Renin-Angiotensin System (RAS) is a critical regulator of blood pressure homeostasis. The protease renin cleaves angiotensinogen into inactive decameric peptide angiotensin-I (Ang-I). Angiotensin-converting enzyme (ACE) then cleaves a C-terminal dipeptide from Ang-I to form an active octamer angiotensin-II (Ang-II), which can contribute to hypertension by promoting vascular smooth muscle vasoconstriction and renal tubule sodium reabsorption. ACE mutant mice display spontaneous hypotension, partial male infertility and kidney malformations. ACE is found in somatic (s-ACE) and testicular/germinal (t-ACE) isoforms. The ACE-2 gene has been mapped at human chromosome Xp22. ACE-2 enzymes from human (805aa) and mouse (798aa) are single chain proteins with 40% seq homology to N- and C-terminal domains of ACE. However, in contrast to s-ACE, which consists of two catalytic sites, ACE-2 contains only one active site. Unlike s-ACE and t-ACE, which are dipeptidyl-carboxypeptidases, ACE-2 acts as a carboxypeptidase, cleaving a single residue from Ang-I, generating Ang1-9 and a single residue from Ang-II to generate Ang1-7. ACE-2 can cleave angiotensin-I but not bradykinin and the enzyme activity is not inhibited by the ACE inhibitors. This enzyme is expressed highly in heart, kidney and testis, and moderately in colon, small intestine and ovary. ACE-2 is an essential regulator of heart function because targeted disruption of this enzyme in mice results in severe cardiac contractility defect, increased angiotensin-II levels and upregulation of hypoxia-induced genes in the heart.
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