FIGURE 5.

Relationship between H₂O₂ production by complex II and reduction of cytochrome b₅₆₆. a, effect of malonate on H₂O₂ production rates in the absence of site III_Qo inhibitors. H₂O₂ production rates were measured in skeletal muscle mitochondria oxidizing glycerol 3-phosphate in the presence of 4 μm rotenone and 2.5 μm antimycin A (site III_Qi inhibitor) (black circles) and in parallel with the further addition of 1 mm malonate (white circles). Data are means only for clarity; S.E. values were similar to other panels (n = 3–5). b, using the conditions in a, the malonate-sensitive component of the H₂O₂ production rates were determined with rac-α/β-glycerol phosphate (25% active substrate sn-glycerol 3-phosphate, white circles, n = 3–5) or rac-α-glycerol phosphate (50% active substrate sn-glycerol 3-phosphate, black squares, n = 2–4). Data are means ± S.E. (error bars). The dashed box encompasses all points for glycerol phosphate ≥20 mm. c, H₂O₂ production rates from b replotted against cytochrome b₅₆₆ values under identical conditions from Fig. 1e and Fig. 2c. The peak H₂O₂ production rate from site II_F in the presence of the site III_Qo inhibitor myxothiazol is replotted from Fig. 3b (black circle). Data are means ± S.E. d, data from c replotted as a unified set omitting the data points for glycerol phosphate ≥20 mm (dashed box in c) and fit to an exponential curve of the equation, Rate of H₂O₂ production driven by electron flow from the Q-pool into complex II in pmol·min⁻¹·mg of protein⁻¹ = 8.0·e^{(0.045·%b566 reduction)}. Data are means ± S.E. Rot, rotenone; myx, myxothiazol; ant A, antimycin A.