Characterization of superoxide production sites in isolated rat brain and skeletal muscle mitochondria.

In this report, we have quantified the superoxide and H(2)O(2) production rates of intact rat brain and skeletal muscle mitochondria under condition of oxygen saturation applying p-hydroxyphenylacetate as fluorescent probe for H(2)O(2) generation and hydroethidine as probe for superoxide formation. The localisation of superoxide producing sites was determined by evaluating the effects of SOD addition. At comparable respiration rates and functional quality of mitochondria, we detected in brain mitochondria, a high reversed electron flow-dependent H(2)O(2) generation while the bc(1)-complex-dependent H(2)O(2) generation in the presence of succinate+antimycin was low. On the other hand, the reversed electron flow-dependent superoxide generation rate was small while the bc(1)-complex-dependent superoxide production was considerable. In contrast, isolated skeletal muscle mitochondria of comparable quality showed at almost comparable reversed electron flow-dependent H(2)O(2) generation more than 10-fold higher bc(1)-complex-dependent H(2)O(2) generation. Our data are compatible with the following suppositions: (i) The major ROS generation site in complex I visible during reversed electron flow (very likely the FMN moiety) is liberating superoxide predominantly to the mitochondrial matrix space. (ii) Similarly, the bc(1)-complex-dependent superoxide generation site (the semiquinone at center 'o') liberates superoxide with preference to the cytosolic space and (iii) Muscle mitochondria, most probably due to their higher endogenous CoQ content, generate at comparable maximal rates of respiration considerable larger amounts of superoxide at center 'o' of complex III.