Purity >95%. The apelin gene encodes a pre-proprotein that is processed to generate bioactive peptides consisting of 36, 17, or 13 amino acids (apelin-36, apelin-17, and apelin-13, respectively). Apelin-13 is the endogenous ligand of the APJ receptor, activating this G protein-coupled receptor with an EC50 value of 0.37 nM (the EC50 values for apelin-17 and apelin-36 are 2.5 and 20 nM, respectively). It acts primarily in the periphery and central nervous system, playing important roles in regulating cardiovascular function, fluid homeostasis, hypertension, and insulin sensitivity. Unlike apelin-36, apelin-13 poorly blocks the entry of human immunodeficiency virus into cells.