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Thymoquinone is a phytochemical compound found in the plant Nigella sativa. It is also found in select cultivated Monarda fistulosa plants which can be steam distilled to produce an essential oil commonly known as black seed oil.

It has been classified as a pan-assay interference compound, which binds indiscriminately to many proteins. It is under preliminary research to identify its possible biological properties.

Biosynthesis
Thymoquinone is biosynthesized by plants through the terpene biosynthetic pathway as a secondary metabolite. It belongs to the class of monoterpenes which stem from condensation of two isoprene units. For thymoquinone, geranyl disphosphate cyclizes and is converted to "Gamma"-terpinene by gamma-terpinene synthase (during seed maturation). Cytochrome p450 monooxygenases (P450s) then oxidizes gamma-terpinene into p-cymene. Hydroxylation leads to the formation of carvacrol and further hydroxylation produces thymohydroquinone. Thymohydroquinone then undergoes oxidation to form thymoquinone.

An alternative biosynthetic pathway may occur if some species of the plant accumulate thymol instead of carvacrol, where p-cymene hydroxylates to thymol.

Chemical Synthesis
Thymoquinone was first extracted and isolated in 1963 by an Egyptian chemist Mostafa M. El-Dakhakhny.

Antioxidant, Anti-Inflammatory Activity
Thymoquinone is considered an anti-oxidant, which defends against reactive oxygen species (ROS) which are continuously produced in cells through aerobic metabolism. The presence of ROS induces oxidation of biomolecules which can be harmful to the host. Thymoquinone neutralizes the free radical-induced oxidative damage as an anti-oxidant enzyme in a variety of organs, including the liver, stomach, and kidney.

Inflammation in the body is mainly mediated by cyclooxygenase and lipoxygenase, enzymes that generate prostaglandins and leukotrienes. Thymoquinone has the ability to inhibit both enzymes in their pathways of arachidonic acid metabolism specifically in rats, causing anti-inflammatory effects.

Effects on the Respiratory System
Known for its effects on alleviating asthma, thymoquinone has been tested in its volatile oil form in guinea pigs and dogs. It was concluded that (i.m) or (i.p) injection of thymoquinone in doses of 200ul/kg induced bronchodilation, reversing the effects of histamine-induced bronchoconstriction. Thymoquinone also induced relaxation of subject's isolated trachea by inhibiting lipoxygenase products and by non-selective blocking of histamine and serotonin receptors.