Auranofin drug interacting with thioredoxin reductase, computer-generated molecular model. Thioredoxin reductase is an enzyme which catalyses the reduction of thioredoxin in the thioredoxin (Trx) system. The Trx system exists in all living cells and is essential for cell growth and survival, and cellular redox (reduction and oxidation) balance. The anti-arthritic gold-containing drug Auranofin is lethal to the protozoan intestinal parasite Entamoeba histolytica, the causative agent of amoebic dysentery. A possible mechanism is the binding of the drug at the active site of thioredoxin reductase, interfering with redox homeostasis in the parasite. Labels indicate molecules of Auranofin, and cofactors FAD (flavin adenine dinucleotide) and NADPH (Nicotinamide adenine dinucleotide phosphate).