Thujone is a ketone and a monoterpene that occurs naturally in two diastereomeric forms: -α-thujone and -β-thujone. It has an odor like that of menthol, which is found in mint. Though it is best known as a chemical compound in the spirit absinthe, it is unlikely to be responsible for absinthe's alleged stimulant and psychoactive effects due to the small quantities present. Thujone acts on GABA as an antagonist. As a competitive antagonist of GABA, 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 used 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.
Biosynthesis
The biosynthesis of thujone is similar to the synthesis of other monoterpenes and begins with the formation of geranyl diphosphate from Dimethylallyl pyrophosphate and isopentenyl diphosphate, catalyzed by the enzyme geranyl diphosphate synthase. Quantitative 13CNMR spectroscopic analysis has demonstrated that the isoprene units used to form thujone in plants are derived from the methylerythritol phosphate pathway. The reactions that generate the thujane skeleton in sabinene from GPP are mediated by the enzyme sabinene synthase which has GPP as its substrate. GPP first isomerizes to linalyl diphosphate and neryl diphosphate . LPP preferentially forms a delocalized allylic cation-diphosphate. The ion-pair intermediate then cyclizes in an electrophilic addition to yield the α-terpinyl tertiary cation. The α-terpinyl cation then undergoes a 1,2 hydride shift via a Wagner–Meerwein rearrangement, leading to the formation of the terpinen-4-yl cation. This cation undergoes a second cyclization to form the thujyl cation intermediate before loss of a proton to form the thujone precursor, -sabinene. From -sabinene, the proposed biosynthetic route to generate thujone follows a three-step pathway: -sabinene is first oxidized to an isomer of -sabinol by a cytochrome P450 enzyme, followed by conversion to -sabinone via a dehydrogenase. Finally, a reductase mediates the conversion to α-thujone and β-thujone. The isomerism of the -sabinol intermediate varies among thujone-producing plants; for instance, in the western redcedar, thujone is derived exclusively from the -trans-sabinol intermediate whereas in the common garden sage, thujone is formed from the -cis-sabinol intermediate.
Pharmacology
Based on studies that looked only at molecular shape, for many years thujone was thought to act similarly to THC on the cannabinoid receptors; however, this has since been proven false. Thujone is a GABAAreceptor antagonist and more specifically, a GABAA receptor competitive antagonist. By inhibiting GABA receptor activation, neurons may fire more easily, which can cause muscle spasms and convulsions. Thujone is also a 5-HT3 antagonist. The median lethal dose, or LD50, of alpha-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, alpha-thujone is metabolized quickly in the liver in mice. Pretreatment with GABA positive allosteric modulators like diazepam, phenobarbital, or 1 g/kg of ethanol protects against a lethal dose of 100 mg/kg. 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 arborvitae are used in herbal medicine, mainly for their 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 famous for being a compound in the spirit absinthe. In the past, absinthe was thought to contain up to 260–350 mg/l thujone, but modern tests have shown this estimate to be far too high. A 2008 study of 13 pre-ban bottles using gas chromatography-mass spectrometry found that the bottles had between 0.5 and 48.3 mg/l and averaged 25.4 mg/l 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. 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. Dr. Valentin Magnan, who studied alcoholism, tested pure wormwood oil on animals and discovered it caused seizures independent from the effects of alcohol. Based on this, 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. 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. 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, 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 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 mg/kg in food prepared with Artemisia species, excluding those prepared with sage and non alcoholic beverages
10 mg/kg in alcoholic beverages not prepared with Artemisia species
25 mg/kg in food prepared with 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, oak moss, tansy, or yarrow, must be thujone-free, which in practice means that they contain less than 10 mg/l thujone. Other herbs that contain thujone have no restrictions. For example, sage and sage oil are on the Food and Drug Administration's list of generally recognized as safe substances. Absinthe offered for sale in the United States must be thujone-free by the same standard that applies to other beverages containing Artemisia, so absinthe with small amounts of thujone may be legally imported.
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.
