From Ghrelin to Ghrelin's O-Acyl Transferase

The hormone ghrelin is a unique signaling peptide with powerful metabolic effects, mediated by its acylated forms. The acyl modification of ghrelin is unique in that it takes place via a susceptible ester linkage in the conserved serine-3 of ghrelin and is composed principally of octanoyl and, to lesser extent, decanoyl fatty acids. The nature of this ester linkage makes it susceptible to esterases, which convert it to its des-acyl forms, and, if not adequately inhibited, the conversion to des-acyl ghrelin, particularly post sample collection, can lead to artifactual and misleading results. Here, we describe sample processing and mass spectrometric methodologies for the accurate and simultaneous quantification of acylated and des-acylated forms of ghrelin. We exploited these methodologies (1) to characterize circulating and tissue-specific forms of acyl and des-acyl ghrelin, (2) to optimize a cell system for acyl ghrelin production and search for the enzyme responsible for ghrelin's acylation, and (3) to demonstrate that GOAT is ghrelin's O-acyl transferase.