Aminopeptidase O/ONPEP: ProductsAminopeptidase O (AP-O; also C9orf3) is a 93 kDa (predicted), intracellular member of the M1 metalloprotease family of enzymes. It is expressed in heart, testis and pancreas where it cleaves N-terminal Arg and Asn residues. Human AP-O is 819 amino acids in length. Aminopeptidase O contains one catalytic domain (aa 167 - 589) with a zinc-binding site (His479 - His483) and a C-terminal ARM region (aa 706 - 819). At least three additional isoforms of Aminopeptidase O exist. Full-length human AP-O is 76% aa identical to mouse AP-O.