Bovine milk xanthine oxidase (XOD, E.C. 1.17.3.2) was covalently immobilized, via glutaraldehyde, on magnetic polysiloxane-polyvinyl alcohol (mPOS-PVA) particles yielding a preparation containing 9.5 +/-0.5 mu g of protein per mg of support and specific activity of 36.3 +/-7.8 mU/mg of protein (55.0 +/- 11.7% of the free enzyme). Optimal pH (8.8) and temperature (60 degrees C) were slightly higher than those established for the free enzyme (8.2 and 55 degrees C, respectively). No decrease of activity was observed after five reuses and only 17% was lost at the tenth reuse. The apparent Michaelis constant estimated for the mPOS-PVA-XOD (8.86 +/- 0.88 mu M) was not statistically different from the free enzyme (7.48 +/- 1.01 mu M). The 6-mercaptopurine oxidation catalyzed by the mPOS-PVA-XOD followed the same pathway described for the free enzyme, namely, 6-mercaptopurine -> 6-mercapto-8-hydroxypurine -> 6-thiouric acid, and no 6-thioxanthine was formed. (C) 2011 Published by Elsevier B.V.