Characterization of protein targets of mammalian thioredoxin reductases

Mammalian thioredoxin reductases (TRs) are members of the pyridine nucleotide-disulfide oxidoreductase family. The main function of these enzymes is to maintain thioredoxins (Trxs) in the reduced state. The accessibility and high reactivity of selenocysteine in the C-terminal tetrapeptide allows mammalian TRs to couple to a range of substrates from proteins, such as Trx, to small molecules, such as selenite and hydroperoxides. However, identification of physiological substrates of TRs remains a challenge, with new targets identified primarily by testing random candidates in in vitro assays. The reaction mechanism of TRs supports a procedure that could trap substrates in a covalent nonproductive complex with TRs. Accordingly, attachment of TRs to affinity matrices allows isolation and identification of these targets. Application of this method revealed that Trxs are the major targets of TRs and supported efficient isolation of Trx substrates on TR affinity columns. We suggest that this procedure may be used as a general method of affinity isolation of Trxs and other TR substrates.