title: Complexities in horseradish peroxidase-catalyzed oxidation of dihydroxyphenoxazine derivatives: appropriate ranges for pH values and hydrogen peroxide concentrations in quantitative analysis.
abstract: The highly sensitive, convenient fluorescence assay, based on the oxidation of nonfluorescent 10-acetyl-3,7-dihydroxyphenoxazine (Amplex Red) to highly fluorescent resorufin, is becoming increasingly popular for hydrogen peroxide quantitation. Yet, the intricacies of the horseradish peroxidase-catalyzed oxidation of the reductant substrate Amplex Red by hydrogen peroxide and the resulting resorufin could complicate the assay design and data interpretation. In particular, substrate inhibition and enzyme inactivation at higher hydrogen peroxide concentrations were known to affect the enzyme kinetics and end-point fluorescence. In addition, here we report the spontaneous transformation of resorufin to less or nonfluorescent product(s) in the absence of hydrogen peroxide and horseradish peroxidase. This spontaneous decay of resorufin fluorescence is most prominent in the pH range 6.2-7.7, likely due to general base-catalyzed de-N-acetylation and polymerization of resorufin. From a practical point of view, precautions for properly designing assays for hydrogen peroxide or characterizing hydrogen peroxide-generating systems are discussed based on the spontaneous transformation of resorufin to less fluorescent compound(s), substrate inhibition and enzyme inactivation at higher (>100 microM) hydrogen peroxide concentrations, and enzymatic oxidation of resorufin to nonfluorescent resazurin.
authors.affiliation: Department of Bioprocess and Bioanalytical Research, Merck Research Laboratories, West Point, PA 19486, USA.
authors.forename: Victoria
authors.initials: V
authors.lastname: Towne
authors.forename: Mark
authors.initials: M
authors.lastname: Will
authors.forename: Brent
authors.initials: B
authors.lastname: Oswald
authors.forename: Qinjian
authors.initials: Q
authors.lastname: Zhao
mesh_terms: D002384:Catalysis; D005456:Fluorescent Dyes / Q000737:chemistry / Q000378:metabolism; D006735:Horseradish Peroxidase / Q000378:metabolism*; D006861:Hydrogen Peroxide / Q000494:pharmacology*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D015394:Molecular Structure; D010078:Oxazines / Q000032:analysis* / Q000737:chemistry / Q000378:metabolism*; D010084:Oxidation-Reduction / Q000187:drug effects; D010619:Phenazines / Q000737:chemistry* / Q000378:metabolism*; D012015:Reference Standards; D013050:Spectrometry, Fluorescence
chemical_list: D005456:Fluorescent Dyes; D010078:Oxazines; D010619:Phenazines; C499011:dihydroxyphenoxazine; C470430:Amplex Red; C014180:resorufin; D006861:Hydrogen Peroxide; D006735:Horseradish Peroxidase