Succinate Can Shuttle Reducing Power from the Hypoxic Retina to the O2-Rich Pigment Epithelium

When O-2 is plentiful, the mitochondrial electron transport chain uses it as a terminal electron acceptor. However, the mammalian retina thrives in a hypoxic niche in the eye. We find that mitochondria in retinas adapt to their hypoxic environment by reversing the succinate dehydrogenase reaction to use fumarate to accept electrons instead of O-2. Reverse succinate dehydrogenase activity produces succinate and is enhanced by hypoxia-induced downregulation of cytochrome oxidase. Retinas can export the succinate they produce to the neighboring O-2-rich retinal pigment epithelium-choroid complex. There, succinate enhances O-2 consumption by severalfold. Malate made from succinate in the pigment epithelium can then be imported into the retina, where it is converted to fumarate to again accept electrons in the reverse succinate dehydrogenase reaction. This malate-succinate shuttle can sustain these two tissues by transferring reducing power from an O-2-poor tissue (retina) to an O-2-rich one (retinal pigment epithelium-choroid).