Catalytic oxidation and reduction reactions of hydrophilic carbon clusters with NADH and cytochrome C: features of an electron transport nanozyme
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Previously, our group reported on the promising efficacy of poly(ethylene glycol)-hydrophilic carbon clusters (PEG-HCCs) to work as broadly active and high capacity antioxidants in brain ischemia and injury models including stroke and traumatic brain injury coupled with hemorrhagic shock. PEG-HCCs are a carbon nanomaterial derived from harsh oxidation of single wall carbon nanotubes and covalently modified with poly(ethylene glycol). They retain no tubular remnants and are composed of a highly oxidized carbon core functionalized with epoxy, peroxyl, quinone, ketone, carboxylate, and hydroxyl groups. HCCs are the redox active carbon core of PEG-HCCs, which have a broad reduction potential range starting at +200 mV and extending to -2 V. Here we describe a new property of these materials: the ability to catalytically transfer electrons between key surrogates and proteins of the mitochondrial electron transport complex in a catalytic fashion consistent with the concept of a nanozyme. The estimated reduction potential of PEG-HCCs is similar to that of ubiquinone and they enabled the catalytic transfer of electrons from low reduction potential species to higher reduction electron transport complex constituents. PEG-HCCs accelerated the reduction of resazurin (a test indicator of mitochondrial viability) and cytochrome c by NADH and ascorbic acid in solution. Kinetic experiments suggested a transient tertiary complex. Electron paramagnetic resonance demonstrated NADH increased the magnitude of PEG-HCCs' intrinsic radical, which then reduced upon subsequent addition of cytochrome c or resazurin. Deconvolution microscopy identified PEG-HCCs in close proximity to mitochondria after brief incubation with cultured SHSY-5Y human neuroblastoma cells. Compared to methylene blue (MB), considered a prototypical small molecule electron transport shuttle, PEG-HCCs were more protective against toxic effects of hydrogen peroxide in vitro and did not demonstrate impaired cell viability as did MB. PEG-HCCs were protective in vitro when cells were exposed to sodium cyanide, a mitochondrial complex IV poison. Because mitochondria are a major source of free radicals in pathology, we suggest that this newly described nanozyme action helps explain their in vivo efficacy in a range of injury models. These findings may also extend their use to mitochondrial disorders.
author list (cited authors)
Derry, P. J., Nilewski, L. G., Sikkema, W., Mendoza, K., Jalilov, A., Berka, V., ... Kent, T. A.