Teaching the fundamentals of electron transfer reactions in mitochondria and the production and detection of reactive oxygen species

摘要

Mitochondria fulfill a number of biological functions which inherently depend on ATP and O2^−•/H2O2 production. Both ATP and O2^−•/H2O2 are generated by electron transfer reactions. ATP is the product of oxidative phosphorylation whereas O2^−• is generated by singlet electron reduction of di-oxygen (O2). O2^−• is then rapidly dismutated by superoxide dismutase (SOD) producing H2O2. O2^−•/H2O2 were once viewed as unfortunately by-products of aerobic respiration. This characterization is fitting considering over production of O2^−•/H2O₂ by mitochondria is associated with range of pathological conditions and aging. However, O₂^−•/H₂O₂ are only dangerous in large quantities. If produced in a controlled fashion and maintained at a low concentration, cells can benefit greatly from the redox properties of O₂^−•/H₂O₂. Indeed, low rates of O₂^−•/H₂O₂ production are required for intrinsic mitochondrial signaling (e.g. modulation of mitochondrial processes) and communication with the rest of the cell. O₂^−•/H₂O₂ levels are kept in check by anti-oxidant defense systems that sequester O₂^−•/H₂O₂ with extreme efficiency. Given the importance of O₂^−•/H₂O₂ in cellular function, it is imperative to consider how mitochondria produce O₂^−•/H₂O₂ and how O₂^−•/H₂O₂ genesis is regulated in conjunction with fluctuations in nutritional and redox states. Here, I discuss the fundamentals of electron transfer reactions in mitochondria and emerging knowledge on the 11 potential sources of mitochondrial O₂^−•/H₂O₂ in tandem with their significance in contributing to overall O₂^−•/H₂O₂ emission in health and disease. The potential for classifying these different sites in isopotential groups, which is essentially defined by the redox properties of electron donator involved in O₂^−•/H₂O₂ production, as originally suggested by Brand and colleagues is also surveyed in detail. In addition, redox signaling mechanisms that control O₂^−•/H₂O₂ genesis from these sites are discussed. Finally, the current methodologies utilized for measuring O₂^−•/H₂O₂ in isolated mitochondria, cell culture and in vivo are reviewed.