N-Ethylmaleimide

N-Ethylmaleimide (NEM) is an electrophilic small molecule, used in biological research to alkylate and covalently modify nucleophilic thiol residues on proteins. Covalent modification of the active-site thiol residue of cysteine proteases ablates enzyme function. N-Ethylmaleimide also demonstrates inhibition of Mg²⁺-dependent internucleosomal DNA fragmentation, a defining occurrence of the apoptotic pathway. Other thiophilic species such as Hg²⁺ and Cd²⁺ inhibit fragmentation as well, indicating that a thiol moiety is crucial for this degradation to occur. NADP-dependent isocitrate dehydrogenase in yeast is disabled by N-Ethylmaleimide, showing alkylation of a cysteine residue followed by a slower alkylation of a lysine residue on the enzyme. N-Ethylmaleimide is an activator of PLA2.