Significance Several functions in cells require reductive processes, i.e., the enzymatic catalysis of a transfer of electrons to specific cellular substrates. One major reductive system in the cytosol of human cells depends upon thioredoxin 1 (Trx1), which in turn is kept reduced by thioredoxin reductase 1 (TrxR1) using NADPH. In the present study it is shown that another protein in addition to Trx1, called thioredoxin-related protein of 14 kDa (TRP14), is highly efficient together with TrxR1 in catalyzing reduction of L-cystine or nitric oxide-derivatized cysteine residues. It is also shown that TRP14, in contrast to Trx1, is resistant to inactivation by hydrogen peroxide. These findings reveal that several TrxR1-dependent functions in cells may not be propelled solely by Trx1, but instead relate to activities of TRP14.