Aconitine is an alkaloid toxin produced by various plant species belonging to the genus Aconitum (family Ranunculaceae), commonly known by the names wolfsbane and monkshood. Aconitine is notorious for its toxic properties.
Structure and reactivity Biologically active isolates from Aconitum and Delphinium plants are classified as norditerpenoid alkaloids,Biogenetically, aconitine is not a 'true' alkaloid, as it is not ultimately derived from amino acids. Aconitine is ultimately derived from isoprene, so it is technically a terpenoid and a pseudoalkaloid. which are further subdivided based on the presence or absence of the C18 carbon. Aconitine is a C19-norditerpenoid, based on its presence of this C18 carbon. It is barely soluble in water, but very soluble in organic solvents such as chloroform or diethyl ether. Aconitine is also soluble in mixtures of alcohol and water if the concentration of alcohol is high enough.
Like many other alkaloids, the basic nitrogen atom in one of the six-membered ring structures of aconitine can easily form salts and ions, giving it affinity for both polar and lipophilic structures (such as cell membranes and receptors) and making it possible for the molecule to pass the blood–brain barrier. The acetoxyl group at the c8 position can readily be replaced by a methoxy group, by heating aconitine in methanol, to produce 8-deacetyl-8-O-methyl derivatives. If aconitine is heated in its dry state, it undergoes pyrolysis to form pyroaconitine ((1α,3α,6α,14α,16β)-20-ethyl-3,13-dihydroxy-1,6,16-trimethoxy-4-(methoxymethyl)-15-oxoaconitan-14-yl benzoate) with the chemical formula C32H43NO9.
Mechanism of action Aconitine can interact with the voltage-dependent sodium-ion channels, which are transmembrane proteins in the cell membranes of excitable tissues, such as cardiac and skeletal muscles and neurons, resulting in the cells becoming non-excitable, and thus, paralysed. These proteins are highly selective for sodium ions. They open to quickly depolarize the cell membrane, causing the upstroke of an action potential. Normally, the sodium channels close again very rapidly, but the action potential causes the opening of potassium channels and potassium efflux, which results in repolarization of the membrane.
Aconitine binds to the channel at the neurotoxin binding site 2 on the alpha subunit (the same site bound by batrachotoxin, veratridine, and grayanotoxin). This binding results in a sodium-ion channel that stays open longer. Aconitine suppresses the conformational change in the sodium-ion channel from the active state to the inactive state. The membrane stays depolarized due to the constant sodium influx (which is 10–1000-fold greater than the potassium efflux). As a result, the membrane cannot be repolarized. The binding of aconitine to the channel also leads to the channel to change conformation from the inactive state to the active state at a more negative voltage. In neurons, aconitine increases the permeability of the membrane for sodium ions, resulting in a huge sodium influx in the axon terminal. As a result, the membrane depolarizes rapidly. Due to the strong depolarization, the permeability of the membrane for potassium ions increases rapidly, resulting in a potassium reflux to release the positive charge out of the cell. Not only the permeability for potassium ions but also the permeability for calcium ions increases as a result of the depolarization of the membrane. A calcium influx takes place. The increase of the calcium concentration in the cell stimulates the release of the neurotransmitter acetylcholine into the synaptic cleft. Acetylcholine binds to acetylcholine receptors at the postsynaptic membrane to open the sodium-channels there, generating a new action potential.
Research with mouse nerve-hemidiaphragm muscle preparation indicate that at low concentrations (<0.1 μM) aconitine increases the electrically evoked acetylcholine release causing an induced muscle tension. Action potentials are generated more often at this concentration. At higher concentration (0.3–3 μM) aconitine decreases the electrically evoked acetylcholine release, resulting in a decrease in muscle tension. At high concentration (0.3–3 μM), the sodium-ion channels are constantly activated, transmission of action potentials is suppressed, leading to non-excitable target cells or paralysis.
Biosynthesis and total synthesis of related alkaloids Aconitine is biosynthesized by the monkshood plant via the terpenoid biosynthesis pathway (MEP chloroplast pathway). Approximately 700 naturally occurring C19-diterpenoid alkaloids have been isolated and identified, but the biosynthesis of only a few of these alkaloids are well understood.
Likewise, only a few alkaloids of the aconitine family have been synthesized in the laboratory. In particular, despite over one hundred years having elapsed since its isolation, the prototypical member of its family of norditerpenoid alkaloids, aconitine itself, represents a rare example of a well-known natural product that has yet to succumb to efforts towards its total synthesis. The challenge that aconitine poses to synthetic organic chemists is due to both the intricate interlocking hexacyclic ring system that makes up its core and the elaborate collection of oxygenated functional groups at its periphery. A handful of simpler members of the aconitine alkaloids, however, have been prepared synthetically. In 1971, the Weisner group discovered the total synthesis of talatisamine (a C19-norditerpenoid). In the subsequent years, they also discovered the total syntheses of other C19-norditerpenoids, such as chasmanine, and 13-deoxydelphonine.
The total synthesis of napelline (Scheme a) begins with aldehyde 100. In a 7 step process, the A-ring of napelline is formed (104). It takes another 10 steps to form the lactone ring in the pentacyclic structure of napelline (106). An additional 9 steps creates the enone-aldehyde 107. Heating in methanol with potassium hydroxide causes an aldol condensation to close the sixth and final ring in napelline (14). Oxidation then gives rise to diketone 108 which was converted to (±)-napelline (14) in 10 steps.
