Characterization of the proximal ligand in the P420 form of inducible nitric oxide synthase

Joseph Sabat, Dennis J. Stuehr, Syun Ru Yeh, Denis L. Rousseau

Research output: Contribution to journalArticlepeer-review

23 Scopus citations


The nitric oxide (NO) produced by inducible nitric oxide synthase (iNOS) up-regulates the expression of heme oxygenase (HO), which in turn produces carbon monoxide (CO) that down-regulates iNOS activity by reducing its expression level or by inhibiting its activity by converting it to an inactive P420 form (iNOSP420). Accordingly, CO has been considered as a potentially important attenuator of inflammation. Despite its importance, the nature of the proximal heme ligand of the iNOSP420 species remains elusive. Here we show that the 221 cm-1 mode of the photoproduct of iNOSP420 does not exhibit any H2O-D2O solvent isotope shift such as that found in the iron-histidine stretching mode of myoglobin, indicating that the proximal ligand of iNOSP420 is not a histidine. The νFe-CO and νC-O data reveal that the proximal heme ligand of iNOSP420 is consistent with a protonated thiol instead of a thiolate anion. Furthermore, the optical absorption properties of iNOSP420 are similar to those of a neutral thiol-heme model complex but not myoglobin. Together the data support the scenario that iNOSP420 is inactivated by protonation of the native proximal thiolate ligand to a neutral thiol, instead of by ligand switching to a histidine, as prior studies have suggested.

Original languageEnglish (US)
Pages (from-to)12186-12192
Number of pages7
JournalJournal of the American Chemical Society
Issue number34
StatePublished - Sep 2 2009

ASJC Scopus subject areas

  • Catalysis
  • General Chemistry
  • Biochemistry
  • Colloid and Surface Chemistry


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