THE MOLYBDENUM CENTERS OF XANTHINE-OXIDASE AND XANTHINE DEHYDROGENASE- DETERMINATION OF THE SPECTRAL CHANGE ASSOCIATED WITH REDUCTION FROMTHE MO(VI) TO THE MO(IV) STATE

The UV-visible absorbance change associated with reduction of the moly bdenum centers of xanthine oxidase and xanthine dehydrogenase has been determined using a double-difference technique, At pH 8.5, the Mo(VI) minus Mo(IV) difference spectrum seen with xanthine oxidase exhibits a positive feature at 420 nm, having an extinction change of similar t o 3,000 M(-1) cm(-1) as well as evidence for a negative feature below 340 nm, In xanthine oxidase this change is found to exhibit a marked p H dependence, implicating protonation/deprotonation events associated with changes in the molybdenum oxidation state, Application of the dou ble-difference protocol to the respective circular dichroism spectra o f xanthine oxidase and xanthine dehydrogenase reveals appreciable CD c hanges at 420 and 580 nm associated with the reduction of the molybden um center. The present results demonstrate a direct spectroscopic hand le on the molybdenum centers of both xanthine oxidase and xanthine deh ydrogenase.