- Additionally, FGF 23 directly inhibits Angiotensin converting enzyme 2 (ACE 2) as the central enzyme of the antifibrotic alternative renin-angiotensin-aldosterone system (RAAS) and shifting the RAAS toward the pro fibrotic Angiotensin II. To assess suppression of ACE 2 we will measure Ang 1-5 and Ang 1-7 levels by quantitative analysis of the individual metabolites of the RAAS using mass spectrometry ("RAAS fingerprint").