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Abstract The kinetics of the reduction by dithionite of several biochemically interesting substances has been investigated under anaerobic conditions by stopped flow spectrophotometry at pH 8.0 and 25°. Solutions of sodium dithionite were prepared and standardized by titration against lumiflavin 3-acetate in the absence of oxygen. Under pseudo-first order conditions (dithionite in excess) the reactions of spinach ferredoxin, horse metmyoglobin, lumiflavin 3-acetate, horse heart ferricytochrome c, and spinach plastocyanin conform to first order kinetics for several half-lives. Dependence of the observed pseudo-first order rate constant (kobs) on the dithionite concentration is described by the equation, kobs = aS2O42-½, where a = 8.6, 100, and 960 m-½ s-1 for ferredoxin, metmyoglobin, and lumiflavin 3-acetate, respectively. These results support a mechanism involving SO2- as the kinetically important reducing species. With cytochome c and plastocyanin, the dependence of kobs on dithionite concentration is described by the equation, kobs = aS2O42-½ + bS2O42-, where a and b are 1450 m-½ s-1 and 1.5 x 104 m-1 s-1 in the case of cytochrome c and 1100 m-½ s-1 and 1.35 x 105 m-1 s-1 in the case of plastocyanin. These two proteins apparently react with both SO2- and S2O42-. Analysis of the reaction of cytochrome c with dithionite in the presence of oxygen gave an estimated rate constant of 47 m-½ s-1 for the oxygen-dithionite reaction. The rate constant of the reaction of ferricyanide with S2O42- is 1.4 x 105 m-1 s-1. Evidence is presented that ferricyanide also reacts rapidly with SO2-. When dithionite reacts with excess ferricytochome c, lumiflavin 3-acetate, or oxygen, the reaction rate is virtually independent of the oxidant concentration and a limiting first order rate constant of 1.7 s-1 is approached in each case. This rate constant has been assigned to the monomerization rate of S2O42-, and its temperature dependence gives an activation energy of 24.1 Cal per mole for the dissociation of S2O42-. Determination of the equilibrium constant (K) for the reaction S2O42- ⇌ 2SO2- by electron paramagnetic resonance spectroscopy has given a value of 1.4 ± 0.4 x 10-9 m. Dividing the above listed values of a by K½ gives the absolute rate constants for the reactions between SO2- and oxidants.
Lambeth et al. (Sat,) studied this question.