Tricarboxylic acid cycle, effect

Senior, A.E. Shenatt, H.S.A. (1968). Biochemical effects of the hypoglycaemic compound pent-4-enoic acid and related non-hypoglycemic fatty acids. Oxidative phosphorylation and mitochondrial oxidation of pyruvate, 3-hydroxybutyrate and tricarboxylic acid-cycle intermediates. Biochem. J. 110,499-509. 

Coenzyme availability can also often have a limiting effect (5). If the coenzyme is regenerated by a second, independent metabolic pathway, the speed of the second pathway can limit that of the first one. For example, glycolysis and the tricarboxylic acid cycle are mainly regulated by the availability of NAD" (see p. 146). Since NAD is regenerated by the respiratory chain, the latter indirectly controls the breakdown of glucose and fatty acids (respiratory control, see p. 144). 

The increased degradation of fat that occurs in insulin deficiency also has serious effects. Some of the fatty acids that accumulate in large quantities are taken up by the liver and used for lipoprotein synthesis (hyperlipidemia), and the rest are broken down into acetyl CoA. As the tricarboxylic acid cycle is not capable of taking up such large quantities of acetyl CoA, the excess is used to form ketone bodies (acetoacetate and p-hydroxy-butyrate see p. 312). As H"" ions are released in this process, diabetics not receiving adequate treatment can suffer severe metabolic acidosis (diabetic coma). The acetone that is also formed gives these patients breath a characteristic odor. In addition, large amounts of ketone body anions appear in the urine (ketonuria). 

With two exceptions (lysine and leucine see below), all of the proteinogenic amino acids are also glucogenic. Quantitatively, they represent the most important precursors for gluconeogenesis. At the same time, they also have an anaplerotic effect—1. e., they replenish the tricarboxylic acid cycle in order to feed the anabolic reactions that originate in it (see p. 138). 

Schweizer, O., Howland, W. S., Sullivan, C., and Vertes, E., The effect of ether and halothane on blood levels of glucose, pyruvate, lactate and metabolites of the tricarboxylic acid cycle in normotensive patients during operation. Anes-thesiology 28, 814-822 (1967). 

This type of effect can occur in all tissues and is caused by a metabolic inhibitor such as azide or cyanide, which inhibits the electron transport chain. Inhibition of one or more of the enzymes of the tricarboxylic acid cycle such as that caused by fluoroacetate (Fig. 6.7) also results in inhibition of cellular respiration (for more details of cyanide and fluoroacetate see chap. 7). 

Toxic compounds, which interfere with major pathways in intermediary metabolism, can lead to depletion of energy-rich intermediates. For example, fluoroacetate blocks the tricarboxylic acid cycle, giving rise to cardiac and CNS effects, which may be fatal (see chap. 7). Another example is cyanide (see chap. 7). 

Fluoroacetate causes inhibition of aconitase, an enzyme in the tricarboxylic acid cycle. This is due to the formation of fluorocitrate, which binds to aconitase and inhibits the enzyme. This is because the fluorine atom cannot be removed from the fluorocitrate unlike the hydrogen atom in the normal substrate, citrate. The result is complete blockade of the cycle and this means tissues become starved of ATP and other vital metabolic intermediates. This causes adverse effects in the heart as the organ is particularly sensitive to deficiency of ATP. 

Notice the intermediate in the reaction of citrate synthase (fig. 13.7). Do you think at some time in the future evolution will produce a variety of citrate synthase that recovers the energy in the thioester, analogous to the production of GTP (ATP) by succinate thiokinase (page 291) Would this energy recovery have any effect on the thermodynamics of the tricarboxylic acid cycle ... 

This method can be used to compensate for inhibition of a biochemical pathway which results in a deficiency of an essential metabolic product. Detailed variations of the method are provided by Dayan et al.7 and Amagasa et al.1 The inhibitor concentration should be no higher than that required for strong herbicidal effect. Metabolite concentrations should be below that which is phytotoxic. For example, certain amino acids such at methionine, are growth inhibitors at relatively low concentrations. So, in preliminary work, dose-response studies should be done with amino acids to find the maximum concentrations that do not inhibit growth. Then, seeds of test plants should be imbibed in solutions of the phytotoxin with and without metabolite solutions. Amino acids, tricarboxylic acid cycle intermediates, vitamins, nucleotides, and reducing agents have all been used in complementation studies to elucidate modes of action of a variety of phytotoxins. Examples of each of these is provided by Dayan et al.7... 

