Thermo Scientific DTT (DL-Dithiothreitol; Clelands reagent) is used to stabilize enzymes and other proteins, which possess free sulfhydryl groups. It has been shown to restore activity lost by oxidation of these groups in vitro. DTT quantitatively reduces disulfide bonds and maintains monothiols in a reduced state (see Reference 1).
At a final 0.1 M concentration, DTT is also widely used for disruption of protein disulfide bonds in SDS-polyacrylamide gel electrophoresis.