Mass spectrometric characterization of modifications to angiotensin II by lipid peroxidation products, 4-oxo-2(E)-nonenal and 4-hydroxy-2(E)-nonenal

The octapeptide angiotensin II (Ang II; Asp¹-Arg ²-Val³-Tyr⁴-Ile⁵-His ⁶-Pro⁷-Phe⁸) is the primary active hormone of the renin/angiotensin system (RAS) and has been implicated in various cardiovascular diseases. Numerous structure-activity relationship studies have identified Asp¹, Arg², and His⁶ of Ang II to be critical for its biological activity and receptor binding. From the reactions of Ang II with lipid peroxidation-derived aldehydes, 4-oxo-2(E)-nonenal (ONE) or 4-hydroxy-2(E)-nonenal (HNE), we have identified the major modifications to the N-terminus, Asp¹, Arg², and His⁶ of Ang II by liquid chromatography/mass spectrometry (LC/MS) and matrix-assisted laser desorption ionization-time-of-flight/MS (MALDI-TOF/MS). The identities of ONE- and HNE-modified Ang II were confirmed by tandem mass spectrometry (MS/MS) and postsource decay (PSD)-TOF/MS before and after the reaction with sodium borohydride. In the reaction with ONE, a pyruvamide-Ang II that formed via oxidative decarboxylation of N-terminal Asp was detected as the most abundant product after 48 h of incubation. It was followed by Arg-modified [Arg ²(ONE-H₂O)]-Ang II and the N-terminal-modified 4-ketoamide form of [N-ONE]-Ang II. The Michael addition products of [His ⁶(HNE)]-Ang II were the most abundant products in the beginning of the reaction with HNE, followed by the dehydrated Michael addition products of [His⁶(HNE-H₂O)]-Ang II. [His⁶(HNE)]-Ang II was dehydrated to [His⁶(HNE-H₂O)]-Ang II during the prolonged incubation, and [His⁶(HNE-H₂O)]-Ang II became the major products after 7 days. The model reactions of N^α-tert- butoxycarbonyl (tBoc)-Arg with ONE and tBoc-His with HNE were performed and compared with the Ang II reaction. tBoc-Arg readily reacted with ONE to produce a compound analogous to [Arg²(ONE-H₂O)]-Ang II, which confirmed Arg as one of the important target nucleophiles of ONE. However, tBoc-His exclusively formed a Michael addition product upon the reaction with HNE. The unexpected formation of [His⁶(HNE-H₂O)]-Ang II can be explained by the proximity of His⁶ to C-terminal carboxylate in the specific conformation of Ang II, which facilitates the dehydration of Michael addition products. Therefore, our results suggest a possible discrepancy in the adduction chemistry of ONE and HNE for model amino acids and endogenous bioactive peptides, which is governed by the microenvironment of peptides, such as the specific amino acid sequence and conformation. Such stable ONE- and HNE-derived modifications to Ang II could potentially modulate its functions in vivo by disrupting the interaction with Ang II type 1 (AT₁) receptor and/or inhibiting the enzyme activity of aminopeptidase A (APA), which cleaves the N-terminal Asp residue of Ang II to generate Ang III.