ACE and ACE2 are both chloride-activated metallopeptidases that are predominantly associated with the cell membrane and are widely distributed in various tissues and vascular beds. In contrast to ACE, which cleaves two amino acid residues from the carboxyl terminus of Ang I to form Ang II, ACE2 hydrolyzes a single amino acid from the carboxyl end of Ang II to form Ang-(1-7) [2] . ACE is considered the primary enzymatic pathway that catalyzes the generation of Ang II in the circulation and tissues. ACE inhibitors, which have become standard therapies in the treatment of hypertension and other cardiovascular disease, have little or no inhibitory activity against ACE2, but they reduce the metabolism of Ang-(1-7) [2] . Circulating levels of ACE activity are readily measurable in humans and other species using synthetic substrates or assessing the direct conversion of Ang I to Ang II.