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.

Discrimination of Aspartic and Isoaspartic Acid Residues in Peptides by Tandem Mass Spectrometry with Hydrogen Attachment Dissociation.

Long-lived proteins undergo chemical modifications that can cause age-related diseases. Among these chemical modifications, isomerization is the most difficult to identify. Isomerization often occurs at the aspartic acid (Asp) residues. In this study, we used tandem mass spectrometry equipped with a newly developed ion activation method, hydrogen attachment dissociation (HAD), to analyze peptides containing Asp isomers. Although HAD preferentially produces [cn + 2H]+ and [zm + 2H]+ via N-Cα bond cleavage, [cn + 58 + 2H]+ and [zm - 58 + 2H]+ originate from the fragmentation of the isoAsp residue. Notably, [cn + 58 + 2H]+ and [zm - 58 + 2H]+ could be used as diagnostic fragment ions for the isoAsp residue because these fragment ions did not originate from the Asp residue. The detailed fragmentation mechanism was investigated by computational analysis using density functional theory. According to the results, hydrogen attachment to the carbonyl oxygen in the isoAsp residue results in the Cα-Cß bond cleavage. The experimental and theoretical joint study indicates that the present method allows us to discriminate Asp and isoAsp residues, including site identification of the isoAsp residue. Moreover, we demonstrated that the molar ratio of peptide isomers in the mixture could be estimated from their fragment ion abundance. Therefore, tandem mass spectrometry with HAD is a useful method for the rapid discrimination and semiquantitative analysis of peptides containing isoAsp residues.

Protective role of madecassoside from Centella asiatica against protein L-isoaspartyl methyltransferase deficiency-induced neurodegeneration.

Protein L-isoaspartyl methyltransferase (PIMT/PCMT1) could repair l-isoaspartate (L-isoAsp) residues formed by deamidation of asparaginyl (Asn) residues or isomerization of aspartyl (Asp) residues in peptides and proteins during aging. Aside from abnormal accumulation of L-isoAsp, PIMT knockout (KO) mice mirrors some neuropathological hallmarks such as anxiety-like behaviors, impaired spatial memory and aberrant synaptic plasticity in the hippocampus of neurodegenerative diseases (NDs), including Alzheimer's disease (AD) and related dementias, and Parkinson's disease (PD). While some reports indicate the neuroprotective effect of madecassoside (MA) as a triterpenoid saponin component of Centella asiatica, its role against NDs-related anxiety and cognitive impairment remains unclear. Therefore, we investigated the effect of MA against anxiety-related behaviors in PIMT deficiency-induced mouse model of NDs. Results obtained from the elevated plus maze (EPM) test revealed that MA treatment alleviated anxiety-like behaviors in PIMT knockout mice. Furthermore, Real-time PCR, electroencephalogram (EEG) recordings, transmission electron microscopy analysis and ELISA were carried out to evaluate the expression of clock genes, sleep and synaptic function, respectively. The PIMT knockout mice were characterized by abnormal clock patterns, sleep disturbance and synaptic dysfunction, which could be improved by MA administration. Collectively, these findings suggest that MA exhibits neuroprotective effects associated with improved circadian rhythms sleep-wake cycle and synaptic plasticity in PIMT deficient mice, which could be translated to ameliorate anxiety-related symptoms and cognitive impairments in NDs.