The glycerol phosphate shuttle is one of several pathways to regenerate cytosolic NAD⁺ for glycolysis.

<p>Glycolytic metabolism of glucose to pyruvate involves the reduction of cytosolic NAD⁺ to NADH at the step catalyzed by glyceraldehyde 3-phosphate dehydrogenase. Several mechanisms exist to regenerate cytosolic NAD⁺ to ensure that a high NADH/NAD⁺ ratio does not limit glucose metabolism. (<b>A</b>) The glycerol phosphate shuttle comprises soluble, NAD⁺-linked cGPDH and the membrane bound, FAD-linked mGPDH. This system regenerates cytosolic NAD⁺ and transfers the reducing equivalents directly into the mobile ubiquinone pool of the mitochondrial electron transport chain. (<b>B</b>) The malate-aspartate shuttle comprises multiple enzymes and mitochondrial carriers that interconvert and transport dicarboxylates and amino acids. In the cytosol, malate dehydrogenase regenerates NAD⁺ during reduction of oxaloacetate to malate. This malate is transported into the mitochondrial matrix where it is oxidized back to oxaloacetate by mitochondrial malate dehydrogenase, producing NADH that is reoxidized by complex I of the electron transport chain. To complete the shuttle, cytosolic oxaloacetate is regenerated by sequential transaminations through aspartate. These transaminations are potently inhibited by aminooxyacetate. <i>cMDH</i>, cytosolic malate dehydrogenase; <i>mMDH</i>, mitochondrial malate dehydrogenase; <i>OAA</i>, oxaloacetate; <i>Mal</i>, malate; <i>Asp</i>, aspartate (<b>C</b>) Lactate dehydrogenase regenerates cytosolic NAD⁺ at the expense of mitochondrial oxidation of pyruvate. <i>LDH</i>, lactate dehydrogenase. In certain systems, such as neuronal synapses, the maintenance of cytosolic NAD⁺ by each of these mechanisms can be made less important by direct uptake and oxidation of pyruvate.