Aminopeptidase O (AOPEP, AP-O) is a member of the M1 family of zinc metallopeptidases or gluzincins. AOPEP contains a large catalytic gluzincin domain which has the archetypal metalloprotease zinc-binding site ending in a family-specific Glu residue (HEXXHXE) typical to this family. AOPEP is able to catalyse the hydrolsis of an arginine residue (L-Arg) from angiotensin III (AGT(35-41)) to form angiotensin IV (AGT(36-41)), a bioactive peptide of the renin-angiotensin pathway. AOPEP does not contain a recognisable signal sequence or type II transmembrane domain, indicating that it likely belongs to the cytoplasmic subfamily of gluzincins (Diaz-Perales et al. 2005). AOPEP mRNA transcripts are predominantly detected in the pancreas, placenta, liver, testis, and heart. Expression of the AOPEP in heart and testis could suggest involvement in the regulation of cardiac and male reproductive physiology (Diaz-Perales et al. 2005).