Though it is best known as achemical compound in the spiritabsinthe, it is only present in trace amounts and is unlikely to be responsible for the spirit's purported stimulant and psychoactive effects.[5][6][7]
Thujone acts on theGABAA receptor as an antagonist. As a competitive antagonist of GABAA receptor, thujone alone is considered to beconvulsant,[8] though by interfering with the inhibitory transmitter GABA, it may convey stimulating, mood-elevating effects at low doses.[citation needed] It is also found in perfumery as a component of several essential oils.[citation needed]
In addition to the naturally occurring (−)-α-thujone and (+)-β-thujone, two other forms are possible:(+)-α-thujone and(−)-β-thujone. In 2016, they were found in nature as well,[9] inSalvia officinalis.
Thujone is found in a number of plants, such asarborvitae (genusThuja, hence the derivation of the name),Nootka cypress, somejunipers,mugwort,oregano,common sage (sage oil contain more than 20% from alpha and beta thujone),tansy, andwormwood, most notably grand wormwood (Artemisia absinthium), usually as a mix of isomers in a 1:2 ratio. It is also found in various species ofMentha (mint).
The reactions that generate the thujone skeleton insabinene from GPP are mediated by the enzymesabinene synthase which has GPP as its substrate.[10] GPP (1) first isomerizes to linalyl diphosphate (LPP) (2) and neryl diphosphate (NPP) (3). LPP preferentially forms a delocalized allylic cation-diphosphate (4). The ion-pair intermediate then cyclizes in an electrophilic addition to yield the α-terpinyl tertiary cation (5).[10]
The conversion of GPP to alpha-terpinyl cation.
The α-terpinyl cation (5) then undergoes a 1,2 hydride shift via aWagner–Meerwein rearrangement, leading to the formation of the terpinen-4-yl cation (6). This cation undergoes a second cyclization to form the thujyl cation intermediate (7) before loss of a proton to form the thujone precursor,(+)-sabinene (8).
The conversion of alpha-terpinyl cation to (+)-sabinene
From(+)-sabinene (8), the proposed biosynthetic route to generate thujone follows a three-step pathway: (+)-sabinene is first oxidized to an isomer of (+)-sabinol (9-1,9-2) by acytochrome P450 enzyme, followed by conversion to (+)-sabinone (10) via adehydrogenase. Finally, areductase mediates the conversion to α-thujone (11-1) and β-thujone (11-2).[12] The isomerism of the (+)-sabinol intermediate varies among thujone-producing plants; for instance, in thewestern redcedar (Thuja plicata), thujone is derived exclusively from the (+)-trans-sabinol intermediate (9-1) whereas in thecommon garden sage (Salvia officinalis), thujone is formed from the (+)-cis-sabinol intermediate (9-2).[13]
Based on a hypothesis that considered only molecular shape, it was speculated that thujone might act similarly toTHC on thecannabinoid receptors;[14] however, thujone failed to evoke a cannabimimetic response in a 1999 investigative study.[15] Thujone is aGABAA receptor antagonist[16] and, more specifically, aGABAA receptor competitive antagonist. By inhibiting GABA receptor activation, neurons may fire more easily, which can cause muscle spasms and convulsions.[17] This interaction with theGABAA receptor is specific to alpha-thujone.[18] Thujone is also a5-HT3 antagonist.[19][20]
Themedian lethal dose, or LD50, of α-thujone, the more active of the two isomers, in mice, is around 45 mg/kg, with 0% mortality rate at 30 mg/kg and 100% at 60 mg/kg. Mice exposed to the higher dose haveconvulsions that lead to death within 1 minute. From 30 to 45 mg/kg, the mice experience muscle spasms in the legs, which progress to general convulsions until death or recovery. These effects are in line with other GABA antagonists. Also, α-thujone is metabolized quickly in the liver in mice.[17] Pretreatment with GABA positive allosteric modulators likediazepam,phenobarbital, or 1 g/kg ofethanol protects against a lethal dose of 100 mg/kg.[citation needed]
Attention performance has been tested with low and high doses of thujone in alcohol. The high dose had a short-term negative effect on attention performance. The lower dose showed no noticeable effect.[5]
Thujone is reported[by whom?] to be toxic to brain, kidney, and liver cells and could cause convulsions if used in too high a dose. Other thujone-containing plants such as the treearborvitae (Thuja occidentalis) are used in herbal medicine, mainly for their alleged immune-system stimulating effects.[citation needed] Side effects from theessential oil of this plant include anxiety, sleeplessness, and convulsions, which confirms the central nervous system effects of thujone.[8][21]
