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

J Biol Inorg Chem. 2018 Aug;23(6):849-860. doi: 10.1007/s00775-018-1581-5. Epub 2018 Jun 19.

Abstract

Cadmium (Cd2+) 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 Cd2+ 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 Cd2+. The structure includes two Cd2+ ions, one coordinated by active site residues and another near a cysteine residue. Cd2+ presumably inactivates IDH due to its high affinity for thiols, leading to a covalent enzyme modification. However, Cd2+ also activates IDH by providing a divalent cation required for catalytic activity. Inactivation of IDH by Cd2+ is less effective when the enzyme is activated with Cd2+ than Mg2+. Although reducing agents cannot restore activity following inactivation by Cd2+, they can maintain IDH activity by chelating Cd2+. Glutathione, a cellular sulphydryl reductant, has a moderate affinity for Cd2+, allowing IDH to be activated with residual Cd2+, unlike dithiothreitol, which has a much higher affinity. In the presence of Cd2+-consuming cellular antioxidants, cells must continually supply reductants to protect against oxidative stress. The ability of IDH to utilise Cd2+ to generate NADPH could allow cells to protect themselves against Cd2+.

Keywords: Cadmium; Crystal structure; Enzyme activity; Glutathione; NADP+-dependent cytosolic isocitrate dehydrogenase.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cadmium / toxicity*
  • Calorimetry
  • Chelating Agents / metabolism*
  • Crystallography, X-Ray
  • Cysteine / chemistry
  • Cytosol / enzymology*
  • Dithiothreitol / pharmacology
  • Enzyme Activation
  • Glutathione / metabolism*
  • Isocitrate Dehydrogenase / antagonists & inhibitors
  • Isocitrate Dehydrogenase / chemistry
  • Isocitrate Dehydrogenase / metabolism*
  • Mice
  • NADP / metabolism*
  • Protein Conformation
  • Spectrophotometry, Ultraviolet

Substances

  • Chelating Agents
  • Cadmium
  • NADP
  • Isocitrate Dehydrogenase
  • Glutathione
  • Cysteine
  • Dithiothreitol