Protein posttranslational modifications (PTMs) play a central role for regulating protein function and cellular processes, with many PTMs arising from reactions with electrophilic metabolites. Here we extend the known landscape of PTMs with the identification of “lysine C 3 -iminylation,” the conjugation of protein lysine residues with propionaldehyde. To stabilize iminylation for mass spectrometric analyses and distinguish it from other isomeric PTMs, we developed a fixation and stable-isotope labeling approach based on parallel reduction of proteome with sodium borohydride and borodeuteride. Analyses of protein hydrolysates confirmed the presence of C 3 -iminylation in Caenorhabditis elegans and mouse. Additionally, proteomics results demonstrated specificity of this PTM in vitro and in vivo and revealed C 3 -iminylation in proteins related to critical metabolic pathways. Importantly, collective evidence from isotope tracing as well as genetic, dietary, and pharmacological manipulation experiments uncovered that C 3 -iminylation originates from cytochrome P450 (CYP)-mediated oxidation of omega-3 fatty acids. Correspondingly, C 3 -iminylation levels were elevated in C. elegans daf-2 ( e1370 ) mutants, an aging model, in which CYP activity is generally increased. These findings not only expand our understanding of the biochemical diversity of PTMs but also underscore the complex interplay between lipid metabolism and protein modifications, enabling further exploration of their biological and clinical implications.
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