Bullfrog ghrelin is modified by n-octanoic acid at its third threonine residue

We have identified the amphibian ghrelin from the stomach of the bullfrog. We also examined growth hormone (GH)-releasing activity of this novel pepti de in both the rat and bullfrog. The three forms of ghrelin identified, eac h comprised of 27 or 28 amino acids, possessed 29% sequence identity to the mammalian ghrelins. A unique threonine at amino acid position 3 (Thr(3)) i n bullfrog ghrelin differs from the serine present in the mammalian ghrelin s; this Thr(3) is acylated by either n-octanoic or n-decanoic acid. The fro g ghrelin-28 has a complete structure of GLT (O-n-octanoyl)FLSPADMQKLAERQSQ NKLRHGNM; the structure of frog ghrelin-27 was determined to be GLT(O-n-oct anoyl)FLSPADMQKLAERQSQNKLRHGN; frog ghelin-27-C10 possessed a structure of GLT(O-n-decanoyl)FLSPADMQKLAERQSQNKLRHGN. Northern blot analysis demonstrat ed that ghrelin mRNA is predominantly expressed in the stomach. Low levels of gene expression were observed in the heart, lung, small intestine, gall bladder, pancreas, and testes, as revealed by reverse transcription polymer ase chain reaction analysis. Bullfrog ghrelin stimulated the secretion of b oth GH and prolactin in dispersed bullfrog pituitary cells with potency 2-3 orders of magnitude greater than that of rat ghrelin. Bullfrog ghrelin, ho wever, was only minimally effective in elevating plasma GH levels following intravenous injection into rats. These results indicate that although the regulatory mechanism of ghrelin to induce GH secretion is evolutionary cons erved, the structural changes in the different ghrelins result in species-s pecific receptor binding.