The oxidation-reduction properties of spinach thioredoxins f and m and of ferredoxin:thioredoxin reductase.

Oxidation-reduction midpoint potentials have been determined, using cyclic voltammetry, for the active-site disulfide/dithiol couples of spinach thioredoxins f and m and of spinach ferredoxin:thioredoxin reductase (FTR) and for a component likely to be the [4Fe-4S] cluster of FTR. Values for the midpoint potentials (n = 2) of -210 +/- 10 mV were determined for both thioredoxins f and m. Two redox centers were detected in FTR, with midpoint potential values of -230 +/- 10 mV (n = 2) and +340 +/- 30 mV, respectively. Alkylation of the active-site cysteines of FTR by treatment of the enzyme with N-ethylmaleimide (NEM) eliminates the component with the -230 mV midpoint potential, allowing one to assign this value to the active site disulfide/dithiol couple. Inasmuch as the only other electron-carrying center known to be present in FTR is the [4Fe-4S] cluster, it appears likely that the high-potential component can be attributed to this redox moiety. The midpoint potential value of the high-potential feature shifts slightly, to +380 +/- 20 mV, in the NEM-treated enzyme.