title: Charge transfer complexes between pteridine substrates and the active center molybdenum of xanthine oxidase.
abstract: Anaerobic addition of lumazine (2,4-dihydroxypteridine) to xanthine oxidase leads to a rapid bleaching of the enzyme with concomitant formation of a long wave-length species with maximum absorbance at 650 nm. At the conclusion of the reaction there is a net increase in absorbance at wavelengths greater than 550 nm. A spectrally similar species is obtained when violapterin is added to dithionite-reduced enzyme. This long wavelength absorbance is not accompanied by an EPR signal characteristic of flavin neutral radical and it is produced equally well in flavin-free xanthine oxidase. Its production is eliminated by the inhibitors, cyanide and allopurinol, which react at the molybdenum center. We conclude that this new species is a charge transfer complex between Mo(IV) and the product of the reaction, violapterin (2,4,7-trihydroxypteridine). A similar, though less intense, absorbance at 650 nm is also obtained when lumazine is bound to the reduced enzyme. These Mo(IV)-pteridine charge transfer compounds are optically active and exhibit intense circular dichroism centered at 630 nm.
mesh_terms: D000693:Anaerobiosis; D000818:Animals; D001665:Binding Sites; D002417:Cattle; D004227:Dithionite / Q000494:pharmacology; D005260:Female; D007700:Kinetics; D008892:Milk / Q000201:enzymology; D008982:Molybdenum; D010084:Oxidation-Reduction; D011621:Pteridines; D013053:Spectrophotometry; D014969:Xanthine Oxidase / Q000378:metabolism*
chemical_list: D011621:Pteridines; D004227:Dithionite; D008982:Molybdenum; D014969:Xanthine Oxidase