Using ferricenium hexafluorophosphate, we attempted to gen erate a radical on the pterin cofactor that bidentately coordinates molybdenum in xanthine oxidase. The resulting ferriceni uni-treated protein gave rise to an EPR signal previously not seen. The signal was distinctly Isotropie with gavg ~ 2.02. Generation of the signal was pH dependent with appreciable signa 1 generation occurring only at pH 10.0, however, once generated, the signal was stable to pH 6.O. The ferriceni urn-derived EPR signal showed no loss of proton hyperfine coupling upon exchange into DiO. Addition of 2-hydroxy-6-methy Ipurine (in the presence of catalase and Superoxide dismutase) generated the hfov "very rapid" EPR signal while preserving the ferriceniurn-derived EPR sig nal. Coupling of the two signals under a variety of conditions was not evident , although power saturation studies provided some indirect evidence for interac tion between the "very rapid" signal and the ferriceni urn-derived EPR signal. E vidence of coupling to either of the two 2Fe/2S centers or the flavin semiquino ne was not seen. Reaction of ferriceni urn-treated xanthine oxidase with xanthin e was studied by stopped-flow spectrophotometry and resulted in an unaltered fcrej. These observations lead us to interpret the new ferriceni urn-derived E PR signal of xanthine oxidase as that of a tyrosine radical.
|State||Published - 1996|