Figure 4.

Sodium dithionite titration of rat SeCys⁴⁹⁸-Cys TrxR. Enzyme (21.2 nmol of subunits) in 0.6 ml of 0.1 M phosphate buffer/1 mM EDTA/pH 7.5 was titrated with 10 mM sodium dithionite dissolved in 50 mM sodium pyrophosphate buffer, pH 8.5, at 25°C in the presence of 150 nM methyl viologen. Spectra show oxidized enzyme (curve 1), the enzyme reduced with 0.94 equivalents (curve 2), 1.4 equivalents (curve 3), 1.9 equivalents (curve 4), and 2.83 equivalents of dithionite (curve 5). A slight artifact in the spectra at 360 nm is because of lamp change at this wavelength. Inset shows the effects of dithionite addition on the absorbances at 464 nm and at 540 nm.