Redox reactions accompanied by the oxidation–reduction of endogenous molecules play important roles in maintaining homeostasis in living organisms. In humans, numerous endogenous molecules that contribute towards maintaining physiological conditions form free radicals via electron transfer. A typical example of this is the mitochondrial electron transport chain, which is involved in energy production. If free radicals derived from endogenous molecules could be visualised and exploited as biological and functional probes, redox reactions mediated by endogenous molecules could be detected non-invasively. We succeeded in visualising the free radicals derived from endogenous molecules using an in vivo dynamic nuclear polarisation (DNP) magnetic resonance imaging (MRI) system. In this review, we describe the visualisation of endogenous redox molecules, such as flavins and ubiquinones, which are mitochondrial electron carriers, as well as vitamin E and vitamin C (ascorbate). In addition, we describe the application of melanin free radicals for the in vivo visualisation of metabola without using probes via in vivo DNP-MRI.
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