Preparation of bovine xanthine oxidase and the subunit structures of some iron flavoproteins.

A new preparative procedure for bovine milk xanthine oxidase avoids exposure of the enzyme to proteases. The enzyme so obtained exhibited a molecular weight of about 300,000 as did chicken liver xanthine dehydrogenase. The former readily yielded species with a molecular weight of 150,000 in guanidine and in acid but could not be resolved into smaller subunits. Aldehyde oxidase of rabbit liver exhibited a molecular weight of 275,000 and again deaggregated into molecules of about half that size in guanidine and in acid but was not further deaggregated. Dihydroorotic dehydrogenase has a molecular weight of 112,000 to 124,000 and deaggregated into 4 identical subunit molecules in guanidine solution. The major features of the optical rotatory dispersion spectra of xanthine oxidase were associated with the large absorption peaks due to its flavin and iron complexes and were abolished by the procedures which led to deaggregation into subunits.