminus. The main diketopiperazine bands from the mass spectral data were due to (pyro)Glu‐His, Trp‐Ser, Tyr‐Gly, Leu‐Arg, Arg‐Pro, Pro‐Gly, and their decomposition. Mass spectral fragmentation of the hormone and its chymotryptic fragments after acetylation and permethylation, before and after methanolysis, yielded orderly patterns, indicating the sequence of the deca‐peptide and also confirming the (pyro)glutamyl N‐terminus and the glycylamide C‐terminus. These studies, in agreement with the conventional chemical sequencing of the hypothalamic peptide after enzymic cleavage, proved that the structure of the hormone is: Glu‐His‐Trp‐Ser‐Tyr‐Gly‐Leu‐Arg‐Pro‐Gly‐NH₂