The thioredoxin system, found in all living creatures, consists of thioredoxin protein (Trx), thioredoxin reductase enzyme (TrxR), and NADPH. In this study, turkey liver mitochondrial TrxR enzyme with 3.07 EU x mg(-1) specific activity was purified 990-fold in a yield of 2.05% using 2',5'-ADP Sepharose 4B affinity chromatography. The purity of the enzyme was measured and the molecular weight of its subunits was determined to be 45.5 kDa by SDSPAGE. The molecular mass of the enzyme's natural state was found to be 88 kDa using Sephadex-G 150 gel filtration chromatography. In addition, characteristic and kinetic properties of the enzyme were determined. Then the inhibitory effects of some heavy metal ions (Ag+, Fe (3+), cd (2+), Cu (2+), zn (2+), Pb (2+), Ni (2+), and Co (2+)) on the activity of TrxR enzyme were examined under in vitro conditions. IC50 values were found with the heavy metal concentration with which 50% of the activity of the TrxR enzyme was inhibited. Finally, K, values for these substances were calculated from the Lineweaver Burk plots. It was determined that Ag (+), Fe (3+), Cd (2+), Cu (2+), and Zn (2+) ions inhibited TrxR enzyme, Pb (2+) ion increased enzyme activity, and Ni (2+) and Co (2+) ions had no effect on enzyme activity.