Detecting apelin isoforms in human plasma using LC-MS/MS

The apelin peptides are a family of widely expressed peptide hormones that act as the ligand for the G-protein coupled receptor APJ. Apelin plays a significant role in cardiovascular health, neuroprotection, glucose metabolism, kidney function, and potentially tumor growth. These pleiotropic biological effects make apelin peptides potentially useful biomarkers or potential therapeutic agents. Signal transduction by apelin’s binding protein APJ can be heavily biased by different apelin isoforms and lead to different biological effects. This isoform-dependent signaling necessitates that apelin peptides are measured using an analytical approach capable of detecting each apelin isoform in a sensitive, specific, and robust manner to understand apelins biological activities. The standard methods for measuring apelin rely on immunoaffinity assays, which are not able to reliably distinguish between apelin isoforms. Here we demonstrate an ultra-high-performance liquid chromatography – tandem mass spectrometry method for the quantification of apelin peptide isoforms apelin-12, apelin-13, [Pyr1]-apelin-13, and apelin-17 in human plasma samples. This method is capable of detecting endogenous apelin in human plasma with a limit of detection in the picomolar range and a coefficient of variation of less than 20%. Using this method specific apelin peptides can be detected in plasma from human subjects allowing for future investigations of apelins in health and disease.