CPN2 is a zinc metalloprotease, and cleaves carboxy-terminal arginines and lysines from peptides found in the bloodstream such as complement anaphylatoxins, kinins, and creatine kinase MM (CK-MM). By removing only one amino acid, CPN has the ability to change peptide activity and receptor binding. It is a 280 kDa tetrameric glycoprotein that is synthesized by the liver and secreted into the plasma. It consists of 2 identical 83 kDa regulatory subunits (CPN2) and 2 identical 50 kDa catalytic subunits (CPN1). CPN2, the 83 kDa subunit, binds and stabilizes the catalytic subunit at 37 degrees Celsius and keeps it in circulation. Under some circumstances it may be an allosteric modifier of the catalytic subunit. CPN is a member of a larger family of carboxypeptidases, many of which also cleave arginine and lysine. Because of the highly conserved active sites and the possible redundant functions of carboxypeptidases, it has been difficult to elucidate the role of CPN in disease processes.