Accurate Proteomewide Measurement of Methionine Oxidation in Aging Mouse Brains

The oxidation of
methionine has emerged as an important post-translational
modification of proteins. A number of studies have suggested that
the oxidation of methionines in select proteins can have diverse impacts
on cell physiology, ranging from detrimental effects on protein stability
to functional roles in cell signaling. Despite its importance, the
large-scale investigation of methionine oxidation in a complex matrix,
such as the cellular proteome, has been hampered by technical limitations.
We report a methodology, methionine oxidation by blocking (MobB),
that allows for accurate and precise quantification of low levels
of methionine oxidation typically observed in vivo. To demonstrate
the utility of this methodology, we analyzed the brain tissues of
young (6 m.o.) and old (20 m.o.) mice and identified over 280 novel
sites for in vivo methionine oxidation. We further demonstrated that
oxidation stoichiometries for specific methionine residues are highly
consistent between individual animals and methionine sulfoxides are
enriched in clusters of functionally related gene products including
membrane and extracellular proteins. However, we did not detect significant
changes in methionine oxidation in brains of old mice. Our results
suggest that under normal conditions, methionine oxidation may be
a biologically regulated process rather than a result of stochastic
chemical damage.