The longer and neurotoxic species of amyloid‐β protein (Aβ), Aβ42 and Aβ43, contribute to Aβ accumulation in Alzheimer's disease (AD) pathogenesis and are considered to be the primary cause of the disease. In contrast, the predominant secreted form of Aβ, Aβ40, inhibits amyloid deposition and may have neuroprotective effects. We have reported that angiotensin‐converting enzyme (ACE) converts Aβ42 to Aβ40 and that Aβ43 is the earliest‐depositing Aβ species in the amyloid precursor protein transgenic mouse brain. Here we found that Aβ43 can be converted to Aβ42 and to Aβ40 in mouse brain lysate. We further identified the brain Aβ43‐to‐Aβ42‐converting enzyme as ACE2. The purified human ACE2 converted Aβ43 to Aβ42, and this activity was inhibited by a specific ACE2 inhibitor, DX600. Notably, the combination of ACE2 and ACE could convert Aβ43 to Aβ40. Our results indicate that the longer, neurotoxic forms of Aβ can be converted to the shorter, less toxic or neuroprotective forms of Aβ by ACE2 and ACE. Moreover, we found that ACE2 activity showed a tendency to decrease in the serum of AD patients compared with normal controls, suggesting an association between lower ACE2 activity and AD. Thus, maintaining brain ACE2 and ACE activities may be important for preventing brain amyloid neurotoxicity and deposition in Alzheimer's disease. © 2014 Wiley Periodicals, Inc.