Malate-aspartate shuttle

Malate-aspartate shuttle The malate-aspartate shuttle (MAS) is a redox process that supports oxidative pathways in the cytosol, and reductive potential in mitochondria. The mitochondrial succinate dehydrogenase (SDH) reaction provides reducing equivalents (electrons) for the respiratory electron transport, with the NADH needed to reduce malate coming from cytosolic processes. There is no NADH equilibrium between cytosol and mitochondria: cytosolic NADH/NAD+ ratio is 0.001, while in mitochondria, it is 0.1. The MAS creates this NADH gradient by reducing oxaloacetate (OA) to malate (MAL), catalyzed by cytosolic MDH1, and exchanging cytosolic MAL with mitochondrial 2-oxoglutarate (2OG, 2-KG), catalyzed by SLC25A11. At the same time, aspartate (L-Asp) gets exported and transaminated to glutamate (L-Glu), which subsequently gets coimported with a proton and transaminated back. In summary, mitochondria take up one proton and one reducing equivalent. The proton import by SLC25A12/13 is irreversible, so the MAS always runs in one direction. Hence, the mitochondrial outward proton-motive force drives the MAS toward cytosolic NADH oxidation. Defects in any of the reactions of this pathway lead to cytosolic NAD+ scarcity, affecting glycolysis, L-Glu, and L-Ser biosynthesis, as well as L-Asp availability. Neurotransmission in the CNS specifically needs L-Asp and L-Glu, and mutations in proteins catalyzing MAS reactions are commonly associated with early infantile epileptic encephalopathy (reviewed in Borst, 2020; Broeks et al., 2021).