Purines oxidation by immobilized xanthine oxidase on magnetic polysiloxane–polyvinyl alcohol composite

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 μg 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 °C) were slightly higher than those established for the free enzyme (8.2 and 55 °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 μM) was not statistically different from the free enzyme (7.48 ± 1.01 μM). 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.

Figure optionsDownload high-quality image (104 K)Download as PowerPoint slideHighlights
► High specific activity retention (half of the free enzyme).
► Optimal pH and temperature slightly higher than those of the free enzyme.
► No decrease of activity after five reuses and only 17% after ten reuses.
► Apparent Michaelis constant equal to that estimated for free enzyme.
► 6-Mercaptopurine oxidation following the same pathway described for the free enzyme.