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⁺ .