Reduction of Methemoglobin by Cobaltocytochrome c Catalyzed by Mediators

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Abstract
The reduction of [human] methemoglobin [metHb] by cobaltocytochrome c (Cocyt c) was measured using 9 mediators of different half-reduction potentials, Em,7. he rate increased with the increase of Em,7 for the mediator but drops precipitously when it becomes more positive than the Em,7 for the metHb/Hb couple. The reaction is most efficient with phenazine methosulfate and was therefore studied in detail. The reaction is first order in the concentrations of Cocyt c and phenazine methosulfate. The average second-order rate constant for Cocyt c + phenazine methosulfate (M) .**GRAPHIC**. Cocyt c+ + .**GRAPHIC**. is 2.9 .times. 104 M-1 s-1 at 25.degree. C, 0.1 M phosphate pH 7.0. There is a slight negative temperature dependence of k1 at low temperature; at higher temperatures the process has .**GRAPHIC**. .apprxeq. 27 kJ mol-1 and .**GRAPHIC**. .apprxeq. -75 J mol-1 K-1. The effect of anions reflects the dependence of Em,7 for the metHb/Hb couple with various anions. There is no significant effect on k1 by the addition of inositol hexakisphosphate. The variation of k1 with pH is complicated. The experimental rate constants are compared with values calculated with the theory of nonadiabatic multiphonon process of electron tunneling.