Hepatic Sulfite Oxidase

Abstract Sulfite oxidase (EC 1.8.3.1), when reduced by sulfite, exhibited the electron paramagnetic resonance spectrum of molybdenum(V). Specific colorimetric analyses demonstrated that the purified enzyme contained 1 molybdenum per heme. The electron paramagnetic resonance signal of molybdenum, which was generated by the addition of sulfite, was enriched by the purification procedure to the same degree as was the sulfite-reducible heme and the sulfite oxidase activity. Incubation of the enzyme with 5 x 10-3 m arsenite produced parallel losses of the sulfite-inducible molybdenum signal and the catalytic activity. Incubation of the enzyme with cyanide in the presence of sulfite resulted in slow, progressive, and irreversible inactivation. This treatment also caused a concomitant and proportional loss of the molybdenum signal. Cyanide had no effect on the absorption spectrum of the enzyme in the visible or near ultraviolet regions of the spectrum. Whereas the heme prosthetic group of the enzyme was oxidized in the steady state achieved by the simultaneous presence of sulfite and ferricyanide, the molybdenum component was in the reduced (pentavalent) state. Examination of the molybdenum signal as a function of pH, showed a shift in the field strength of maximum energy absorption, and a change in the shape of the signal. The electron paramagnetic resonance spectra of the enzyme in D2O at pD 7.5 and at pD 9.6 demonstrated that the environment of the unpaired electron was axially symmetrical at the lower pD but was asymmetrical at the more alkaline pD.