Abstract 242: The pH Influences Angiotensin-(1-7) Forming Enzyme Activities in the Kidney
The kidney plays a pivotal role in acid-base balance. Hydrogen and bicarbonate titration through the nephron produces ranges in urinary pH from 4.5 to 8. All of the components of the renin angiotensin system (RAS) are expressed in the kidney and intrarenal angiotensin II (Ang II) and Ang-(1-7) are formed by multiple independent enzymatic pathways. Most enzymes function optimally at the pH of their native environment. Since small changes in pH can have significant effects on enzyme activities, we investigated the influence of pH on renal Ang II processing to Ang-(1-7). Wild type (WT) and angiotensin converting enzyme 2 (ACE2) knock out (KO) mouse kidney homogenates (20-30 μg protein) were incubated for 30 min at 37°C in 50 mM citrate, phosphate or glycine buffer (pH 4-10) containing 50 μM substrate. First, activity was analyzed using the fluorogenic substrate MCA-Ala-Pro-Lys(Dnp)-OH. A second approach, matrix assisted laser desorption ionization mass spectrometry ((MALDI MS), allowed for the use of the natural substrate, Ang II. In the WT, the fluorogenic assay system detected activity at pH 4, 5, 5.5, 6, 7, 8, 9, and 10 with minimal differences. Using MALDI MS, renal enzymatic Ang-(1-7) formation was observed at a pH range between 4 and 9 with two peaks at pH 5.5 and 8. At pH < 7, conversion of the fluorogenic substrate as well as Ang-(1-7) formation was also detected in KO mice. The activity at pH < 7 was not blocked in KO and WT by ACE2 inhibitor MLN-4760 but was quenched in only WT at pH ≥ 7. The combined prolyl carboxypeptidase (PCP)/prolyl endopeptidase (PEP) inhibitor [Z-pro-prolinal (ZPP)] reduced Ang-(1-7) formation in WT and KO kidney homogenates at pH < 7. Results suggest enzymes responsible for renal Ang-(1-7) formation shift with changes in pH. ACE2 independent, pH dependent formation of Ang-(1-7) from Ang II has not been described before and underscores the usefulness of the MALDI MS approach.
- © 2012 by American Heart Association, Inc.