FIGURE 5.

Characterization of the SQR-sulfite CT complex. A, a CT complex is formed upon the addition of 400 μm sulfite to 20 μm SQR and is resolved in the presence of 600 μm sulfide. The final spectrum is that of the reduced flavin (because sulfide is in excess) as observed previously (18). B, titration of 19 μm SQR with sulfite (0–3 mm) in 50 mm Tris-Cl buffer, pH 8.0, containing 0.03% DHPC (n = 3). The starting and final spectra are shown in black. C, the dependence of the CT absorbance change (average of three experiments) on sulfite concentration. D, DTNB (0.33 mm) treatment of the CT complex formed in A, after washing to remove excess sulfite, leads to the disappearance of CT complex, suggesting that a thiol in SQR participates in the CT complex. The decrease in absorbance of the “washed” CT complex likely resulted from a partial reversal to the oxidized protein during the handling step. Note that spectrophotometer was blanked with a DTNB solution, which itself absorbs in the visible region (400–500 nm).