Fig. 4.

Top: room temperature EPR spectra of the sulfite radical adduct of DMPO, DMPO-${\mathrm{SO}}_3^{-}$. All the reactions were performed in 50 mM phosphate buffer (pH=7.4) containing 0.1 mM DTPA. Spectrum A: DMPO (0.1 M), Fe³⁺cyt c (0.1 mM), sulfite (0.05 mM), and H₂O₂ (0.1 mM). Spectrum B: DMPO (0.1 M), Fe³⁺cyt c (0.1 mM), sulfite (0.1 mM), and H₂O₂ (0.1 mM). Spectrum C: DMPO (0.1 M), Fe³⁺cyt c (0.1 mM), sulfite (0.25 mM), and H₂O₂ (0.1 mM). Spectrum D: DMPO (0.1 M), Fe³⁺cyt c (0.1 mM), sulfite (0.5 mM), and H₂O₂ (0.1 mM). EPR instrument parameters used were as described in the Materials and Methods section. Each EPR spectrum is a single scan. Bottom: Plot of initial rate of formation of DMPO-${\mathrm{SO}}_3^{-}$ versus sulfite concentration. Rates were obtained from the initial slope of the formation of DMPO-${\mathrm{SO}}_3^{-}$. EPR spectra were quantified by computer simulation of the spectra and comparison of the double integral of the observed signal with that of a TEMPO standard (1 μM) measured under the identical conditions. Data represent means±SE (n=3).