Modifications of redox equilibria with semiquinone stabilization upon pyruvate binding to L-lactate cytochrome c oxidoreductase (flavocytochrome b2)

Spectral redox titrations of flavin and cytochrome b₂ moieties of flavocytochrome b₂ were achieved in the absence and in the presence of pyruvate under equilibrium conditions at 18° C; direct measurements of spin flavosemiquinone proportions have been carried out by EPR determinations at the same temperature. Our results show that the equilibria involving flavin are largely affected by the presence of pyruvate; the semiquinone proportion markedly increases almost till unit near half-reduction of cytochrome b₂; at 10 mM pyruvate, the dismutation constant, $\text{K}{}_{\mathrm{dism}}\text{=}\text{(}\text{F}{}_{\mathrm{s}}\text{)}{}^2\text{(}\text{F}{}_{\mathrm{o}}\text{)}{}^1\text{(}\text{F}{}_{\mathrm{r}}\text{)}$ increases by a factor ≥ 10.