Thujone ([Derived from the Ancient Greek θυία, thuj(a), a kind of Cedrus + -ωνη, -one, feminine patronymic for a chemical relative of acetone]) is a ketone and a terpene that occurs predominantly in two () forms: (−)-α-thujone and (+)-β-thujone.
Though it is best known as a chemical compound in the spirit absinthe, it is only present in trace amounts and is unlikely to be responsible for the spirit's purported stimulant and psychoactive effects.[ Absinthe Myths Finally Laid To Rest][ Chemical Composition of Vintage Preban Absinthe with Special Reference to Thujone, Fenchone, Pinocamphone, Methanol, Copper, and Antimony Concentrations]
Thujone acts on the GABAA receptor as an antagonist. As a competitive antagonist of GABAA receptor, thujone alone is considered to be convulsant,
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,
Sources
Thujone is found in a number of plants, such as
Thuja (genus
Thuja, hence the derivation of the name), Nootka cypress, some
,
mugwort,
oregano,
common sage (sage oil contain more than 20% from alpha and beta thujone),
tansy, and wormwood, most notably grand wormwood (
Artemisia absinthium), usually as a mix of isomers in a 1:2 ratio. It is also found in various species of
Mentha (mint).
Biosynthesis
The biosynthesis of thujone is similar to the synthesis of other monoterpenes and begins with the formation of geranyl diphosphate (GPP) from dimethylallyl pyrophosphate (DMAPP) and isopentenyl diphosphate (IPP), catalyzed by the enzyme geranyl diphosphate synthase.
Quantitative
13C NMR spectroscopic analysis has demonstrated that the
Isoprene rule used to form thujone in plants are derived from the
MEP pathway (MEP).
The reactions that generate the thujone skeleton in sabinene from GPP are mediated by the enzyme sabinene synthase which has GPP as its substrate.
The α-terpinyl cation ( 5) then undergoes a 1,2 hydride shift via a Wagner–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).
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 a cytochrome P450 enzyme, followed by conversion to (+)-sabinone ( 10) via a dehydrogenase. Finally, a reductase mediates the conversion to α-thujone ( 11-1) and β-thujone ( 11-2).
Pharmacology
Based on a hypothesis that considered only molecular shape, it was speculated that thujone might act similarly to THC on the
cannabinoid receptors;
[Conrad, Barnaby, III; (1988). Absinthe: History in a Bottle. Chronicle Books. p. 152] however, thujone failed to evoke a cannabimimetic response in a 1999 investigative study.
Thujone is a GABA
A receptor antagonist
and, more specifically, a
GABAA receptor. By inhibiting GABA receptor activation, neurons may fire more easily, which can cause muscle spasms and convulsions.
This interaction with the
GABAA receptor is specific to alpha-thujone.
Thujone is also a 5-HT
3 antagonist.
[ Modulation of Ionotropic GABA Receptors by Natural Products of Plant Origin]
The median 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 have convulsions 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.
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.
Thujone is reported 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 tree Thuja ( Thuja occidentalis) are used in herbal medicine, mainly for their alleged immune-system stimulating effects. Side effects from the essential oil of this plant include anxiety, sleeplessness, and convulsions, which confirms the central nervous system effects of thujone.
In absinthe
Thujone is most commonly known for being a compound in the spirit
absinthe. In the past, absinthe was thought to contain up to 260–350 mg/L thujone,
[ Absinthism: a fictitious 19th-century syndrome with present impact, Padosch et al. Retrieved Oct. 28, 2006.] but modern tests have shown this estimate to be far too high. A 2008 study of 13 pre-ban (1895–1910) bottles using gas chromatography–mass spectrometry (GC-MS) found that the bottles had between 0.5 and 48.3 mg/L and averaged 25.4 mg/L
[ Chemical Composition of Vintage Preban Absinthe with Special Reference to Thujone, Fenchone, Pinocamphone, Methanol, Copper, and Antimony Concentrations] 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.
[ Thujone—Cause of absinthism? Lachenmeier, Emmert et al. Retrieved Oct. 28, 2006.] GC–MS testing is important in this capacity because gas chromatography alone may record an inaccurately high reading of thujone as other compounds may interfere with and add to the apparent measured amount.
[ Determination of α-/β-Thujone and Related Terpenes in Absinthe using Solid Phase Extraction and Gas Chromatography , Emmert et al. Retrieved Oct. 28, 2006.]
History
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 caused seizures 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 and
. The seizures are caused by the
(+)-α-thujone interacting with the GABA receptors, causing epileptic activity.
In light of modern evidence, these conclusions are questionable, as they are based on a poor understanding of other compounds and diseases,
and clouded by Magnan's belief that alcohol and absinthe were degenerating the French race.
[Conrad III, Barnaby; (1988). Absinthe: History in a Bottle. Chronicle books. Pg. 101-105]
After absinthe was banned, research dropped off until the 1970s, when the British scientific journal Nature published an article comparing the molecular shape of thujone to tetrahydrocannabinol (THC), the primary psychoactive substance found in cannabis, and hypothesized it would act the same way on the brain, sparking the myth that thujone was a cannabinoid.
More recently, following European Council Directive No. 88/388/EEC (1988) allowing certain levels of thujone in foodstuffs in the EU,[ European Council Directive No. 88/388/EEC, 22 June 1988.] the studies described above were conducted and found only minute levels of thujone in absinthe.
Regulations
European Union
Maximum thujone levels in the EU are:
[ Regulation (EC) No 1334/2008 of the European Parliament and Council of 16 December 2008, European Commission.][ Opinion of the Scientific Committee on Food on Thujone Scientific Committee on Food (2003) Retrieved Oct 28, 2006.]
-
0.5 mg/kg in food prepared with Artemisia species, excluding those prepared with common sage and non-alcoholic beverages
-
10 mg/kg in alcoholic beverages not prepared with Artemisia species
-
25 mg/kg in food prepared with common sage
-
35 mg/kg in alcoholic beverages prepared with Artemisia species
United States
In the United States, the addition of pure thujone to foods is not permitted.
Foods or beverages that contain
Artemisia species,
white cedar,
oakmoss,
tansy, or
yarrow, must be thujone-free,
[ FDA Regulation 21 CFR 172.510 – Food Additives Permitted for Direct Addition to Food for Human Consumption. Food and Drug Administration (2003). Retrieved Oct 28, 2006.] which in practice means that they contain less than 10 parts per million thujone.
[ Department of the Treasury Alcohol and Tobacco Tax and Trade Bureau Industry Circular 2007-5 October 17, 2007. Retrieved May 5, 2009] Other herbs that contain thujone have no restrictions. For example,
Common sage and
sage oil (which can be up to 50% thujone) are on the Food and Drug Administration's list of generally recognized as safe (GRAS) substances.
[ Substances generally recognized as safe. Food and Drug Administration (2003). Retrieved Oct 28, 2006.]
Absinthe offered for sale in the United States must be thujone-free by the same standard that applies to other beverages containing Artemisia,
Canada
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; 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.
See also
-
Piołunówka – Polish alcoholic preparation with thujone content higher than in absinthe
Further reading
External links
