Electron paramagnetic resonance properties and oxidation-reduction potentials of the molybdenum, flavin, and iron-sulfur centers of chicken liver xanthine dehydrogenase

Abstract

The oxidation-reduction potentials of the various prosthetic groups in the native and desulfo forms of chicken liver xanthine dehydrogenase, determined by potentiometric titration in 0.05 m potassium phosphate buffer, pH 7.8, are: Mo(VI)/Mo(V) (native), −357 mV; Mo(VI)/Mo(V) (desulfo), −397 mV; Mo(V)/Mo(IV) (native), −337 mV; Mo(V)/Mo(IV) (desulfo), −433 mV; FAD/FADH · −345 mV; FADH · FADH 2, − 377 mV; (Fe/S)I ox/(Fe/S)I red, −280 mV; (Fe/S)II ox/(Fe/S)II red, − 275 mV. Titration at pH 6.8 revealed that the Mo and FAD centers but not the Fe/S centers are in prototropic equilibrium. Spectroscopic studies on the native and deflavinated enzymes show that environment of the flavin in xanthine dehydrogenase differs from that in bovine milk xanthine oxidase.