Aminopeptidase N (APN, ANPEP, aminopeptidase M, alanyl aminopeptidase) hydrolyzes angiotensin-(2-8) (angiotensin III) to yield angiotensin-(3-8) (angiotensin IV) (see the positive control reactions in Diaz-Perales et al. 2005). Aminopeptidase O (AOPEP) also hydrolyzes angiotensin-(2-8) to angiotensin-(3-8) in vitro (Diaz-Perales et al. 2005) but AOPEP is located in the nucleolus in vivo (Axton et al. 2008) and angiotensin-(2-8) has not been observed in the nucleus.