Purification, crystallization and preliminary X-ray diffraction studies ofxanthine dehydrogenase and xanthine oxidase isolated from bovine milk

Xanthine dehydrogenase catalyzes the oxidation of hypoxanthine to xanthine and the further oxidation of xanthine to uric acid. The enzyme is the targe t of the anti-gout drug allopurinol and its involvement in postischemic rep erfusion injury is presently being defined. Each subunit of the homodimeric 290 kDa enzyme contains four cofactors: one Mo-pterin, two [2Fe-2S] cluste rs and one FAD. Both the dehydrogenase (XDH) and the proteolytically modifi ed oxidase form (XO) of the enzyme from bovine milk have been crystallized. XO crystals belong to space group C222(1), with unit-cell parameters a = 1 16.3, b = 164.4, c = 153.2 Angstrom at room temperature and a = 117.8, b = 165.4, c = 154.5 Angstrom when flash-frozen. They allow data collection to 3.3 and 2.5 Angstrom, respectively. In addition, a data set was collected f rom frozen XDH crystals and processed to 2.1 Angstrom. These crystals belon g to space group C2, with unit-cell parameters a = 169.9, b = 124.8, c = 14 8.6 Angstrom, beta = 90.9 degrees. The unit-cell volumes and Matthews param eters are similar for the two crystal forms. There is one monomer per asymm etric unit in the XO crystals and a complete native dimer per asymmetric un it in the XDH crystals.