title: Formation of a porphyrin pi-cation radical in the fluoride complex of horseradish peroxidase.
abstract: Horseradish peroxidase (HRP) was oxidized by IrCl6(2-) to a mixture of compounds I and II, the rate of oxidation and the ratio of the mixture being greatly affected by pH (Hayashi & Yamazaki, 1979). Oxidation of HRP by IrCl6(2-) in the presence of fluoride was significantly accelerated. This resulted in the formation of a new compound which is a ferric fluoride complex containing a porphyrin pi-cation radical. The spectrum of the new compound showed a decreased absorption band in the Soret region and a broad band at 570 nm; which was converted to that of the original ferric fluoride complex by addition of ascorbate or hydroquinone. Addition of cyanide slowed down the oxidation of HRP by IrCl6(2-), and the oxidation product was the same as that obtained in the absence of cyanide. Compound I was formed when H2O2 was added to HRP in the presence of fluoride or cyanide. The one-electron reduction potential (Eo') of the oxidized HRP-fluoride complex was measured at several pH values, the Eo' value at pH 7 being 861 +/- 4 mV. The ratio of delta Eo' to delta pH was 49 mV/pH unit.
authors.affiliation: National Center for the Design of Molecular Function, Utah State University, Logan 84322-4630, USA.
authors.forename: Z S
authors.initials: ZS
authors.lastname: Farhangrazi
authors.forename: R
authors.initials: R
authors.lastname: Sinclair
authors.forename: L
authors.initials: L
authors.lastname: Powers
authors.forename: I
authors.initials: I
authors.lastname: Yamazaki
mesh_terms: D001205:Ascorbic Acid / Q000494:pharmacology; D005459:Fluorides / Q000378:metabolism*; D005609:Free Radicals; D006735:Horseradish Peroxidase / Q000737:chemistry / Q000378:metabolism*; D006861:Hydrogen Peroxide / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D006873:Hydroquinones / Q000494:pharmacology; D007495:Iridium / Q000737:chemistry; D010084:Oxidation-Reduction; D011166:Porphyrins / Q000737:chemistry / Q000378:metabolism*; D011190:Potassium Cyanide / Q000494:pharmacology; D012969:Sodium Fluoride / Q000494:pharmacology; D013053:Spectrophotometry
chemical_list: D005609:Free Radicals; D006873:Hydroquinones; D011166:Porphyrins; C010978:hexachloroiridic acid; D007495:Iridium; D012969:Sodium Fluoride; D006861:Hydrogen Peroxide; D006735:Horseradish Peroxidase; D011190:Potassium Cyanide; D001205:Ascorbic Acid; D005459:Fluorides; C031927:hydroquinone