Pyruvate dehydrogenase stimulation

As discussed in the previous problem, the phosphatase activates pyruvate dehydrogenase, stimulating the rate of both glycolysis and the citric acid cycle. Calcium-mediated activation of pyruvate dehydrogenase therefore promotes increased production of ATP, which is then available for muscle contraction. 

Insulin stimulates lipogenesis by several other mechanisms as well as by increasing acetyl-CoA carboxylase activity. It increases the transport of glucose into the cell (eg, in adipose tissue), increasing the availability of both pyruvate for fatty acid synthesis and glycerol 3-phosphate for esterification of the newly formed fatty acids, and also converts the inactive form of pyruvate dehydrogenase to the active form in adipose tissue but not in liver. Insulin also—by its ability to depress the level of intracellular cAMP—inhibits lipolysis in adipose tissue and thereby reduces the concentration of... 

Where two enzymes compete for the same substrate, we expect to see some form of metabolic control and in this case the concentrations of NADH and acetyl-CoA are the key controlling factors (Figure 6.44). When glucose is not available as a fuel, metabolism switches to 3- oxidation of fatty acids, which generates more than sufficient quantities of both NADH and acetyl-CoA to drive the TCA cycle and to maintain oxidative phosphorylation. Pyruvate dehydrogenase activity is suppressed and pyruvate carboxylase is stimulated by ATP, NADH and acetyl-CoA (strictly speaking by low mitochondrial ratios of ADP/ATP, NAD+/NADH and coenzyme A/acetyl-CoA), so... 

Acetyl-CoA is a critical regulator of the fate of pyruvate it allosterically inhibits pyruvate dehydrogenase and stimulates pyruvate carboxylase (see Fig. 15-20). In these ways acetyl-CoA prevents it own further production from pyruvate while stimulating the conversion of pyruvate to oxaloacetate, the first step in gluconeo-genesis. 

Deficiency of pyruvate dehydrogenase is the most frequent cause of lactic acidemia ac Since this enzyme has several components (Fig. 15-15), a number of forms of the disease have been observed. Patients are benefitted somewhat by a high-fat, low-carbohydrate diet. Transient lactic acidemia may result from infections or from heart failure. One treatment is to administer dichloroacetate, which stimulates increased activity of pyruvate dehydrogenase, while action is also taken to correct the underlying illness.d Another problem arises if a lactate transporter is defective so that lactic acid accumulates in muscles.6... 

Answer Fatty acid catabolism increases the level of acetyl-CoA, which stimulates pyruvate carboxylase. The resulting increase in oxaloacetate concentration stimulates acetyl-CoA consumption through the citric acid cycle, causing the citrate and ATP concentrations to rise. These metabolites inhibit glycolysis at PFK-1 and inhibit pyruvate dehydrogenase, effectively slowing the utilization of sugars and pyruvate. 

Pyruvate dehydrogenase, especially in the adipose tissue, is stimulated by a high insulin/glucagon ratio. This leads to the production of acetyl-CoA, which may enter the Krebs cycle in the fed state. The more likely possibility is the biosynthesis of fatty acids from acetyl-CoA. The latter requires NADPH, and for this reason, the hexose monophosphate shunt is also activated. 

There have been reports of other glucagon actions in the liver which can be related to the elevation of cAMP, but whose molecular mechanisms are not well defined. Examples are the stimulations of ketogenesis, ureogenesis, amino acid transport, respiration and ion fluxes, the rapid changes in pyruvate dehydrogenase and pyruvate carboxylase, and the induction of P-enolpyruvate carboxykinase and other enzymes. 

Figure 7-2. Reactions of the pyruvate dehydrogenase (PDU) multienzyme complex (PDC). Pyruvate is decarboxylated by the PDH subunit (I ,) in the presence of thiamine pyrophosphate (TPP). The resulting hydroxyethyl-TPP complex reacts with oxidized lipoamide (LipS3), the prosthetic group of dehydrolipoamide transacetylase (Ii2), to form acetyl lipoamide. In turn, this intermediate reacts with coenzyme A (CoASH) to yield acetyl-CoA and reduced lipoamide [Lip(SH)2]. The cycle of reaction is completed when reduced lipoamide is reoxidized by the flavoprotein, dehydrolipoamide dehydrogenase (E3). Finally, the reduced flavoprotein is oxidized by NAD+ and transfers reducing equivalents to the respiratory chain via reduced NADH. PDC is regulated in part by reversible phosphorylation, in which the phosphorylated enzyme is inactive. Increases in the in-tramitochondrial ratios of NADH/NAD+ and acetyl-CoA/CoASH also stimulate kinase-mediated phosphorylation of PDC. The drug dichloroacetate (DCA) inhibits the kinase responsible for phosphorylating PDC, thus locking the enzyme in its unphosphory-lated, catalytically active state. Reprinted with permission from Stacpoole et al. (2003).

DCA is an inhibitor of pyruvate dehydrogenase kinase, the enzyme that regulates pyruvate dehydrogenase (PDH) activity by phosphorylation of the Ex subunit, leading to a decrease in PDH activity (see Chapter 7). DCA has been used to lower lactate levels in some patients with MELAS. Stimulation of PDH activity reduces the release of lactate from peripheral tissues and enhances its metabolism by the liver. Since prolonged use of DCA may lead to sensory neuropathy, treatment must be closely monitored. 

Insulin is an antilipolytic hormone, and its effect on adipose tissue is to increase the transport of glucose into the fat cell, to stimulate lipogenesis and inhibit lipolysis. Thus, pyruvate dehydrogenase and acetyl-CoA carboxylase are activated, and the hormone-sensitive lipase is inactivated. In the normal, well-fed state insulin stimulates the deposition of fat. 

Pyruvate dehydrogenase phosphatase (PDH) assay was determined using the method of Caro (1). The effects of selected experimental agents in stimulating the insulin-dependent enzyme PDH at 100 and 0.1 p,M, 1.0 and 10pM are provided in Tables 1 and 2, respectively. 

Lamer, J., Huang, L.C., Schwartz, C.EW., Oswald, A.S., Shen, T.-Y., Kinter, M., Tang, G., and Zeller, K., 1988, Rat liver insulin mediator which stimulates pyruvate dehydrogenase phosphatase contains galatosamine and D-chiroinositol. Biochem. Biophys. Res. Commun. 151 1416-1426. 

Sugden MC, Bulmer K, Augustine D, and Holness MJ (2001a) Selective modification of pyruvate dehydrogenase kinase isoform expression in rat pancreatic islets elicited by starvation and activation of peroxisome proliferator-activated receptor-alpha implications for glucose-stimulated insulin secretion. Diabetes 50, 2729-36. 

A. Insulin stimulates activation of pyruvate kinase, pyruvate dehydrogenase, and phospho-fructokinase 2 (PFK2). PFK2 then catalyzes formation of fructose 2,6-bisphosphate, which is an activator of PFK1 and an inhibitor of fructose 1,6-bisphosphatase, a gluconeogenic enzyme. 

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