Location of alkali metal binding sites in endothelin a selective receptor antagonists, cyclo(D-Trp-D-Asp-Pro-D-Val-Leu) and cyclo(D-Trp-D-Asp-Pro-D-Ile-Leu), from multistep collisionally activated decompositions

We previously showed by using mass spectrometry that endothelin A selective receptor antagonists BQ123 and JKC301 form novel coordination compounds wi th sodium ions. This property may underlie the ability of an ETA antagonist to induce net tubular sodium reabsorption in the proximal tubule cells and reverse acute renal failure induced by severe ishemia, We have now defined the metal binding sites on BQ123 and JKC301 by subjecting the metal-contai ning peptides to multiple stages of collisionally activated decomposition ( CAD) in an ion trap mass spectrometer, When submitted to low-energy CAD, th e ring opens at the Asp-Pro amide bond. The metal ion, which bonds, inter a lia, to the carbonyl oxygen of the proline residue, acts as a fixed charge site, and directs a charge-remote, sequence-specific fragmentation of the r ing-opened peptide. Amino acid residues are sequentially cleaved from the C -terminal end, and the terminal aziridinone structure moves one step toward the N-terminus with each C-terminal amino acid residue removed. These obse rvations are the basis of a new method to sequence cyclic peptides. Amino a cid residues are observed as sets of three ions, a(nPD)*, b(nPD)*, and c(nP D)*, where n is the number of amino acid residues in the peptide, Copyright (C) 2000 John Wiley & Sons, Ltd.