DehydroalanylGly,a new post translational modification resulting from the breakdown of glutathione

Background

The human body contains numerous long-lived proteins which deteriorate with age, typically by racemisation, deamidation, crosslinking and truncation. Previously we elucidated one reaction responsible for age-related crosslinking, the spontaneous formation of dehydroalanine (DHA) intermediates from phosphoserine and cysteine. This resulted in non-disulphide covalent crosslinks. The current paper outlines a novel posttranslational modification (PTM) in human proteins, which involves the addition of dehydroalanylglycine (DHAGly) to Lys residues.

Methods

Human lens digests were examined by mass spectrometry for the presence of (DHA)Gly (+144.0535?Da) adducts to Lys residues. Peptide model studies were undertaken to elucidate the mechanism of formation.

Results

In the lens, this PTM was detected at 18 lysine sites in 7 proteins. Using model peptides, a pathway for its formation was found to involve initial formation of the glutathione degradation product, γ-Glu(DHA)Gly from oxidised glutathione (GSSG). Once the Lys adduct formed, the Glu residue was lost in a hydrolytic mechanism apparently catalysed by the ε-amino group of the Lys.

Conclusions

This discovery suggests that within cells, the functional groups of amino acids in proteins may be susceptible to modification by reactive metabolites derived from GSSG.

General significance

Our finding demonstrates a novel +144.0535?Da PTM arising from the breakdown of oxidised glutathione.