Pyroglutamyl apelin-13 identified as the major apelin isoform in human plasma

Apelin is emerging as an important hormone regulator of cardiovascular homoeostasis and an important biomarker for heart failure. Apelin concentrations have historically been measured by immunoassays; however, reported apelin concentrations measured in healthy volunteers show a large disparity from a few picograms per milliliter (pg/ml) to several nanograms per milliliter (ng/ml). Apelin exists in several isoforms ranging in size from 12 to 36 residues, and immunoassays generally cannot distinguish the specific forms present. In this study, an optimized method for enriching apelin peptides with cation-exchange beads followed with mass spectrometry analysis is presented. Apelin peptides are labile in plasma at physiological conditions; however, by lowering the plasma pH to 4.5, the recovery of apelin peptides can be increased significantly. Through optimizing the cation-exchange extraction process, we improved the lower limit of detection for most of the apelin peptides monitored to a few pg/ml. Using the improved method, we detected pyroglutamyl apelin-13 [(pyr)apelin-13] as the major apelin isoform present in plasma from several healthy volunteers at concentrations ranging from 7.7 to 23.3 pg/ml.