A novel approach to the C-terminal determination of peptides: selective 2H- and 3H-labelling reaction of C-terminal amino acids through oxazolone

Deuterium Labeling of Isoaspartic and Isoglutamic Acids for Mass Spectrometry Analysis

Isoaspartic acid (isoAsp) is a common protein modification that spontaneously arises from asparagine or aspartic acid and has been linked to various diseases and health conditions. However, current methods for identifying isoAsp sites in proteins often suffer from ambiguity and have not gained widespread adoption. We developed a novel method that exclusively labels isoAsp with deuterium. This method capitalizes on the unique structural characteristics of isoAsp residues, which possess a free α-carboxyl group and can form an oxazolone ring. Once the oxazolone ring forms, it facilitates racemization at the Cα-position, incorporating a deuteron from a D2O solvent. The sites of deuterium-incorporated isoAsp in proteins can be unequivocally determined by comparing the precursor and product ion masses of the peptides from proteins reacted in H2O and D2O. The effectiveness of this method has been demonstrated through its application to model proteins lysozyme and rituximab. Furthermore, we have confirmed that the isoAsp deuterium-labeling reaction efficiently labels both l- and d-isoAsp without distinction, as well as isoglutamic acid (isoGlu), for which no effective detection methods currently exist.

C-terminal sequencing method for proteins in polyacrylamide gel by the reaction of acetic anhydride

A successive C-terminal amino acid truncation reaction with acetic anhydride was applied on proteins in polyacrylamide gel. Protein bands separated by conventional SDS-PAGE were excised, partially fixed in the gel with glutaraldehyde ethanol solution, dehydrated with ACN and subjected to the truncation reaction with acetic anhydride formamide solution. Pre-treatment of the gel with pyridine aqueous solution was found to enhance the truncation reaction yields. After the truncation reaction, the products were treated with an aqueous solution of dimethylaminoethanol to hydrolyze oxazolone rings at the C termini of the truncated products and O-acetylated products of serine, threonine and/or tyrosine. Several commercially available proteins of 10-40 kDa, as determined by SDS-PAGE, such as myoglobin, trypsin inhibitor, alpha-hemolysin, cytochrome c, chymotrypsin C chain, elastase, acylase and histone H4, were subjected to the C-terminal analysis. The truncated proteins were in-gel digested with trypsin and the extracted peptides were analyzed by MALDI-TOF MS, giving rise to a series of molecular mass ions of the C-terminal truncated fragments corresponding to the C-terminal amino acid sequence of the relevant protein.

Amino acid sequence and D/L-configuration determination methods for D-amino acid-containing peptides in living organisms

D-amino acid-containing peptides with biological activities have been isolated from invertebrates and amphibians, and partial racemization of amino acid residues in mammalian peptides associated with aging and diseases have been discussed. Here, we review the amino acid configuration determination methods in these peptides and recent progress of simultaneous determination method for sequence and configuration of amino acid residues. The applicability of C-terminus sequence analysis and mass spectrometry to configuration determination of amino acids is also discussed.

C-terminal sequencing of protein. A novel partial acid hydrolysis and analysis by mass spectrometry

Peptides or proteins were hydrolyzed by vapors of 90% pentafluoropropionic acid or heptafluorobutyric acid at 90 degrees C for various time periods. The hydrolyzate mixtures analyzed by both fast-atom-bombardment and electrospray ionization mass spectrometry showed a series of C-terminal successive degradation molecular ions. The degradation reaction may be due to the selective formation of an oxazolone ring at the C-terminal amino acid, followed by hydrolytic removal of the C-terminal amino acid. The major side reactions were cleavages of the peptide bonds at the C side of the internal aspartic acid residue and the N side of serine residue.