Detection and identification of arginine modifications on methylglyoxal-modified ribonuclease by mass spectrometric...

Analysis of the broad range of trace chemical modifications of proteins in biological samples is a significant challenge for modern mass spectrometry. Modification at lysine and arginine residues, in particular, causes resistance to digestion by trypsin, producing large tryptic peptides that are not readily sequenced by mass spectrometry. In this work, we describe the analysis of ribonuclease (RNase) modified by methylglyoxal (MGO) under physiological conditions. For detection of modifications, we use comparative analysis of the single combined spectra extracted from the full-scan MS data of the tryptic digests from native and modified proteins. This approach revealed 11 ions unique to MGO-modified RNase, including a 32-amino acid peptide containing a modified Arg-85 residue. Sequential digestion of MGO-modified RNase by endoproteinase Glu-C and trypsin was required to obtain peptides that were amenable to sequencing analysis. Arg-39 was identified as the main site of modification (35% modification) on MGO-modified Rnase, and the dihydroxyimidazolidine and hydroimidazolone derivatives were the main adducts formed, with minor amounts of the tetrahydropyrimidine and argpyrimidine derivatives. For identification of these products, we used variations in source voltage and collision energy to obtain the dehydration and decarboxylation products of the tetrahydropyrimidine-containing peptides and dehydration of the dihydroxyimidazoline-containing peptides. The resultant spectra were dependent on the cone voltage and collision energy, and analysis of spectra at various settings permitted structural assignments. These studies illustrate the usefulness of single combined mass spectra extracted from full-scan data and variations in source and collision cell voltages for detection and structural characterization of chemical adducts on proteins.