Abstract

Bovine milk xanthine oxidase (xanthine:oxidase oxygen oxidoreductase, EC 1.2.3.2) utilizes anthracycline antibiotics including daunorubicin and adriamycin in its electron transfer system both aerobically and anaerobically and generates drug semiquinone free radicals. Under anaerobic conditions and in the presence of several electron donors, xanthine oxidase metabolizes daunorubicin and adriamycin. The resultant metabolite from daunorubicin was identified as 7-deoxydaunorubicin aglycone by chromatography and mass spectrometry. NADH is the preferred electron donor for the cleavage reaction, although xanthine, purine, and glycolaldehyde also function. The cleavage reaction has the highest activity in phosphate buffer with a wide optimal pH range. With NADH as electron donor the concentration that gave 50% of the maximum cleavage rate is 250 µM for daunorubicin. Oxygen consumption by xanthine oxidase was stimulated by anthracyclines when NADH and NADPH were the electron donors, but it was inhibited if other electron donors were used. Both cleavage reaction and oxygen uptake are inhibited by allopurinol. Daunorubicin semiquinone free radical (g value, 2.0035 ± 0.0001) was detected by electron paramagnetic resonance after a lag period when xanthine oxidase was incubated with NADH or xanthine. The same free radical signal was detected without the lag period when the system was kept anaerobic. This lag period is precisely the time required for the depletion of oxygen as indicated by measuring the oxygen uptake. We propose that the semiquinone free radical form is an intermediate for the reductive glycosidic cleavage of anthracycline.