Xanthine oxidase/hydrogen peroxide generates sulfur trioxide anion radical (SO^.-₃) from sulfite (SO^2-₃)

In the presence of hydrogen peroxide (H₂O₂), xanthine oxidase has been found to catalyze sulfur trioxide anion radical (SO^.-₃) formation from sulfite anion (SO^2-₃). The SO^.-₃ radical was identified by ESR (electron spin resonance) spin trapping, utilizing 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) as the spin trap. Inactivated xanthine oxidase does not catalyze SO^.-₃ radical formation, implying a specific role for this enzyme. The initial rate of SO^.-₃ radical formation increases linearly with xanthine oxidase concentration. Together, these observations indicate that the SO^.-₃ generation occurs enzymatically. These results suggest a new property of xanthine oxidase and perhaps also a significant step in the mechanism of sulfite toxicity in cellular systems.