Degradation of gonadotropin-releasing hormones in the gilthead seabream, Sparus aurata: I. Cleavage of native salmon GnRH and mammalian LHRH in the pituitary

Abstract

The pattern and kinetics of degradation of native salmon gonadotropin-releasing hormone (sGnRH) and mammalian luteinizing hormone-releasing hormone (LHRH) by pituitary bound enzymes were studied in the gilthead seabream, Sparus aurata. sGnRH and LHRH were incubated for different periods of time with membrane or cytosolic fractions of pituitary homogenates. At the end of the incubation, the degradation mixture was fractionated on reverse-phase high-pressure liquid chromatography. The degradation products were identified by comparing their retention times to those of synthesized GnRH fragments and by analyzing their amino acid composition. The main GnRH degradative activity resides in the cytosolic fraction of the pituitary homogenate. Both sGnRH and LHRH are rapidly degraded by pituitary cytosol, with 78.3 and 87.7% of the peptides, respectively, cleaved after 3 hr of incubation. Maximal degradation of sGnRH occurred at a pH range of 7 to 8. The main initial products of degradation of sGnRH and LHRH are the 1–5, 6–10, and 1–9 fragments. This suggests the involvement of two site-specific peptidases, a Tyr 5-Gly 6 endopeptidase and a Pro 9-Gly 10NH 2 peptidase or postproline cleaving enzyme. While the 1–6 and 1–9 fragments undergo rapid secondary degradation, the 1–5 is relatively stable. Competition experiments suggest that the endopeptidase cleaving the sGnRH at the Tyr 5-Gly 6 bond is not specific to the neuropeptide and is probably a general proteolitic enzyme. However, the cleavage at the 9–10 bond has a high degree of specificity to the Pro 9-Gly 10NH 2 sequence found in sGnRH. The two proposed pituitary peptidases of S. aurata have some characteristics similar to those of rat hypophyseal and hypothalamic GnRH cleaving enzymes. No differences are found in hypophyseal GnRH degradative activity between females with occytes undergoing previtellogenesis or advanced stages of vitellogenesis.