Demonstration that histidine-25, but not histidine-132, is the axial heme ligand in rat heme oxygenase-1

Mariko Itomaki, Kazunobu Ishikawa, Kathryn Mansfield Matera, Michihiko Sato, Masao Ikeda-saito, Tadashi Yoshida

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71 Citations (Scopus)


A truncated, soluble rat heme oxygenase-1 lacking its C-terminal, membrane-anchoring segment, and its His25 → Ala and His132 → Ala mutants have been prepared by site-directed mutagenesis and expression in Escherichia coli. We found that wild-type enzyme can degrade heme to biliverdin, but its specific activity was about one-fifth that of the native, full-length enzyme, suggesting that the C-terminal segment is important for accepting electrons from NADPH cytochrome P450 reductase. His132 → Ala mutant had an enzyme activity comparable to that of the wild-type enzyme; hence, the highly conserved His132 is not essential for the display of the heme oxygenase activity. In contrast, His25 → Ala mutation completely abolished the enzyme′s catalytic activity. A five-coordinate type ferrous NO EPR spectrum was observed for the heme-heme oxygenase H25A complex. Hence, we conclude that His25 is the proximal axial ligand of the heme iron and is essential for the heme degradation activity of the enzyme.

Original languageEnglish
Pages (from-to)253-258
Number of pages6
JournalArchives of Biochemistry and Biophysics
Issue number1
Publication statusPublished - 1995 Feb 20


  • BiJiverdin
  • EPR
  • Heme
  • Heme oxygenase
  • Hemoprotein
  • O activation
  • P450 reductase

ASJC Scopus subject areas

  • Biophysics
  • Biochemistry
  • Molecular Biology


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