Mammalian blood plasma contains considerable activity of soluble copper-containing amine oxidase (AOC) referred to as plasma or serum amine oxidase (SAO). The identity and origin of SAO was investigated based on the recent characterization of four porcine AOC genes with AOC1 encoding diamine oxidase (DAO), AOC2 retina-specific amine oxidase (RAO), AOC3 vascular adhesion protein-1 (VAP-1), and AOC4 a VAP-1 homologue that is expressed mainly in the liver and has a signal peptide sequence instead of a transmembrane domain at its N-terminus. Purification and characterization of the major amine oxidase activity from porcine serum showed that it is the product of the AOC4 gene. Intriguingly, all mammals possessing a functional AOC4 gene exhibit high plasma amine oxidase activity. Humans and rodents lack a functional AOC4 gene and have comparably low plasma amine oxidase activity that is probably derived from partial proteolytic release of the membrane-associated AOC3 gene product VAP-1.