Reverse Electron Transfer is a More Dominant Source of Mitochondrial ROS Production in the Heart and Kidney Outer Medulla than in the Kidney Cortex
Authors: Sadri Shima , Tomar Namrata , Zhang Xiao , Yang Chun , Audi Said H. , Cowley Allen W. , Dash Ranjan K. 
Published: May 2022

Rationale

Reactive oxygen species (ROS; e.g. O2·– and H2O2) play important roles in both physiological and pathophysiological processes. ROS in low concentrations contribute to physiological processes, such as cellular redox signaling and phagocytosis, whereas ROS in high concentrations are toxic to the cell causing tissue injury contributing to the pathogenesis of cardiovascular and chronic renal diseases, including salt‐sensitive hypertension. Mitochondria, which produce ROS as byproducts of aerobic respiration via both forward electron transfer (FET) and reverse electron transfer (RET) are known to be one of the major cellular sources of ROS. Although it is recognized that the RET mechanism in which electrons flow back from complex II to complex I contribute significantly to ROS production in cardiac mitochondria, the mechanisms of ROS production and the role of RET in kidney mitochondria has remained poorly understood.

Method

We evaluated the relative contributions of FET and RET towards overall ROS production in mitochondria isolated from the heart and kidney cortex and outer medulla (OM) of adult Sprague‐Dawley rats. H2O2 emission was measured by a spectrofluorometer in isolated mitochondria in the presence of either Succinate (Suc) simulating RET or succinate+rotenone (Suc+Rot) simulating FET. Furthermore, we measured mitochondrial rates of H2O2 production along with respiration and membrane potential under three respiratory states namely (i) leak state (state 2; after substrate addition), (ii) oxidative phosphorylation (OxPhos) state (state 3; after ADP addition), and (iii) maximum respiratory state (state 5; after the addition of the uncoupler FCCP).

Results

It was found that mitochondria isolated from the heart and kidney cortex produced the least and the most ROS, respectively. The rate of ROS production in the presence of Suc+Rot compared to Suc alone decreased significantly in the heart and to a lesser extent in OM, indicating significant contribution of RET to overall ROS production in the heart and slightly in the OM. In contrast, there was not significant difference in ROS production rates in the presence of Suc and Suc +Rot in mitochondria from kidney cortex, showing that RET is not predominant in the kidney cortex. Also, we observed significant reduction in the ROS production rate in state 4 compared to state 2 in the heart mitochondria compared to kidney cortex and OM. A possible explanation for these differential results is that oxaloacetate (OAA), produced by the tricarboxylic acid cycle, accumulates resulting in succinate dehydrogenase (SDH) inhibition more rapidly in the heart than in the kidney affecting mitochondrial ROS production, respiration, and bioenergetics.

Conclusion

RET mechanism contributes to mitochondrial ROS production significantly in the heart and slightly in the kidney OM, but not in the kidney cortex. OAA accumulation contributes to SDH inhibition significantly in the heart than in the kidney.