The molybdoenzyme, xanthine oxidoreductase (XOR), has been much studied, largely because of its ready availability on a large scale from cows' milk [ l ] . It catalyses the oxidation of a wide range of substrates, most notably hypoxanthine and xanthine, generating xanthine and uric acid respectively, in the process of purine catabolism. The enzyme is a homodimer of 150 kDa subunits and exists i...