Degradation of gonadotropin-releasing hormones in the gilthead seabream, Sparus aurata: II. Cleavage of native salmon GnRH, mammalian LHRH, and their analogs in the pituitary, kidney, and liver

Abstract

The pattern and kinetics of degradation of native salmon gonadotropin-releasing hormone (sGnRH), mammalian luteinizing hormone-releasing hormone (LHRH), and some of their analogs by cytosolic enzymes of pituitary, kidney, and liver were studied in the gilthead seabream, Sparus aurata. The native peptides sGnRH and LHRH are rapidly degraded by all three tissues, LHRH being degraded faster than sGnRH. The kinetics of production of the peptide fragments suggest that initial cleavage of sGnRH and LHRH in the three studied tissues occurs at the 5–6 and 9–10 bonds. This indicates the initial activity of a Tyr 5-Gly 6 endopeptidase and a Pro 9-Gly 10NH 2 peptidase or postproline cleaving enzyme. Secondary degradation of the main initial fragments (1–5, 6–10, and 1–9) is more intensive in the kidney than in the pituitary or liver. Substitution of the position 6 amino acid glycine by a dextrorotatory ( d) amino acid such as in the d-Trp 6-LHRH renders the 5–6 bond resistant to cleavage. However, whereas [ d-Trp 6]-LHRH is intensively cleaved at the Pro 9-Gly 10NH 2 bond by the pituitary, its cleavage at this site by the kidney and liver is slow. This suggests a low activity of the Pro 9-Gly 10NH 2 peptidase in the kidney and liver as compared to the pituitary. When, in addition to the position 6 substitution, the carboxy terminus Pro 9-Gly 10NH 2 is modified to Pro 9NET, such as in the [ d-Ala 6-Pro 9NET]-LHRH and the [ d-Arg 6-Pro 9NET]-sGnRH, the 9–10 cleavage site is also blocked, resulting in GnRH analogs highly resistant to degradation. The relationships between susceptibility of the different forms of GnRH to enzymatic degradation by the pituitary, kidney, and liver and their relative biological activities in S. aurata are discussed. We conclude that increased resistance of GnRH analogs to enzymatic degradation contributes to their superactivity.