title: Photoreactivation of alloxanthine-inhibited xanthine oxidase.
abstract: Alloxanthine-inhibited xanthine oxidase (XOD) was found to be photoreactivated by irradiation of light of wavelengths in the range of 340-430 nm. The enzyme activity can be fully controlled to be on or off by many dark-light cycles. Electron spin resonance measurement shows the appearance of the molybdenum (V) ion and the reduced form of flavin adenine dinucleotide (FADH.) radical signals after irradiation of the alloxanthine-XOD complex. Electronic-absorption spectrum also shows the bleaching of Fe/S and flavin adenine dinucleotide chromophores at 375 and 450 nm as well as broad-band absorption of FADH. in the range of 500-700 nm. The quantum yield of photoreactivation of the enzyme activity is approximately 0.06. A photoinduced intraenzyme electron-transfer model is proposed to rationalize the photoreactivation process.
authors.affiliation: Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30043, ROC.
authors.forename: L A
authors.initials: LA
authors.lastname: Tai
authors.forename: K C
authors.initials: KC
authors.lastname: Hwang
mesh_terms: D000493:Allopurinol / Q000378:metabolism; D004307:Dose-Response Relationship, Radiation; D004578:Electron Spin Resonance Spectroscopy; D004789:Enzyme Activation; D005182:Flavin-Adenine Dinucleotide / Q000378:metabolism; D007700:Kinetics; D008954:Models, Biological; D015394:Molecular Structure; D008982:Molybdenum; D010117:Oxypurinol / Q000378:metabolism / Q000494:pharmacology; D010777:Photochemistry; D019820:Xanthine / Q000378:metabolism; D014969:Xanthine Oxidase / Q000037:antagonists & inhibitors / Q000378:metabolism / Q000528:radiation effects*
chemical_list: D005182:Flavin-Adenine Dinucleotide; D019820:Xanthine; D000493:Allopurinol; D008982:Molybdenum; D014969:Xanthine Oxidase; D010117:Oxypurinol