Structure of mammalian and nonmammalian ghrelins.

The discovery of ghrelin has elucidated the role of the stomach as an important organ in the regulation of growth hormone (GH) release and energy homeostasis. Ghrelin is a peptide hormone in which Ser3 Thr3 in frogs) is modified by an n-octanoic acid; this modification is essential for ghrelin's activity. Ghrelin and motilin, another gastric peptide, structurally and functionally define a peptide superfamily; these two factors may have evolved from a common ancestral peptide. Ghrelin is found in both mammalian species as well as nonmammalian species, such as frogs, birds, and fish. Moreover, ghrelin structure, particularly that of the acyl-modification regions, is highly conserved throughout vertebrate species. All of the ghrelin peptides that have been identified are modified by a fatty acid, primarily n-octanoic acid. These discoveries implicate ghrelin as an essential hormone in the maintenance of GH release and energy homeostasis in vertebrates.