Steady-state redox behavior of cytochrome c, cytochrome a, and CuA of cytochrome c oxidase in intact rat liver mitochondria.

We have examined the steady-state redox behavior of cytochrome c (Fec), Fea, and CuA of cytochrome c oxidase during steady-state turnover in intact rat liver mitochondria under coupled and uncoupled conditions. Ascorbate was used as the reductant and TMPD (N,N,N',N'-tetramethyl-1,4-phenylenediamine) as the redox mediator. After elimination of spectroscopic interference from the oxidized form of TMPD, we found that Fea remains significantly more oxidized than previously thought. During coupled turnover, CuA always appears to be close to redox equilibrium with Fec. By increasing the amount of TMPD, both centers can be driven to fairly high levels of reduction while Fea remains relatively oxidized. The reduction level at Fea is close to a linear function of the enzyme turnover rate, but the levels at Fec and CuA do not keep pace with enzyme turnover. This behavior can be explained in terms of a redox equilibrium among Fec, CuA, and Fea, where Fea is the electron donor to the oxygen reduction site, but only if Fea has an effective Em (redox midpoint potential) of 195 mV. This is too low to be accounted for on the basis of nonturnover measurements and the effects of the membrane potential. However, if there is no equilibrium, the internal CuA----Fea electron-transfer rate constant must be slow in the time average (about 200 s-1). Other factors which might contribute to such a low Em are discussed. In the presence of uncoupler, this situation changes dramatically. Both Fec and CuA are much less reduced; within the resolution of our measurements (about 10%), we were unable to measure any reduction of CuA. Fea and CuA remain too oxidized to be in redox equilibrium with Fec during steady-state turnover. Furthermore, our results indicate that, in the uncoupled system, the (time-averaged) internal electron-transfer rate constants in cytochrome oxidase must be of the order of 2500 s-1 or higher. When turnover is slowed by azide, the relative redox levels at Fea and Fec are much closer to those predicted from nonturnover measurements. In presence of uncouplers, Fea is always more reduced than Fec, but in the absence of uncouplers, the two centers track together. Unlike the uninhibited, coupled system, the redox behavior here is consistent with the known effect of the electrical membrane potential on electron distribution in the enzyme. Interestingly, in these circumstances (azide and uncoupler present), Fea behaves as if it were no longer the kinetically controlling electron donor to the bimetallic center.