The effect of calcium ions has been studied on three different isoforms of thioredoxin reductase. The cytosolic (TrxR1), mitochondrial (TrxR2), and the Escherichia coli enzymes were examined and compared. In our condition, TrxR1 appears extremely sensitive to Ca2+ showing an IC50 of about 160 nM, while Ca2+ exerts only a weak inhibitory effect on the mitochondrial isoform. The thioredoxin reductase purified from E coli is almost completely insensitive to calcium ions. Circular dichroism analysis of highly Purified mitochondrial and cytosolic thioredoxin reductases reveals that Ca2+ induces conformational alterations that are particularly relevant only in the cytosolic isoform. These observations are discussed with reference to the physiological role and, in particular, to the regulatory functions of the thioredoxin system. (c) 2006 Elsevier Inc. All rights reserved.