Thioredoxin Reductase 1 (TXNRD1) is a selenium-containing enzyme part of the thioredoxin system responsible for regulating oxidative stress and redox signaling via reduction of disulfide bonds. The dimer, Thioredoxin Reductase 1, reduces thioredoxin and other substrates using NADPH and an FAD cofactor. Thioredoxin Reductase 1 involvement in protecting against oxidative stress and injury, regulation of cellular development and growth, and various other cellular processes make it an interesting target for studies of various cancers, AIDS, and other diseases. Humans express three different isozymes of thioredoxin reductase, isoform 1 is the cytosolic form, isoform 2 is the mitochondrial form, and isoform 3 is a testes specific form.

The enzyme activity is determined by following the NADPH-assisted reduction of 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) to 5-thio-2-nitrobenzoic acid (TNB) which leads to increased absorbance at 412 nm. Two moles of TNB are formed from the oxidation of one mole of NADPH according to the reaction shown below.

DTNB + NADPH + H⁺ ↔ 2 TNB + NADP⁺

The molar extinction coefficient of TNB is 14.15 X 10³ mM^-1cm^-1. Thioredoxin Reductase 1 activity is controlled by enzyme amount. An antibody specific to isoform 1 of thioredoxin reductase is used to isolate the enzyme from a bulk protein source for isoform-specific enzymatic activity measurements. Because the activity measured is specific to the isolated enzyme, a thioredoxin reductase inhibitor is not required for background signal subtraction.