Aconite, toxicity

Citrate is isomerized to isocitrate by the enzyme aconitase (aconitate hydratase) the reaction occurs in two steps dehydration to r-aconitate, some of which remains bound to the enzyme and rehydration to isocitrate. Although citrate is a symmetric molecule, aconitase reacts with citrate asymmetrically, so that the two carbon atoms that are lost in subsequent reactions of the cycle are not those that were added from acetyl-CoA. This asymmetric behavior is due to channeling— transfer of the product of citrate synthase directly onto the active site of aconitase without entering free solution. This provides integration of citric acid cycle activity and the provision of citrate in the cytosol as a source of acetyl-CoA for fatty acid synthesis. The poison fluo-roacetate is toxic because fluoroacetyl-CoA condenses with oxaloacetate to form fluorocitrate, which inhibits aconitase, causing citrate to accumulate. 

Liver tissue B12 is assayed by suspending 50 mg of lyophilized human liver in 100 ml of aconitate buffer to which 100 mg sodium metabisulfite is added. The suspension is autoclaved for 30 minutes at 16 psi to liberate the bound forms of the vitamin. Aliquots are diluted 1 10 after autoclaving to reduce metabisulfite toxicity on microorganism growth. Each milliliter now contains 50 pg of liver (w/v) 1.0, 1.5, and 2.0 ml of this hydrolyzate is added. After dilution to a volume of 5 ml, this represents an assay of 10, 15, and 20 ig of liver per milliliter of final solution. Normal liver contains between 2-14 mpg of vitamin B /mg liver powder. 

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). 

Aconite alkaloids Aconitine, molecular formula C34H47NO11, is an example of an aconite alkaloid. It is soluble in organic solvents, e.g. CHCI3 and CeHg, and sfightly soluble in alcohol or ether, but insoluble in water. Aconitine is an extremely toxic substance obtained from the plants of the genus Aconitum (family Ranunculaceae), commonly known as aconite or monkshood . It is a neurotoxin, and used for creating models of cardiac arrhythmia. 

Actaea asiatica . Lai Ye Sheng Ma (Asian baneberry) (aerial part) frans-Aconitic acid. This herb is toxic.51 A prophylactic against pestilence, malaria, evil miasma. 

In contrast, selective inhibition of enzyme activity involves highly specific interactions between the protein and chemical groups on the xenobiotic. An excellent example of this type of inhibition is seen in the toxic effect of fluoroacetate, which is used as a rodenticide. Although fluoroacetate is not directly toxic, it is metabolized to fluoroacetyl-CoA, which enters the citric acid cycle due to its structural similarity to acetyl-CoA (Scheme 3.5). Within the cycle, fluoroacetyl-CoA combines with oxalo-acetate to form fluorocitrate, which inhibits the next enzyme, aconitase, in the cycle [42]. The enzyme is unable to catalyze the dehydration to cis-aconitate, as a consequence of the stronger C-F bond compared with the C-H bond. Therefore, fluorocitrate acts as a pseudosubstrate, which blocks the citric acid cycle and, subsequently, impairs ATP synthesis. 

Among the most deadly of simple compounds is sodium fluoroacetate. The LD50 (the dose lethal for 50% of animals receiving it) is only 0.2 mg/kg for rats, over tenfold less than that of the nerve poison diisopropylphosphofluoridate (Chapter 12).a b Popular, but controversial, as the rodent poison "1080," fluoroacetate is also found in the leaves of several poisonous plants in Africa, Australia, and South America. Surprisingly, difluoroacetate HCF2-COO is nontoxic and biochemical studies reveal that monofluoroacetate has no toxic effect on cells until it is converted metabolically in a "lethal synthesis" to 2R,3R-2-fluorocitrate, which is a competitive inhibitor of aconitase (aconitate hydratase, Eq. 13-17).b This fact was difficult to understand since citrate formed by the reaction of fluorooxalo-acetate and acetyl-CoA has only weak inhibitory activity toward the same enzyme. Yet, it is the fluorocitrate formed from fluorooxaloacetate that contains a fluorine atom at a site that is attacked by aconitase in the citric acid cycle. 

