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

Abstract

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+ .