Profiling Carbonylated Proteins in Human Plasma

This study reports the first proteomic-based identification and characterization of oxidized proteins in human plasma. The study was conducted by isolating carbonylated proteins from the plasma of male subjects (age 32−36) with avidin affinity chromatography subsequent to biotinylation of carbonyl groups with biotin hydrazide and sodium cyanoborohydride reduction of the resulting Schiff’s bases. Avidin selected proteins were digested with trypsin, and the peptide fragments were separated by C18 reversed phase chromatography and identified and characterized by both electrospray ionization and matrix assisted laser desorption ionization mass spectrometry. Approximately 0.2% of the total protein in plasma was selected with this method. Sixty-five high, medium, and low abundance proteins were identified, the majority appearing in all subjects. An interesting feature of the oxidized proteins isolated was that in addition to carbonylation they often bore other types of oxidative modification. Twenty-four oxidative modifications were mapped in 14 proteins. Fifteen carbonylation sites carried on 7 proteins were detected. Methionine oxidation was the most frequent single type of oxidative modification followed by tryptophan oxidation. Apolipoprotein B-100 had 20 oxidative modifications, the largest number for any protein observed in this study. Among the organs contributing oxidized proteins to plasma, kidney, liver, and soft tissues were the most frequent donors. One of the more important outcomes of this work was that mass spectral analysis allowed differentiation between different biological mechanisms of oxidation in individual proteins. For the first time, oxidation products arising from direct ROS oxidation of amino acid side chains in proteins, formation of advanced glycation endproducts (AGEs) adducts, and formation of adducts with lipid peroxidation products were simultaneously recognized and assigned to specific sites in proteins.