The effect of nonfatal injuries such as a 2-hour period of bilateral hind-limb ischemia or a full-thickness scald of 20% of skin surface on the LDso of DNOC and its hyperthermic effect were evaluated in male rats (Stoner 1969). The intraperitoneal LDs° of DNOC was significantly (p<0.001) reduced from 24.8 to 26.2 mg/kg to 14 mg/kg DNOC when DNOC was given 1.5- 24 hours after either type of nonfatal injury. The authors concluded that the toxicity of DNOC was increased by previous trauma. These investigators proposed that this interaction was associated with sequential blocking of the tricarboxylic acid cycle with inhibition of citrate synthetase reaction during the early part of the response to the injury. Because DNOC acts as an uncoupler of oxidative phosphorylation, less ATP is produced. Therefore, the effects of trauma will be enhanced by an uncoupling agent such as DNOC. 

This finding ruled out citrate as a direct intermediate in the tricarboxylic acid cycle since isotopic citric acid, which is a symmetrical compound, could only give rise to a-ketoglutaric acid containing isotope distributed equally in both carboxyl groups (Fig. 5). Addition of citrate to their system (Evans and Slotin), furthermore, did not affect the specific radioactivity of the o-ketoglutarate. This effect would have been obtained had citrate been an intermediate. 

Although several mechanisms have been proposed to be responsible for causing CRS, none has been extensively studied. One hypothesis has been that the effects are due to an immediate hypersensitivity reaction. Since no IgE-mediated reaction has been documented, there is no direct evidence that this is the case. Another hypothesis is that vitamin Bg deficiency plays a role in the response because the symptoms were prevented by supplementing individuals with the vitamin. Since glutamate can be converted to acetylcholine by the tricarboxylic acid cycle, it has also been proposed that the effects are due to an increase in acetylcholine levels. It has been noted that after MSG ingestion, there is a decrease in cholinesterase levels. Due to inadequate investigations, it is not currently known if any or all of these mechanisms are responsible for CRS. The neurotoxicity of MSG, demonstrated after exposure... 

Differential outcomes are evident depending on the preparation used to study the in vitro effects of sodium fluoroacetate. Mitochondria-free preparations exhibit a Ki value of 22-45 pM for inhibition of aconitase by fluoroacetate. Aconitase bound to mitochondria appears to be much more sensitive to the effects of fluoroacetate fluoroacetate inhibits aconitase in these preparations in the picomolar concentration range. Inhibition of the tricarboxylic acid cycle in actrocytes results in a depletion of ATP and a... 

Acetylcoenzyme A supplies the main fuel for the tricarboxylic acid cycle (see Fig. 5), which is the principal route by which carbohydrate is oxidized to carbon dioxide and water. The effect of one turn of the cycle is the simple oxidation of a molecular unit of acetate. 

Much has been published on the controversial subject of the control of glycolysis. The following brief summary of some of the controls responsible for the Pasteur effect in yeasts is based mainly on a review by Sols and coworkers144 (see also, Fig. 7). (i) Isocitrate dehydrogenase (NAD ) (EC 1.1.1.41), one of the controlling enzymes of the tricarboxylic acid cycle (see Fig. 5), catalyzes the reaction... 

Buchanan, R. L. and Lewis, D. F. 1984. Regulation of aflatoxin biosynthesis Effect of glucose on the activities of various glycolytic enzymes. Appl. Environ. Microbiol. 48, 306-310. Buchanan, R. L., Federowicz, D., and Stahl, H. G. 1985. Activities of tricarboxylic acid cycle... 

Shiraishi, F. Savageau, M. A. (1993). The tricarboxylic acid cycle in Dictyostelium discoideum Systemic effects of including protein turnover in the current model. J. Biol. Chem. 268, 16917-16928. 

Aspartase deficiency in Y. pestis is another example of a substantial effect of a single base transversion (in this case a missense mutation) that nevertheless results in 99.99% reduction in enzyme activity [32], AspA activity catalyzes the deamination of L-aspartate to form fumarate, a component of the tricarboxylic acid cycle. Comparison of aspA in Y. pestis and closely related Y. pseudotuberculosis defines only a single base transversion (G.C-T.A) at a.a. position 363. This causes exchange of valine (GUG) in... 

Phlorizin has been reported to have a marked effect on biliary excretion of BSP and bilirubin it usually increases bile flow, but excretion of both compounds is reduced (J6, S40, V3). The action of this compound may be nonspecific, since it is believed that the drug inhibits the oxidation of all substrates of the tricarboxylic acid cycle (L16) by alteration of the permeability of cell membranes (K5). Phlorizin is also excreted in bile (S34), and its choleretic action is possibly due to the osmotic effect of excretion of this organic anion. 

F15. Frohman, C. E., and Orten, J. M., Tracer studies of acids of the tricarboxylic acid cycle II. Effect of fructose and bicarbonate on acetate metabolism in liver of diabetic rats. J. Biol. Chem. 220, 315 (1956). 

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