Structural and functional significance of arginine at position 2 of angiotensin II: Implications for oxidant-mediated amino acid residue conversions

Angiotensin II (Ang II) is a major mediator of pathophysiological processes. Angiotensin-converting enzyme 2 (ACE2) regulates Ang II levels by cleaving it into Ang 1-7. Our previous study showed that arginine at the second position (Arg2) of Ang II plays an important role in regulating ACE2 peptidase activity. The present study examined the mechanisms and functions of Arg2 in Ang II. Cleavage of Arg2 from Ang II, or single amino acid replacement of a positively charged Arg with other amino acids, particularly negatively charged amino acids such as glutamic acid (Glu), eliminated Ang IIs ability to compete with the fluorogenic substrate used in the ACE2 activity assay. Arg2-to-Glu substitution also affected the ability of angiotensin I and angiotensin III to interact with ACE2. These results confirmed the important roles of Arg2 in the ACE2 enzymatic mechanism. Since Arg and proline (Pro) residues are susceptible to protein carbonylation, and both residues produce glutamic semialdehyde, which can be further oxidized to Glu, we previously proposed that Arg, Pro, and Glu could be interchangeable, and that ROS promote amino acid conversions within protein structures post-translationally in biological systems. Thus, Arg2 of Ang II could undergo oxidant-mediated amino acid residue conversion to become Glu or Pro. The presence of both circulating Ang II (Arg2Glu) and Ang II (Arg2Pro) in rats were confirmed by mass spectrometry. Global gene expression analyses revealed that Arg2Glu- and Arg2Pro-converted Ang II peptides elicit different cellular signaling responses compared with wild-type Ang II, suggesting that Arg2-modifed Ang II peptides exhibit distinct biological functions.