This is supplied by glutamate, which in the process is transformed into alpha-ketoglutarate by the same enzyme. Since aspartate is an amino acid, an amino radical needs to be added to the oxaloacetate. Oxaloacetate is then transformed into aspartate (since oxaloacetate cannot be transported into the cytosol) by mitochondrial aspartate aminotransferase. After malate reaches the mitochondrial matrix, it is converted by mitochondrial malate dehydrogenase into oxaloacetate, during which NAD + is reduced with two electrons to form NADH. Once malate is formed, the first antiporter (malate- alpha-ketoglutarate) imports the malate from the cytosol into the mitochondrial matrix and also exports alpha-ketoglutarate from the matrix into the cytosol simultaneously. In this process, two electrons generated from NADH, and an accompanying H +, are attached to oxaloacetate to form malate. The two malate dehydrogenases are differentiated by their location and structure, and catalyze their reactions in opposite directions in this process.įirst, in the cytosol, malate dehydrogenase catalyses the reaction of oxaloacetate and NADH to produce malate and NAD +. Malate dehydrogenase is present in two forms in the shuttle system: mitochondrial malate dehydrogenase and cytosolic malate dehydrogenase. The primary enzyme in the malate-aspartate shuttle is malate dehydrogenase. glutamate-aspartate antiporter in the inner membrane.

malate-alpha-ketoglutarate antiporter in the inner membrane.aspartate aminotransferase in the mitochondrial matrix and intermembrane space.malate dehydrogenase in the mitochondrial matrix and intermembrane space.Accordingly, pyruvate- and lactate-elicited glucagon secretion remains unaffected since their signalling is independent of mitochondrial shuttles.The shuttle consists of four protein parts: Thus, suppressed glycerol phosphate shuttle activity in the α-cell may prevent a high rate of glycolysis and consequently glucagon secretion in response to glucose. Furthermore, expression of the rate-limiting enzyme of the glycerol phosphate shuttle was higher in sorted primary β- than in α-cells. These results were confirmed in mouse islets, where PhS abrogated secretion of glucagon but not insulin. Our data suggest that the glycerol phosphate shuttle augments the malate–aspartate shuttle in INS-1 832/13 but not αTC1-6 cells. Accordingly, relative expression of constituents in the glycerol phosphate shuttle compared with malate–aspartate shuttle was lower in αTC1-6 cells. Blocking the malate–aspartate shuttle increased levels of glycerol 3-phosphate only in INS-1 832/13 cells. Inhibition of the malate–aspartate shuttle, using phenyl succinate (PhS), abolished glucose-provoked ATP production and hormone secretion from αTC1-6 but not INS-1 832/13 cells. Although glycolytic metabolism was similar in the two cell lines, TCA (tricarboxylic acid) cycle metabolism, respiration and ATP levels were less glucose-responsive in αTC1-6 cells. In contrast, tight coupling of glycolytic and mitochondrial metabolism was observed only in INS-1 832/13 cells. We found that INS-1 832/13 and αTC1-6 cells respectively secreted insulin and glucagon concentration-dependently in response to glucose. Therefore, we studied the regulation of glucagon secretion from αTC1-6 (αTC1 clone 6) cells and compared it with insulin release from INS-1 832/13 cells. Altered secretion of insulin as well as glucagon has been implicated in the pathogenesis of Type 2 diabetes (T2D), but the mechanisms controlling glucagon secretion from α-cells largely remain unresolved.

Malate aspartate shuttle