PEPCK mitochondrial

Phosphoenolpyruvate carboxykinase (PEPCK) deficiency is distinctly rare and even more devastating clinically than deficiencies of glucose-6-phosphatase or fructose-1,6-bisphosphatase. PEPCK activity is almost equally distributed between a cytosolic form and a mitochondrial form. These two forms have similar molecular weights but differ by their kinetic and immunochemical properties. The cytosolic activity is responsive to fasting and various hormonal stimuli. Hypoglycemia is severe and intractable in the absence of PEPCK [12]. A young child with cytosolic PEPCK deficiency had severe cerebral atrophy, optic atrophy and fatty infiltration of liver and kidney. 

Figure 18.8 indicates that oxaloacetate may proceed in three directions toward glucose biosynthesis. It may be converted to phosphoenolpyruvate by a mitochondrial PEPCK, as shown in Equation (18.7) ... 

A species specificity mi ht be pointed out with respect to the location of PE PC K, an enzyme used for gluconeogenesis, OAA is converted to PEP by PEPCK in the liver Cytoplasm of rats and mice because PEPCK is a cytosolic enzyme in these animals. In the livers of chickens and rabbits, however, OAA is converted to PEP in the mitochondria, because PEKK is mitochondrial in these animals, PEP must then be transported to the cytosol. PEPCK is evenly distributed between the mitochondria and the cytosol in human and cow livers (Pilkis et itf., 1988). 

Pyruvate carboxylase is a mitochondrial enzyme in animal cells, whereas PEPCK is almost exclusively mitochondrial in some species (e.g., pigeons) and cytosolic in others (e.g., rats and mice). In humans (and guinea pigs), PEPCK occurs in both mitochondria and cytosol. An interesting consequence of this species differences is that quinolinate, an inhibitor of cytoplasmic PEPCK, causes hypoglycemia in rats but is less active in humans and guinea pigs. 

In humans, oxaloacetate must be transported out of the mitochondrion to supply the cytosolic PEPCK. Because there is no mitochondrial carrier for oxaloacetate and its diffusion across the mitochondrial membrane is slow, it is transported as malate or asparate (Figure 15-2). The malate shuttle carries oxaloacetate and reducing equivalents, whereas the aspartate shuttle, which does not require a preliminary reduction step, depends on the availability of glutamate and a-ketoglutarate in excess of tricarboxylic acid (TCA) cycle requirements. 

By decreasing the mitoehondrial concentration of glutamate, an inhibitor of pyruvate carboxylase, through stimulation of the TCA cycle (secondary to the increase in mitochondrial acetyl-CoA) and the aspartate shuttle (secondary to the increase in cytosolic PEPCK induced by glucagon). 

The increase in glutamate favors transamination of oxaloacetate and limits oxaloacetate availability for phosphoenolpyruvate synthesis. When the [NADH]/[NAD+] ratio is low, malate formation occurs more readily. The cytosolic PEPCK is relatively unaffected by the mitochondrial [NADH]/[NAD+] ratio. Once malate and aspartate are transported to the cytosol and they are reconverted to oxaloacetate, cytosolic PEPCK can convert it to phosphoenolpyruvate. 

Oxaloacetate, generated from pyruvate by pyruvate carboxylase or from amino acids that form intermediates of the TCA cycle, does not readily cross the mitochondrial membrane. It is either decarboxylated to form PEP by the mitochondrial PEPCK or it is converted to malate or aspartate (see Figs. 31.7B and 31.7C). The conversion of oxaloacetate to malate requires NADH. PEP, malate, and aspartate can be transported into the cytosol. 

The metabolic steps in gluconeogenesis occur in two intracellular compartments (Fig. 3.2) the cytosol and the mitochondrial matrix. The enzymes of the tricarboxylic acid cycle reside in the mitochondrial matrix, apart from succinate dehydrogenase which is present in the inner mitochondrial membrane, whereas most of the enzymes of the gluconeogenic pathway are present in the cytosol. Transaminases, such as alanine aminotransferase and aspartate aminotransferase, are present both in mitochondria and cytosol of the domestic fowl liver (Sarkar, 1977). One of the control enzymes in gluconeogenesis, PEPCK, has a different intracellular distribution in avian liver compared with mammalian liver (Table 3.3). PEPCK in both pigeon and domestic fowl liver is present almost exclusively (> 99%) in mitochondria (Soling et al.. 1973), whereas in most mammals that have been studied, it is present mainly in the cytosol, and only present, if at all, in smaller amounts in... 

PEPCK requires, in addition to oxaloacetate, its second substrate GTP or ITP. If the enzyme is located in the mitochondria, then GTP will also be necessary in the mitochondria. GTP is generated in substrate level phosphorylation by the citric acid cycle enzyme succinyl-CoA synthetase (reaction 3.9), which is present in the mitochondrial matrix. 

Soling (1982) has studied the intracellular distribution of NDP kinase in rat and pigeon livers. The NDP kinase activity in the rat liver mitochondrial matrix is so low that he concludes that all the GTP required in the matrix must come from succinyl-CoA synthetase. By contrast, pigeon liver mitochondria have significant NDP kinase activity, which may generate the GTP required for PEPCK. 

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