Fig. 5.

Monitoring intra-protein disulfides by AMS addition. 4′-acetamido-4′-maleimidylstilbene-2,2′-disulfonic acid (AMS) reacts with reduced protein thiols resulting in protein-AMS adducts which increases the mass by ~0.5 kDa. This additional molecular weight leads to a molecular shift on SDS-PAGE, generating an additional upper band on a gel. In contrast, if a protein thiol is oxidized it will not react with AMS and so no shift will be observed.