N-Ethylmaleimide (NEM) is a chemical derivative of maleic acid imide or maleic acid. It is a thiol reactive compound commonly used to modify cystine residues in proteins and peptides.^[1] It can be used to irreversibly inhibit the formation of cystine linking in proteins, whereas bifunctional analogues such as p-NN'-phenylenebismaleimide can be used as cross-linking reagent for cystine residues.^[2] While reaction with thiols takes place in the pH range 6.5–7.5, NEM may react with amines or undergo hydrolysis at a more alkaline pH.^[3]

It has been observed that after addition of NEM vesicular transport was blocked. After identification of the responsible protein it was called N-ethylmaleimide-sensitive factor (NSF). In lysis buffers, 20 to 25 mM of NEM is used to inhibit de-sumoylation of proteins for Western Blot analysis. NEM is also used as an inhibitor of deubiquitinases.

N-Ethylmaleimide was used by Arthur Kornberg and colleagues to knock out DNA polymerase III in order to compare its activity to that of DNA polymerase I (pol III and I, respectively). Kornberg had been awarded the Nobel Prize for discovering pol I, then believed to be the mechanism of bacterial DNA replication, though in this experiment he showed that pol III was the actual replicative machinery.

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