[Analysis of variant and modified structures of proteins by mass spectrometry--application for clinical laboratory...

We have successfully applied soft ionization MS for the analysis of proteins in blood and tissues. This article is a summary of a lecture presented on March 8, 2006 in the Hall of Osaka Medical College at the time of the author's retirement from Osaka Medical College. The article addresses the detection and characterization of hemoglobin (Hb) variants, an improved reference method for HbAlc measurement, identification of variants of transthyretin (TTR) and Cu/Zn-superoxide dismutase (SOD-1) and the diagnostic application of the signals of modified forms of TTR. During the process of TTR analysis, we found unique isoforms of TTR, which showed changes of the cysteine (10th from amino terminal) residue to glycine, dehydroalanine, and S-sulfocysteine residues. Without the addition of sulfuric acid, the S-sulfonated adduct was generated, namely, sulfur was generated from the peptide or protein itself via dimer formation. These experiments suggest that transformation starts from beta-elimination of disulfide linkage to dehydroalanine and S-thiocysteine. Dehydroalanine reacts easily with H2O, generating serine, which changes to glycine. S-thiocysteine is oxidized easily to S-sulfocysteine. Such modified structures were never seen in SOD-1 and Hb in our extensive analyses by MS, although these molecules have free cysteine residue. Susceptibility to beta-elimination may depend on adjacent amino acids in the stereochemical structure of the protein. Basic amino acids located near cysteine 10, lysine residues at 9 and/or 15, may promote the reaction. As dehydroalanine in protein reacts strongly with other amino acids either in the molecule or between molecules, the reaction may generate cross-linking covalently or noncovalently, causing amyloidosis. Dehydroalanine reacts with cysteine, forming a thiazolidine ring, followed by cleavage of the peptide-bond at the N-terminal side of dehydroalanine. This type of non-enzymatic cleavage may occur in amyloidogenic precursor protein before fiber formation or in amyloid fibers.