Aconites, commonly called wolfsbane or monkshood, are species of Aconitum (Ranunculaceae), valued ornamental herbaceous plants, grown for their showy blue or purple flowers, which are shaped like a monk s cowl. Their alkaloid content, mainly in the roots, makes them some of the most toxic plants commonly encountered. The dried roots of Aconitum napellus were once used, mainly externally for relief of pain, e.g. in rheumatism. The toxic alkaloids (0.3-1.5%) are complex diterpene-derived esters. Aconitine (Figure 6.124) is the principal component (about 30%) and is a diester of aconine with acetic and benzoic acids. Hydrolysis products benzoylaconine and aconine are also present in dried plant material. These alkaloids appear to behave as neurotoxins by acting on sodium channels. All species of Aconitum and Delphinium are potentially toxic to man and animals and must be treated with caution. 

Other mechanisms have been considered. Clarke has proposed that the acute toxicity of fluorocitrate may be caused by irreversible inhibition of aconitase by (2is,4R)-4-fluoro-aconitate, formed by the dehydration of fluorocitrate by aconitase59. Hornfeldt and Larson reported evidence that seizures accompanying fluorocitrate toxicity may result from Ca+2 chelation in the spinal cord60. 

Toxicity. Aconitine affects both the heart and the central nervous system and is one of the most potent and quick-acting poisons. It is well absorbed from the gastro-intestinal tract and death may occur within a few minutes. The estimated minimum lethal dose is 2 mg of aconitine, 5 ml of aconite tincture (25% v/v) or 1 g of aconite root, although recovery after ingestion of 10 mg of aconitine has been reported. 

The activity of an enzyme may be inhibited by the presence of a toxic metabolite. Sodium fluoroacetate, known as rat poison 1080, is extremely toxic to animals. The toxic action, however, is not due to sodium fluoroacetate itself but to a metabolic conversion product, flu-orocitrate, formed through a reaction commonly known as "lethal synthesis," as shown in Figure 5.3. The resulting fluorocitrate is toxic because it is inhibitory to aconitase, the enzyme responsible for the conversion of citrate into czs-aconitate and then into isocitrate in the tricarboxylic acid cycle. Inhibition of aconitase results in citrate accumulation. The cycle stops for lack of metabolites, leading to disruption of energy metabolism. 

A powerful competitive inhibitor (highly toxic) of aconitate dehydratase is fluorocitrate, an analogue of citrate ... 

Fluoroacetic acid has been identifled as the toxic component of the South African plant gijblaar (Dichapetalum cymosum) [34]. Its mechanism of action is based on inhibition of the citric acid cycle, the main source of metabolic energy in all animals [35]. In this cyde, fluoroacetate can replace acetate as a substrate of aconi-tase, an enzyme complex which usually forms dtrate by addition of acetate to a-oxoglutarate. The resulting fluorocitrate is binds tightly to the enzyme, but cannot be further converted to ds-aconitate and isocitrate [36], thus inhibiting aconitase. 

Aconite is sometimes added to liquors in India to increase their intoxicating effect, and this has been known to lead to fatal poisoning. Numerous species of Aconitum occur in various parts of the world, varying considerably in their toxicity and alkaloidal content. Poisoning in livestock is not common, though horses have been known to crop the plant (238, 239) and cattle have been poisoned by plants thrown out of the garden in autumn (24). 

Long chain hydrocarbons and fatty acids are best known as constitutents of waxes and lipophilic compounds. Some representatives of this natural product group show high antiprotozoal activity but mostly combined with a high levels of toxicity to mammalian cells. One example is traws-aconitic acid (2) that was used in combination with sodium stibogluconate, allopurinol, or pentamidine for experimental visceral leishmaniasis to determine synergistic effects [34]. When these three drugs (50, 15, 8 mM/kg/day, respectively) were used with trans-aconitic acid (5 mM/kg/day) the parasite load in BALB/c mice was inhibited by 100, 88, and 100%, respectively. At tested concentration trans-aconitic acid itself showed an inhibition of 59.2 %. 

Aconite (monkshood, wolfsbane) Analgesic (topical and oral) Cardiac and CNS toxicity with oral use... 

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