Diphenylene Iodonium Is a Noncovalent MAO Inhibitor: A Biochemical and Structural Analysis

ChemMedChem. 2020 Aug 5;15(15):1394-1397. doi: 10.1002/cmdc.202000264. Epub 2020 Jun 10.


Diphenylene iodonium (DPI) is known for its inhibitory activities against many flavin- and heme-dependent enzymes, and is often used as an NADPH oxidase inhibitor. We probed the efficacy of DPI on two well-known drug targets, the human monoamine oxidases MAO A and B. UV-visible spectrophotometry and steady-state kinetics experiments demonstrate that DPI acts as a competitive and reversible MAO inhibitor with Ki values of 1.7 and 0.3 μM for MAO A and MAO B, respectively. Elucidation of the crystal structure of human MAO B bound to the inhibitor revealed that DPI binds deeply in the active-site cavity to establish multiple hydrophobic interactions with the surrounding side chains and the flavin. These data prove that DPI is a genuine MAO inhibitor and that the inhibition mechanism does not involve a reaction with the reduced flavin. This binding and inhibitory activity against the MAOs, two major reactive oxygen species (ROS)-producing enzymes, will have to be carefully considered when interpreting experiments that rely on DPI for target validation and chemical biology studies on ROS functions.

Keywords: diphenylene iodonium; flavoenzyme; iodonium compounds; monoamine oxidases; reactive oxygen species.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Dose-Response Relationship, Drug
  • Humans
  • Molecular Structure
  • Monoamine Oxidase / metabolism*
  • Monoamine Oxidase Inhibitors / chemistry
  • Monoamine Oxidase Inhibitors / pharmacology*
  • Onium Compounds
  • Structure-Activity Relationship


  • Monoamine Oxidase Inhibitors
  • Onium Compounds
  • diphenyleneiodonium
  • Monoamine Oxidase