A similar process is demonstrated in Karel Wiesner's synthesis of 13-desoxydelphinone (Scheme c). The first step of this synthesis is the generation of a conjugated dienone 112 from 111 in 4 steps. This is followed by the addition of a benzyl vinyl ether to produce 113. In 11 steps, this compound is converted to ketal 114. The addition of heat, DMSO and o-xylene rearranges this ketol (115), and after 5 more steps (±)-13-desoxydelphinone (15) is formed.
Lastly, talatisamine (Scheme d) is synthesized from diene 116 and nitrile 117. The first step is to form tricycle 118 in 16 steps. After another 6 steps, this compound is converted to enone 120. Subsequently, this allene is added to produce photoadduct 121. This adduct group is cleaved and rearrangement gives rise to the compound 122. In 7 steps, this compound forms 123, which is then rearranged, in a similar manner to compound 114, to form the aconitine-like skeleton in 124. A racemic relay synthesis is completed to produce talatisamine (13).
More recently, the laboratory of the late David Y. Gin completed the total syntheses of the aconitine alkaloids nominine and neofinaconitine.
Metabolism
Aconitine is metabolized by cytochrome P450 isozymes (CYPs). There has been research in 2011 in China to investigate in-depth the CYPs involved in aconitine metabolism in human liver microsomes. It has been estimated that more than 90 percent of currently available human drug metabolism can be attributed to eight main enzymes (CYP 1A2, 2C9, 2C8, 2C19, 2D6, 2E1, 3A4, 3A5). The researchers used recombinants of these eight different CYPs and incubated it with aconitine. To initiate the metabolism pathway the presence of NADPH was needed. Six CYP-mediated metabolites (M1–M6) were found by liquid chromatography, these six metabolites were characterized by mass-spectrometry. The six metabolites and the involved enzymes are summarized in the following table: Metabolite Name Involved CYPs M1 O-Demethyl-aconitine CYP3A4, CYP3A5, CYP2D6, CYP2C8 M2 16-O-Demethyl-aconitine CYP3A4, CYP3A5, CYP2D6, CYP2C9 M3 N-deethyl-aconitine CYP3A4, CYP3A5, CYP2D6, CYP2C9 M4 O-didemethyl-aconitine CYP3A5, CYP2D6 M5 3-Dehydrogen-aconitine CYP3A4, CYP3A5 M6 Hydroxyl-aconitine CYP3A5, CYP2D6
Selective inhibitors were used to determine the involved CYPs in the aconitine metabolism. The results indicate that aconitine was mainly metabolized by CYP3A4, 3A5 and 2D6. CYP2C8 and 2C9 had a minor role to the aconitine metabolism, whereas CYP1A2, 2E1 and 2C19 did not produce any aconitine metabolites at all. The proposed metabolic pathways of aconitine in human liver microsomes and the CYPs involved to it are summarized in the table above.
Uses Aconitine was previously used as an antipyretic and analgesic and still has some limited application in herbal medicine, although the narrow therapeutic index makes calculating appropriate dosage difficult.
In Ming China, iron tools with aconitine residue from the tomb of a doctor named Xia Quan (1348–1411 CE) suggests the use of wolfsbane as anesthetic. Aconitine is also present in Yunnan Baiyao, a proprietary traditional Chinese medicine.
Toxicity Consuming as little as 2 milligrams of pure aconitine or 1 gram of the plant itself may cause death by paralyzing respiratory or heart functions. Toxicity may occur through the skin; even touching the flowers can numb finger tips.
The toxic effects of aconitine have been tested in a variety of animals, including mammals (dog, cat, guinea pig, mouse, rat and rabbit), frogs and pigeons. Depending on the route of exposure, the observed toxic effects were local anesthetic effect, diarrhea, convulsions, arrhythmias or death. According to a review of different reports of aconite poisoning in humans, the following clinical features were observed:
Neurological: paresthesia and numbness of face, perioral area and four limbs; muscle weakness in four limbs Cardiovascular: hypotension, palpitations, chest pain, bradycardia, sinus tachycardia, ventricular ectopics and other arrhythmias, ventricular arrhythmias, and junctional rhythm Gastrointestinal: nausea, vomiting, abdominal pain, and diarrhea Others: dizziness, hyperventilation, sweating, difficulty breathing, confusion, headache, and lacrimation
Progression of symptoms: the first symptoms of aconitine poisoning appear approximately 20 minutes to 2 hours after oral intake and include paresthesia, sweating and nausea. This leads to severe vomiting, colicky diarrhea, intense pain and then paralysis of the skeletal muscles. Following the onset of life-threatening arrhythmia, including ventricular tachycardia and ventricular fibrillation, death finally occurs as a result of respiratory paralysis or cardiac arrest.
values for mice are 1 mg/kg orally, 0.100 mg/kg intravenously, 0.270 mg/kg intraperitoneally and 0.270 mg/kg subcutaneously. The lowest published lethal dose (LDLo) for mice is 1 mg/kg orally and 0.100 mg/kg intraperitoneally. The lowest published toxic dose (TDLo) for mice is 0.0549 mg/kg subcutaneously. LD50 value for rats is 0.064 mg/kg intravenously. The LDLo for rats is 0.040 mg/kg intravenously and 0.250 mg/kg intraperitoneally. The TDLo for rats is 0.040 mg/kg parenterally. For an overview of more test animal results (LD50, LDLo and TDLo) see the following table.
Species observed Type of test Route of exposure Dose data (mg/kg) Toxic effects Human LDLo Oral 0.028 Behavioral: excitement Gastrointestinal: hypermotility, diarrhea