The formation of sulfur trioxide radical anion during the prostaglandin hydroperoxidase-catalyzed oxidation of bisulfite (hydrated sulfur dioxide).

The mechanism of prostaglandin synthase-dependent (bi)sulfite (hydrated sulfur dioxide) oxidation was investigated using an enzyme preparation derived from ram seminal vesicles. The horseradish peroxidase-catalyzed oxidation of (bi)sulfite was used as a model system. Incubation of (bi)sulfite with prostaglandin synthase and arachidonic acid, 15-hydroperoxyarachidonic acid, or H2O2 results in the formation of the reactive sulfur trioxide anion radical (SO3(-)). The horseradish peroxidase/H2O2 system also oxidizes (bi)sulfite to SO3(-). This free radical reacts with oxygen resulting in oxygen consumption by these incubations. The free radical was detected with the indirect electron spin resonance technique of spin trapping. The SO3(-) radical adduct formed by the reaction of SO3(-) with the spin trap, 5,5-dimethyl-1-pyrroline-N-oxide, gives a nitroxide free radical with a nearly unique electron spin resonance spectrum (aH = 16.0 G and aN = 14.7 G). Using the spin-trapping technique, the SO3(-) could be detected even in incubations of guinea pig lung microsomes. When arachidonic acid-derived prostaglandin G2 was the source of hydroperoxide, formation of SO3(-) could be inhibited by indomethacin. When 15-hydroperoxyarachidonic acid or hydrogen peroxide was used to drive the enzymatic oxidation of (bi)sulfite, indomethacin had no effect. This hydroperoxidase activity was not nearly as heat-labile as the cyclo-oxygenase reaction which forms prostaglandin G2. Finally, the peroxidatic oxidation of (bi)sulfite may occur in vivo in competition with the mitochondrial sulfite oxidase, which oxidizes (bi)sulfite to sulfate without the formation of free radicals.