FIGURE 7. Relationship between the rate of superoxide production by site III_Qo and the reduction state of cytochrome b₅₆₆.

(a) Dependence of the rate of myxothiazol-sensitive H₂O₂ production on succinate concentration at fixed succinate+malonate concentration in the presence of rotenone. Data were corrected for the contribution of site I_F (Fig. 6c and Fig. 5c). (b) Dependence of cytochrome b₅₆₆ reduction on succinate concentration in parallel incubations (100% reduction was subsequently established by addition of 2 μM Antimycin A). (c) Final calibration of the relationship between the rate of superoxide production from site III_Qo and cytochrome b₅₆₆ reduction state, obtained by combining panels (a) and (b). Filled symbols: control mitochondria; open symbols: control values after correction to CDNB-treated mitochondria using Eq. 1, assuming that 50% of superoxide from site III_Qo was produced in the matrix (see [26]). Where not visible, error bars are contained within the points. Lines show exponential relationships (for simplicity), fitted by non-linear regression to give the parameter values in Eq. 3. See MATERIALS AND METHODS. Data are means ± SEM (n = 9).