Malate dehydrogenase mouse

In pigeon, chicken, and rabbit liver, phospho-enolpymvate carboxykinase is a mitochondrial enzyme, and phosphoenolpyruvate is transported into the cytosol for gluconeogenesis. In the rat and the mouse, the enzyme is cytosolic. Oxaloacetate does not cross the mitochondrial inner membrane it is converted to malate, which is transported into the cytosol, and convetted back to oxaloacetate by cytosolic malate dehydrogenase. In humans, the guinea pig, and the cow, the enzyme is equally disttibuted between mitochondria and cytosol. 

Fig. 3. Specific activities (ytmoles substrate/minute/mg protein) of malate dehydrogenase and cytochrome oxidase of mitochondria from whole mouse fetuses and young.

However, when Syrian (golden) hamster melanoma cells were fused with non-melanin-producing C3H mouse cells [104], and the hybrid line was isolated by the appropriate selection, no melanin production was observed in the hybrid cells, nor were tyrosinase and dopa oxidase activities detected in their extracts [104]. The parental melanoma cells had the enzymic activities and produced pigment. Nevertheless, in the hybrid cells, which possessed the expected hybrid karyotype, the lactate and malate dehydrogenase isozyme patterns were combinations of both parental types [104]. Thus, one might conclude that in the non-melanin-producing C3H parental cells there existed a specific diffusible enzyme... 

Unlike glycolysis, which occurs strictly in the cell cytosol, gluconeogen-esis involves a complex interaction between the mitochondrion and the cytosol. This interaction is necessitated by the irreversibility of the pyruvate kinase reaction, by the relative impermeability of the inner mitochondrial membrane to oxaloacetate, and by the specific mitochondrial location of pyruvate carboxylase. Compartmentation within the cell has led to the distribution of a number of enzymes (aspartate and alanine aminotransferases, and NAD -malate dehydrogenase) in both the mitochondria and the cytosol. In the classical situation represented by the rat, mouse, or hamster hepatocyte, the indirect "translocation" of oxaloacetate—the product of the pyruvate carboxylase reaction—into the cytosol is effected by the concerted action of these enzymes. Within the mitochondria oxaloacetate is converted either to malate or aspartate, or both. Following the exit of these metabolites from the mitochondria, oxaloacetate is regenerated by essentially similar reactions in the cytosol and is subsequently decarboxylated to P-enolpyruvate by P-enol-pyruvate carboxykinase. Thus the presence of a membrane barrier to oxaloacetate leads to the functioning of the malate-aspartate shuttle as an important element in gluconeogenesis. 

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