Angiotensin I-converting enzyme (ACE) plays a key role in the renin-angiotesin aldosterone cascade. We analysed the secondary structure and structural organization of a purified 65kDa N-domain ACE (nACE) from Wistar rat mesangial cells, a 90 kDa nACE from spontaneously hypertensive rats and a 130 kDa somatic ACE. The C-terminal alignment of the 65 kDa nACE with rat ACE revealed that the former was truncated at Ser(482), and the sequence of the 90 kDa nACE ended at Pro(629). Protein's secondary structure consisted predominantly of alpha-helices. The 90 and 65 kDa isoforms were the most stable in guanidine and at low pH, respectively. Enzymatic activity decreased with loss in secondary structure, except in the case of guanidine HCl where the 90 kDa fragment loses its secondary structure faster than its enzymatic activity. We identified and characterized the activity and stability of these isoforms and these findings would be helpful on the understanding of the role of nACE isoforms in hypertension.
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