Itaconation and Succination as Metabolite-Derived Cysteine Modifications in Inflammatory Signalling: Biochemical...

Inflammatory activation rewires cellular metabolism and generates electrophilic metabolites that can modify reactive cysteine residues. This review focuses on two metabolite-derived cysteine modifications: itaconation, driven by aconitate decarboxylase 1 (ACOD1)-dependent itaconate production, and succination, driven by fumarate accumulation. Although both involve cysteine engagement, they differ in chemical stability, metabolite source, cellular distribution, glutathione competition, and functional outcome. Itaconation is linked to Kelch-like ECH-associated protein 1 (KEAP1)-nuclear factor erythroid 2-related factor 2 (NRF2) signalling, glycolytic control, kinase regulation, innate immune sensing, interferon responses, and inflammatory cell death, whereas succination is most strongly associated with stable S-(2-succino)cysteine (2SC) formation, fumarate excess, mitochondrial dysfunction, redox stress, inflammasome regulation, and pyroptosis. A central theme of this review is that endogenous itaconate and fumarate must be distinguished from electrophilic derivatives such as 4-octyl itaconate, dimethyl itaconate, and dimethyl fumarate, because these compounds differ in uptake, reactivity, target engagement, and pharmacological interpretation. We distinguish direct adduct mass spectrometry from competitive cysteine profiling and metabolite-mapping approaches, which can quantify bulk adduct burden, relative cysteine engagement, metabolite distribution, or compartment-specific cysteine state, but do not necessarily establish endogenous site occupancy. Proteomic and chemoproteomic approaches can prioritise metabolite-responsive cysteines, but functional relevance requires site-level validation, residue perturbation, and biochemical rescue. By comparing itaconation and succination across chemistry, target selection, inflammatory signalling, and therapeutic translation, this review defines the evidence needed to identify metabolite-sensitive cysteines as genuine regulatory nodes rather than detectable covalent adducts.