Malate-aspartate carrier system

The malate-aspartate carrier system acts across the inner mitochondrial membrane as a means of:

• replenishing supplies of oxidized NAD within the cytoplasm in order to permit continued glycolysis
• passing energy in the form of reduced NAD across the mitochondrial membrane, across which it cannot physically pass, for conversion to ATP within the mitochondria
• transferring oxaloacetate from mitochondrion to cytoplasm during gluconeogenesis

Cytoplasmic malate dehydrogenase oxidizes NADH to NAD in tandem with reduction of oxaloacetate to malate. Malate is capable of diffusion across the inner mitochondrial membrane where the reverse reaction takes place with the formation of NADH and oxaloacetate.

Oxaloacetate is then converted to aspartate within the mitochondrion by the action of aspartate aminotransferase. Aspartate, unlike oxaloacetate, is capable of diffusing outwards where it then undergoes the reverse reaction to regenerate oxaloacetate.

During gluconeogenesis, the reverse series of reactions take place. These permit mitochondrial oxaloacetate, derived from pyruvate, to cross the inner mitochondrial membrane.