Liver malate-aspartate shuttle

A similar shuttle, the malate-aspartate shuttle, operates in heart and liver (Fig. 6). Oxaloacetate in the cytosol is converted to malate by cytoplasmic malate dehydrogenase, reoxidizing NADH to NAD+ in the process. The malate enters the mitochondrion via a malate-a-ketoglutarate carrier in the inner mitochondrial membrane. In the matrix the malate is reoxidized to oxaloacetate by NAD+ to form NADH. Oxaloacetate does not easily cross the inner mitochondrial membrane and so is transaminated to form aspartate which then exits from the mitochondrion... 

Sugano, T, Handler, J. A., Yoshihara, H., Kizaki, Z., and Thurman, R. G. (1990). Acute and chronic ethanol treatment in vivo increases malate-aspartate shuttle capacity in perfused rat liver. J. Biol. Chem. 265, 21549-21553. 

Two shuttles allow for more ATP to be generated under aerobic conditions the glycerol 3-phosphate shuttle, which functions primarily in skeletal mnscle and brain, and the malate-aspartate shuttle, primarily in the heart, liver, and kidney. 

FIGURE 19-27 Malate-aspartate shuttle. This shuttle for transporting reducing equivalents from cytosolic NADH Into the mitochondrial matrix is used in liver, kidney, and heart. (T) NADH in the cytosol (intermembrane space) passes two reducing equivalents to oxaloacetate, producing malate. (2) Malate crosses the inner membrane via the malate-a-ketoglutarate transporter. In the matrix, malate passes... 

Figure 15.11b shows the malate/aspartate shuttle system, which is particularly active in liver and heart. It uses malate, aspartate, and oxaloacetate to shuttle cytoplasmic electrons from NADH into the mitochondrial matrix. In this shuttle, NADH reduces oxaloacetate to malate, which travels through an inner membrane transport system that ultimately exchanges the malate for an ot-ketoglutarate. To do... 

A more complex and more efficient shutde mechanism is the malate-aspartate shuttle, which has been found in mammalian kidney, liver, and heart. This shuttle uses the fact that malate can cross the mitochondrial membrane, while oxaloacetate cannot. The noteworthy point about this shuttle mechanism is that the transfer of electrons from NADH in the cytosol produces NADH in the mitochondrion. In the cytosol, oxaloacetate is reduced to malate by the cytosolic malate dehydrogenase, accompanied by the oxidation of cytosolic NADH to NAD+ (Figure 20.24). The malate then crosses the mitochondrial membrane. In the mitochondrion, the conversion of malate back to oxaloacetate is catalyzed by the mitochondrial malate dehydrogenase (one of the enzymes of the citric acid cycle). Oxaloacetate is converted to aspartate, which can also cross the mitochondrial membrane. Aspartate is converted to oxaloacetate in the cytosol, completing the cycle of reactions. 

Oxaloacetate is formed from pyruvate by pyruvate carboxylase, located in the mitochondria. Pyruvate carboxylase is a biotin-requiring enzyme its kinetic mechanism has been studied in detail using the enzyme purified from domestic fowl liver (Attwood Graneri, 1992). In tissues where PEPCK is located in the cytosol, its substrate oxaloacetate is required in the cytosol for the formation of PEP for gluconeogenesis. The malate-aspartate shuttle is required for gluconeogenesis in avian kidney, according to the scheme in Fig. 3.3, but whether or not it is also required for gluconeogenesis in avian liver is unresolved. There is evidence for the existence of a... 

Aminooxyacetate, an inhibitor of glutamate— oxalacetate transaminase, inhibits the formation of aspartate. Soling Kleinicke (1976) observed that aminooxyacetate did not inhibit the formation of glucose from lactate and, therefore, concluded that the malate-aspartate shuttle was not essential for the lactate gluconeogenesis in avian liver. However, Ochs Harris (1980) found that aminooxyacetate did block lactate gluconeogenesis when lower concentrations of pyruvate were used and incubation was for longer than 15 min. They concluded that the malate-aspartate shuttle was required. 

An increased intramitochondrial NADH concentration leads to an increase in cytosolic NADH through shuttles in the mitochondrial inner membrane, i.e. the malate-aspartate shuttle. The raised NADH/NAD" ratio in the cytoplasm shifts the lactate dehydrogenase equilibrium in the direction of lactate. An increased lactate/pyruvate ratio can be found in most but not in all patients with a mitochondrial disorder. In the case of a disturbed respiratory chain in liver mitochondria, the reduced intramitochondrial redox state leads to an increased 3-OH-butyrate/acetoacetate ratio. 

The glycerophosphate shuttle is important in muscle in which there is a very high rate of glycolysis (especially insect flight muscle) the malate-aspartate shuttle is especially important in heart and liver. 

The malate—aspartate shuttle is the mechanism by which electrons from NADH produced in the cytosol are transported into mitochondria, as the inner membrane is impermeable to NADH itself. Oxaloacetate is reduced to malate in the cytosol by malate dehydrogenase, in the process oxidizing NADH to replenish cytosolic NAD. The malate-aspartate shuttle is found mainly in cardiac muscle and liver cells, while the glycerol 3-phosphate shuttle operates mainly in brain and skeletal muscle cells. Once malate has entered the mitochondria it is oxidized to oxaloacetate, generating NADH within the mitochondrial matrix. Oxaloacetate is then converted to aspartate, which is transported out of the mitochondria in exchange for glutamate. 

The final reactions to be considered in the metabolism of ethanol in the liver are those involved in reoxidation of cytosolic NADH and in the reduction of NADP. The latter is achieved by the pentose phosphate pathway which has a high capacity in the liver (Chapter 6). The cytosolic NADH is reoxidised mainly by the mitochondrial electron transfer system, which means that substrate shuttles must be used to transport the hydrogen atoms into the mitochondria. The malate/aspartate is the main shuttle involved. Under some conditions, the rate of transfer of hydrogen atoms by the shuttle is less than the rate of NADH generation so that the redox state in the cytosolic compartment of the liver becomes highly reduced and the concentration of NAD severely decreased. This limits the rate of ethanol oxidation by alcohol dehydrogenase. 

In the malate shuttle (left)—which operates in the heart, liver, and kidneys, for example-oxaloacetic acid is reduced to malate by malate dehydrogenase (MDH, [2a]) with the help of NADH+HT In antiport for 2-oxogluta-rate, malate is transferred to the matrix, where the mitochondrial isoenzyme for MDH [2b] regenerates oxaloacetic acid and NADH+HT The latter is reoxidized by complex I of the respiratory chain, while oxaloacetic acid, for which a transporter is not available in the inner membrane, is first transaminated to aspartate by aspartate aminotransferase (AST, [3a]). Aspartate leaves the matrix again, and in the cytoplasm once again supplies oxalo-acetate for step [2a] and glutamate for return transport into the matrix [3b]. On balance, only NADH+H"" is moved from the cytoplasm into the matrix ATP is not needed for this. 

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