A cytochrome-c-functionalized amperometric sensor (Henderson et al., 2009) made it possible to monitor superoxide (O2∙−O_2^{\bullet-}) release from isolated mitochondrial fractions continuously, in real time, during inhibition of respiratory-chain complexes I and III. That demonstration left a coherent bridging objective open: establishing, mechanistically and quantitatively, what such isolated-organelle superoxide fluxes mean for superoxide dynamics in intact cells. Three linked facets remain unresolved: (i) the origin of the modest externally detected superoxide signal observed after complex I inhibition, given that complex-I superoxide is released almost exclusively to the matrix side of the inner membrane; (ii) the quantitative relationship between electrode-measured fluxes on isolated fractions and in-situ cellular superoxide dynamics, given that isolation removes cytosolic antioxidant sinks such as Cu/Zn superoxide dismutase; and (iii) the control strength (quantitative regulatory influence) of individual complexes over whole-chain electron flow and superoxide production. Later work built parallel in-cell and in-vivo measurement technologies (mitochondria-targeted mass-spectrometry probes, genetically encoded fluorescent redox probes, and improved implantable superoxide biosensors), but none of these families has been cross-validated against the isolated-organelle electrode measurement, so the bridging relationship is still unestablished.

Background

Reactive oxygen species (ROS) are partially reduced oxygen derivatives; the proximal one is the superoxide anion radical O2∙−O_2^{\bullet-}, produced when single electrons leak from the mitochondrial electron transport chain (mETC) to molecular oxygen. The mETC comprises four multi-subunit complexes (I–IV) embedded in the mitochondrial inner membrane; complexes I (NADH:ubiquinone oxidoreductase) and III (ubiquinol:cytochrome c oxidoreductase) are the classical superoxide-producing sites. Rotenone blocks electron exit from complex I and antimycin A blocks electron exit from complex III; both drive superoxide production at their respective sites, but at different sides of the inner membrane. Topology studies (St-Pierre et al. 2002; Han et al. 2001; Brand 2010; Quinlan et al. 2013) established that complex I generates superoxide almost exclusively toward the matrix (the compartment enclosed by the inner membrane), whereas the quinol-oxidizing (QoQ_o) site of complex III can release superoxide toward the intermembrane space, i.e. the cytosol-facing side. Superoxide is membrane-impermeant and extremely short-lived (it is removed by superoxide dismutases with near-diffusion-limited rate constants), so a cytosolic detection system should, in principle, see complex-III-derived but not complex-I-derived superoxide. Henderson et al. (2009) introduced a cytochrome-c-functionalized amperometric microelectrode — an electrode coated with cytochrome c, whose superoxide-dependent reduction is re-oxidized at the electrode to produce a real-time current proportional to external superoxide flux — and applied it to isolated mitochondrial fractions from mammalian cells during rotenone and antimycin A inhibition. Two antioxidants matter for interpreting such experiments: MnSOD (manganese superoxide dismutase) in the matrix and Cu/Zn-SOD (copper/zinc superoxide dismutase) in the cytosol and intermembrane space. When mitochondria are isolated, the surrounding cytosolic Cu/Zn-SOD and other cellular sources and sinks of ROS are removed, and the organelles experience non-physiological substrate supply and membrane potential, so measured fluxes are not automatically transferable to living cells. Quantifying how strongly each complex governs a flux is the domain of metabolic control analysis (MCA), which expresses regulation through control coefficients (control strength: the fractional change in flux caused by a fractional change in an individual enzyme's activity); applying this framework to superoxide production would require graded perturbation of complex activity analyzed within MCA rather than single-dose inhibitor comparisons. Candidate routes exist for matrix superoxide to become externally detectable — the voltage-dependent anion channel (VDAC) of the outer membrane, the TOM translocase of the outer membrane, and inner-membrane anion-channel-coupled ROS fluxing described in mitochondrial-oscillator studies — but none has been quantitatively tied to the electrode observation. In parallel, in-situ measurement advanced along other lines: mitoB, a mitochondria-targeted boronic acid ratiometric mass-spectrometry probe that reports matrix hydrogen peroxide in living organisms; genetically encoded fluorescent probes such as mito-roGFP2-Orp1, a redox-sensitive green fluorescent protein fused to the yeast peroxidase Orp1 that reports matrix H2O2H_2O_2 ratiometrically in intact cells; and improved electrochemical superoxide biosensors. These families measure different chemical species in different compartments, and no study has yet connected them quantitatively to the isolated-organelle electrode fluxes that motivated the 2009 work (Murphy 2009; Brand & Nicholls 2011).

Problem Statement

Establish what direct, real-time superoxide measurements on isolated mitochondria mean for superoxide dynamics in intact cells. Concretely, for the cytochrome-c-electrode measurement of external superoxide flux from isolated mitochondrial fractions under defined inhibition of complexes I and III, determine: (1) the mechanistic origin of the externally detected superoxide signal under complex I inhibition — whether it arises from rotenone-sensitive sites within complex I made externally detectable by transmembrane efflux (via outer-membrane channels such as VDAC or TOM, or inner-membrane anion flux), from secondary production at cytosol-facing sites such as the complex III QoQ_o site, from other intramitochondrial redox processes, or from preparation artifacts such as damaged or inside-out mitochondrial membranes — with the accounting grounded in the established topology that complex-I superoxide is released to the matrix; (2) the quantitative relationship between fluxes measured on isolated mitochondrial fractions and in-situ cellular superoxide dynamics, accounting for the removal of cytosolic Cu/Zn-SOD and other ROS sources and sinks during isolation and for differing substrate supply, membrane potential, and oxygen tension; and (3) the control strength of individual respiratory complexes (complex I versus complex III) over whole-chain electron flow and superoxide production, expressed as quantitative control coefficients in the metabolic-control-analysis sense for the cell types and physiological conditions examined. The sensitivity, dynamic range, interferences, and oxygen-tension dependence of the electrode measurement in complex biological matrices such as intact cells and tissue preparations — which recent implantable and SOD-based microbiosensors have begun to address but not completed — are known measurement limitations that bear on all three facets rather than a separate deliverable.

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