Krebs cycle inhibition

H. Koenig, A. Patel, Biochemical Basis For Fluorouracil Neurotoxicity. The Role of Krebs Cycle Inhibition by Fluoroacetate , Arch. Neurol. 1970, 23, 155-160. 

Krebs cycle Inhibition of aconitase by superoxide and fluoroacetate, of succinate dehydrogenase by methamphetamine and mal-onate, of alpha-ketoglutarate dehydrogenase by salicylic add... 

Zieve, L., Lyftogt, C., Draves, K. (1983). Toxicity of a fatty acid and ammonia interactions with h poglycemia and Krebs cycle inhibition. J. Lab. Clin. Med. 101 930-9. 

Koenig H, Patel A. Biochemical basis for fluorouracil neurotoxicity. The role of Krebs cycle inhibition by flnor-oacetate. Arch Neurol 1970 23(2) 155-60. 

Internally converted to fluoroacetate, which may block citrate in Krebs cycle inhibiting cell metabolism, but this mechanism has been challenged... 

Insects poisoned with rotenone exhibit a steady decline ia oxygen consumption and the iasecticide has been shown to have a specific action ia interfering with the electron transport iavolved ia the oxidation of reduced nicotinamide adenine dinucleotide (NADH) to nicotinamide adenine dinucleotide (NAD) by cytochrome b. Poisoning, therefore, inhibits the mitochondrial oxidation of Krebs-cycle iatermediates which is catalysed by NAD. 

Competitive inhibitors bind to specific groups in the enzyme active site to form an enzyme-inhibitor complex. The inhibitor and substrate compete for the same site, so that the substrate is prevented from binding. This is usually because the substrate and inhibitor share considerable stmctural similarity. Catalysis is diminished because a lower proportion of molecules have a bound substrate. Inhibition can be relieved by increasing the concentration of substrate. Some simple examples are shown below. Thus, sulfanilamide is an inhibitor of the enzyme that incorporates j9-aminobenzoic acid into folic acid, and has antibacterial properties by restricting folic acid biosynthesis in the bacterium (see Box 11.13). Some phenylethylamine derivatives, e.g. phenelzine, provide useful antidepressant drags by inhibiting the enzyme monoamine oxidase. The cA-isomer maleic acid is a powerful inhibitor of the enzyme that utilizes the trans-isomer fumaric acid in the Krebs cycle. 

The toxicity of fluoroacetic acid and of its derivatives has played an historical decisive role at the conceptual level. Indeed, it demonstrates that a fluorinated analogue of a natural substrate could have an activity profile that is far different from that of the nonfluorinated parent compound. The toxicity of fluoroacetic acid is due to its ability to block the citric acid cycle (Krebs cycle), which is an essential process of the respiratory chain. The fluoroacetate is transformed in vivo into 2-fluorocitrate by the citrate synthase. It is generally admitted that aconitase (the enzyme that performs the following step of the Krebs cycle) is inhibited by 2-fluorocitrate the formation of aconitate through elimination of the water molecule is a priori impossible from this substrate analogue (Figure 7.1). 

Welsh, N., Eizirik, D. L., Bendtzen, K., and Sandler, S. (1991a). Interleukin-1/3-induced nitric oxide production in isolated rat pancreatic islets requires gene transcription and may lead to inhibition of Krebs cycle enzyme aconitase. Endocrinology (Baltimore) 129, 3167-3173. 

Krebs cycle is inhibited at the points where a-ketoglutarate dehydrogenase and succinate dehydrogenase operate. This causes an increase in organic acids and an accumulation of glutamate. 

The mechanism of toxicity of ethylene glycol involves metabolism, but unlike previous examples, this does not involve metabolic activation to a reactive metabolite. Thus, ethylene glycol is metabolized by several oxidation steps eventually to yield oxalic acid (Fig. 7.84). The first step is catalyzed by the enzyme alcohol dehydrogenase, and herein lies the key to treatment of poisoning. The result of each of the metabolic steps is the production of NADH. The imbalance in the level of this in the body is adjusted by oxidation to NAD coupled to the production of lactate. There is thus an increase in the level of lactate, and lactic acidosis may result. Also, the intermediate metabolites of ethylene glycol have metabolic effects such as the inhibition of oxidative phosphorylation, glucose metabolism, Krebs cycle, protein synthesis, RNA synthesis, and DNA replication. 

There are two Krebs cycle inhibitors that are worth mentioning. Malonate inhibits succinate dehydrogenase because of its very similar structure. Fluoro-acetate inhibits cis-aconitase, which is an Fe-S enzyme. The fluoroacetate replaces acetate as a substrate in the citrate synthase reaction when this combines with cis-aconitase, however, no further reaction becomes possible. 

However, an exception to this situation is the formation of the toxin fluoroacetate, which inhibits the Krebs cycle. Moreover, O Hagan and co-workers have successfully identified the first fluorinase enzyme, in the bacterium Streptomyces cattleya, which catalyses the formation of a C-F bond [88] (Figure 1.6). 

FIGURE 4.47 Regulation of fatty acid oxidation by malonyl-CoA. Carboxylase catalyzes the first step of fatty acid synthesis (ttl). An increase in its activity results in an increase in the levels of malonyl-CoA in the cell (X2). Increased levels of malonyl-CoA inhibit the transport of fatty acids intn the mitnehondria f 3), which limits or controls their oxidation. The fatty acid oxidation pathway involves the degradation to units of acetyl-CoA ( 4), followed by conversion to COj in the Krebs cycle (S5). 

Many metabolic processes such as glycolysis, Krebs cycle reactions, photosynthesis, protein synthesis, and lipid metabolism are affected by exposure to F. Much of the action of F on these processes can be attributed to F-dependent inhibition of enzymes. Examples of enzymes shown to be inhibited by F include enolase, phosphoglucomutase, phosphatase, hexokinase, PEP carboxylase, pyruvate kinase, succinic dehydrogenase, malic dehydrogenase, pyrophosphatase, phytase, nitrate reductase, mitochondrial ATPase, urease (Miller et al. 1983), lipase (Yu et al. 1987), amylase (Yu et al. 1988), invertase (Yu 1996 Ouchi et al. 1999), and superoxide dismutase (SOD) (Wilde and Yu 1998). 

Biotransformation, especially phase I metabolic reactions, cannot be assumed to be synonymous with detoxification because some drugs (although a minority) and xenobiotics are converted to potentially toxic metabolites (e.g. parathion, fluorine-containing volatile anaesthetics) or chemically reactive intermediates that produce toxicity (e.g. paracetamol in cats). The term lethal synthesis refers to the biochemical process whereby a non-toxic substance is metabolically converted to a toxic form. The poisonous plant Dichapetalum cymosum contains monofluoroacetate which, following gastrointestinal absorption, enters the tricarboxylic acid (Krebs) cycle in which it becomes converted to monofluorocitrate. The latter compound causes toxicity in animals due to irreversible inhibition of the enzyme aconitase. The selective toxicity of flucytosine for susceptible yeasts (Cryptococcus neoformans, Candida spp.) is attributable to its conversion (deamination) to 5-fluorouracil, which is incorporated into messenger RNA. 

Salicylates directly stimulate the central respiratory center and thereby cause hyperventilation and respiratory alkalosis. Moreover, salicylates cause uncoupling of oxidative phosphorylation. As a result, heat production (hyperthermia), oxygen consumption, and metabolic rate may be increased. In addition, salicylates enhance anaerobic glycolysis but inhibit Krebs cycle and transaminase enzymes, all of which lead to accumulation of organic acids and thus to metabolic acidosis. ... 

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