Thujone () is a ketone and a monoterpene that occurs predominantly in two diastereomeric (epimeric) 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.

Thujone acts on the GABA<sub>A</sub> receptor as an antagonist. As a competitive antagonist of GABA<sub>A</sub> receptor, thujone alone is considered to be convulsant, though by interfering with the inhibitory transmitter GABA, it may convey stimulating, mood-elevating effects at low doses. It is also found in perfumery as a component of several essential oils.

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, in Salvia officinalis.

<gallery>

File:(-)-alpha-Thujon.svg|(−)-α-thujone

File:Alpha-(+)-Thujon.svg|(+)-α-thujone

File:Beta-(+)-Thujon.svg|(+)-β-thujone

File:Beta-(-)-Thujon.svg|(−)-β-thujone

</gallery>

Sources

Thujone is found in a number of plants, such as arborvitae (genus Thuja, hence the derivation of the name), Nootka cypress, some junipers, 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 <sup>13</sup>C NMR spectroscopic analysis has demonstrated that the isoprene units used to form thujone in plants are derived from the methylerythritol phosphate 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 isomerism of the (+)-sabinol intermediate varies among thujone-producing plants; for instance, in the western redcedar (Thuja plicata), thujone is derived exclusively from the (+)-trans-sabinol intermediate (9-1) whereas in the common garden sage (Salvia officinalis), thujone is formed from the (+)-cis-sabinol intermediate (9-2).

center|700px|Proposed synthesis of thujone from sabinene

Pharmacology

thumb|right|Research-grade thujone

Based on a hypothesis that considered only molecular shape, it was speculated that thujone might act similarly to THC on the cannabinoid receptors; however, thujone failed to evoke a cannabimimetic response in a 1999 investigative study. Thujone is a GABA<sub>A</sub> receptor antagonist and, more specifically, a GABA<sub>A</sub> receptor competitive antagonist. By inhibiting GABA receptor activation, neurons may fire more easily, which can cause muscle spasms and convulsions. This interaction with the GABA<sub>A</sub> receptor is specific to alpha-thujone. Thujone is also a 5-HT<sub>3</sub> antagonist.

The median lethal dose, or LD<sub>50</sub>, of α-thujone, the more active of the two isomers, in mice, is around 45&nbsp;mg/kg, with 0% mortality rate at 30&nbsp;mg/kg and 100% at 60&nbsp;mg/kg. Mice exposed to the higher dose have convulsions that lead to death within 1 minute. From 30 to 45&nbsp;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.

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&nbsp;mg/L thujone, 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&nbsp;mg/L and averaged 25.4&nbsp;mg/L 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.

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 hallucinations. The seizures are caused by the (+)-α-thujone interacting with the GABA receptors, causing epileptic activity. and clouded by Magnan's belief that alcohol and absinthe were degenerating the French race.

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, 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:

  • 0.5&nbsp;mg/kg in food prepared with Artemisia species, excluding those prepared with sage and non-alcoholic beverages
  • 10&nbsp;mg/kg in alcoholic beverages not prepared with Artemisia species
  • 25&nbsp;mg/kg in food prepared with sage
  • 35&nbsp;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, which in practice means that they contain less than 10&nbsp;parts per million thujone. Other herbs that contain thujone have no restrictions. For example, 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.

Absinthe offered for sale in the United States must be thujone-free by the same standard that applies to other beverages containing Artemisia,