NADP(+)-dependent cytosolic isocitrate dehydrogenase provides NADPH in the presence of cadmium due to the moderate chelating effect of glutathione

Cadmium (Cd(2+)) is toxic to living organisms because it causes the malfunction of essential proteins and induces oxidative stress. NADP(+)-dependent cytosolic isocitrate dehydrogenase (IDH) provides reducing energy to counteract oxidative stress via oxidative decarboxylation of isocitrate. Intriguingly, the effects of Cd(2+) on the activity of IDH are both positive and negative, and to understand the molecular basis, we determined the crystal structure of NADP(+)-dependent cytosolic IDH in the presence of Cd(2+). The structure includes two Cd(2+) ions, one coordinated by active site residues and another near a cysteine residue. Cd(2+) presumably inactivates IDH due to its high affinity for thiols, leading to a covalent enzyme modification. However, Cd(2+) also activates IDH by providing a divalent cation required for catalytic activity. Inactivation of IDH by Cd(2+) is less effective when the enzyme is activated with Cd(2+) than Mg(2+). Although reducing agents cannot restore activity following inactivation by Cd(2+), they can maintain IDH activity by chelating Cd(2+). Glutathione, a cellular sulphydryl reductant, has a moderate affinity for Cd(2+), allowing IDH to be activated with residual Cd(2+), unlike dithiothreitol, which has a much higher affinity. In the presence of Cd(2+)-consuming cellular antioxidants, cells must continually supply reductants to protect against oxidative stress. The ability of IDH to utilise Cd(2+) to generate NADPH could allow cells to protect themselves against Cd(2+).