Thujone is most commonly known for being a compound in the spiritabsinthe. In the past, absinthe was thought to contain up to 260–350 mg/L thujone,[22] but modern tests have shown this estimate to be far too high. A 2008 study of 13 pre-ban (1895–1910) bottles usinggas chromatography–mass spectrometry (GC-MS) found that the bottles had between 0.5 and 48.3 mg/L and averaged 25.4 mg/L[6][23] A 2005 study recreated three 1899 high-wormwood recipes and tested with GC–MS, and found that the highest contained 4.3 mg/L thujone.[24] GC–MS testing is important in this capacity becausegas chromatography alone may record an inaccurately high reading of thujone as other compounds may interfere with and add to the apparent measured amount.[25]
The compound was discovered after absinthe became popular in the mid-19th century.Valentin Magnan, who studied alcoholism, tested pure wormwood oil on animals and discovered it causedseizures independent from the effects of alcohol. On this basis, absinthe, which contains a small amount of wormwood oil, was assumed to be more dangerous than ordinary alcohol. Eventually, thujone was isolated as the cause of these reactions. Magnan went on to study 250 abusers of alcohol and noted that those who drank absinthe had seizures andhallucinations. The seizures are caused by the(+)-α-thujone interacting with the GABA receptors, causing epileptic activity.[18] In light of modern evidence, these conclusions are questionable, as they are based on a poor understanding of other compounds and diseases,[26] and clouded by Magnan's belief that alcohol and absinthe were degenerating the French race.[27]
After absinthe was banned, research dropped off until the 1970s, when the British scientific journalNature published an article comparing the molecular shape of thujone totetrahydrocannabinol (THC), the primarypsychoactive substance found incannabis, and hypothesized it would act the same way on the brain, sparking the myth that thujone was acannabinoid.[14][28]
More recently, following European Council Directive No. 88/388/EEC (1988) allowing certain levels of thujone in foodstuffs in the EU,[29] the studies described above were conducted and found only minute levels of thujone in absinthe.
In the United States, the addition of pure thujone to foods is not permitted.[32] Foods or beverages that containArtemisia species,white cedar,oakmoss,tansy, oryarrow, must be thujone-free,[33] which in practice means that they contain less than 10 parts per million thujone.[34] Other herbs that contain thujone have no restrictions. For example,sage andsage oil (which can be up to 50% thujone) are on theFood and Drug Administration's list ofgenerally recognized as safe (GRAS) substances.[35]
Absinthe offered for sale in the United States must be thujone-free by the same standard that applies to other beverages containingArtemisia,[34] so absinthe with small amounts of thujone may be legally imported.
In Canada, liquor laws are the domain of the provincial governments. Alberta, Ontario, and Nova Scotia allow 10 mg/kg thujone; Quebec allows 15 mg per kg;[citation needed] Manitoba allows 6–8 mg thujone per litre; British Columbia adheres to the same levels as Ontario. However, in Saskatchewan and Quebec, one can purchase any liquor available in the world upon the purchase of a maximum of one case, usually 12 750-ml bottles or 9 L. The individual liquor boards must approve each product before it may be sold on shelves.
^Williams, Jack D.; Yazarians, Jessica A.; Almeyda, Chelcie C.; Anderson, Kristin A.; Boyce, Gregory R. (23 May 2016). "Detection of the Previously Unobserved Stereoisomers of Thujone in the Essential Oil and Consumable Products of Sage (Salvia officinalis L.) Using Headspace Solid-Phase Microextraction–Gas Chromatography–Mass Spectrometry".Journal of Agricultural and Food Chemistry.64 (21):4319–4326.Bibcode:2016JAFC...64.4319W.doi:10.1021/acs.jafc.6b01065.PMID27181395.
^Umlauf, Dirk; Zapp, Josef (September 2004). "Biosynthesis of the irregular monoterpene artemisia ketone, the sesquiterpene germacrene D and other isoprenoids inTanacetum vulgare L. (Asteraceae)".Phytochemistry.65 (17):2463–2470.Bibcode:2004PChem..65.2463U.doi:10.1016/j.phytochem.2004.08.019.PMID15381410.
^Del Castillo J.; Anderson M.; Rubottom G.M. (1975). "Letters to Nature: Marijuana, absinthe and the central nervous system".Nature.253 (5490):365–366.doi:10.1038/253365a0.PMID1110781.S2CID4245058.
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