Cucurbitacin is any of a class of biochemical compounds that some plants — notably members of the family Cucurbitaceae, which includes the common pumpkins and gourds — produce and which function as a defence against herbivores. Cucurbitacins are chemically classified as triterpenes, formally derived from cucurbitane, a triterpene hydrocarbon—specifically, from the unsaturated variant cucurbit-5-ene, or 19(10→9β)-abeo-10α-lanost-5-ene. They often occur as glycosides. They and their derivatives have been found in many plant families (including Brassicaceae, Cucurbitaceae, Scrophulariaceae, Begoniaceae, Elaeocarpaceae, Datiscaceae, Desfontainiaceae, Polemoniaceae, Primulaceae, Rubiaceae, Sterculiaceae, Rosaceae, and Thymelaeaceae), in some mushrooms (including Russula and Hebeloma) and even in some marine mollusks.
Cucurbitacins may be a taste deterrent in plants foraged by some animals and in some edible plants preferred by humans, like cucumbers. In laboratory research, cucurbitacins have cytotoxic properties and are under study for their potential biological activities.
The biosynthesis of cucurbitacin C has been described. Zhang et al. (2014) identified nine cucumber genes in the pathway for biosynthesis of cucurbitacin C and elucidated four catalytic steps. These authors also discovered the transcription factors Bl (Bitter leaf) and Bt (Bitter fruit) that regulate this pathway in leaves and fruits, respectively. The Bi gene confers bitterness to the entire plant and is genetically associated with an operon-like gene cluster, similar to the gene cluster involved in thalianol biosynthesis in Arabidopsis. Fruit bitterness requires both Bi and the dominant Bt (Bitter fruit) gene. Nonbitterness of cultivated cucumber fruit is conferred by bt, an allele selected during domestication. Bi is a member of the oxidosqualene cyclase (OSC) gene family. Phylogenetic analysis showed that Bi is the ortholog of cucurbitadienol synthase gene CPQ in squash (Cucurbita pepo) 
The cucurbitacins include:
- Cucurbitacin A found in some species of Cucumis :1
- Cucurbitacin B from Hemsleya endecaphylla (62 mg/72 g):4 and other plants (e.g. Cucurbita andreana); anti-inflammatory, any-hepatotoxic :2
- Cucurbitacin B 2-O-glucoside, from Begonia heracleifolia :3
- 23,24-Dihydrocucurbitacin B from Hemsleya endecaphylla, 49 mg/72 g:5
- 23,24-Dihydrocucurbitacin B 2-O-glucoside from roots of Picrorhiza kurrooa :4
- Deacetoxycucurbitacin B 2-O-glucoside from roots of Picrorhiza kurrooa :5
- Isocucurbitacin B, from Echinocystis fabacea :6
- 23,24-Dihydroisocucurbitacin B 3-glucoside from Wilbrandia ebracteata :7
- 23,24-Dihydro-3-epi-isocucurbitacin B, from Bryonia verrucosa :8
- Pentanorcucurbitacin B or 3,7-dioxo-23,24,25,26,27-pentanorcucurbit-5-en-22-oic acid, C
4, white powder:2
- Cucurbitacin D, from Trichosanthes kirilowii and many other plants  (e.g. Cucurbita andreana):12
- 3-Epi-isocucurbitacin D, from species of Physocarpus :14 and Phormium tenax
- 22-Deoxocucurbitacin D from Hemsleya endecaphylla, 14 mg/72 g:6
- 23,24-Dihydrocucurbitacin D from Trichosanthes kirilowii :13 also from H. endecaphylla, 80 mg/72 g:3
- 23,24-Dihydro-epi-isocucurbitacin D, from Acanthosicyos horridus :20
- 22-Deoxocucurbitacin D from Wilbrandia ebracteata :21
- Anhydro-22-deoxo-3-epi-isocucurbitacin D from Ecballium elaterium :22
- 25-O-Acetyl-2-deoxycucurbitacin D (amarinin) from Luffa amara :24
- 2-Deoxycucurbitacin D, from Sloanea zuliaensis :23
- Cucurbitacin E (α-Elaterin), from roots of Wilbrandia ebracteata. Strong antifeedant for the flea beetle, inhibits cell adhesion  (also in e.g. Cucurbita andreana):27
- 22,23-Dihydrocucurbitacin E from Hemsleya endecaphylla, 9 mg/72 g,:8 and from roots of Wilbrandia ebracteata :28
- 22,23-Dihydrocucurbitacin E 2-glucoside from roots of Wilbrandia ebracteata :29
- Isocucurbitacin E, from Cucumis prophetarum :30
- 23,24-Dihydroisocucurbitacin E, from Cucumis prophetarum :31
- Cucurbitacin F from Elaeocarpus dolichostylus :33
- Cucurbitacin F 25-acetate from Helmseya graciliflora :34
- 23,24-Dihydrocucurbitacin F from Helmseya amabilis :35
- 25-Acetoxy-23,24-dihydrocucurbitacin F from Helmseya amabilis (hemslecin A) :36
- 23,24-Dihydrocucurbitacin F glucoside from Helmseya amabilis :40
- Cucurbitacin II glucoside from Helmseya amabilis :41
- Hexanorcucurbitacin F from Elaeocarpus dolichostylus :43
- Perseapicroside A from Persea mexicana :44
- Scandenoside R9 from Hemsleya panacis-scandens :45
- 15-Oxo-cucurbitacin F from Cowania mexicana :46
- 15-oxo-23,24-dihydrocucurbitacin F from Cowania mexicana :47
- Datiscosides B, D, and H, from Datisca glomerata :48–50
- Cucurbitacin G from roots of Wilbrandia ebracteata :52
- 3-Epi-isocucurbitacin G, from roots of Wilbrandia ebracteata :54
- Cucurbitacin I (elatericin B) from Hemsleya endecaphylla, 10 mg/72 g,:7 also from Ecballium elaterium, Citrullus colocynthis, Cucurbita andreana, deters feeding by flea beetle :55
- Hexanorcucurbitacin I from Ecballium elaterium :56
- 23,24-Dihydrocucurbitacin I see Cucurbitacin L
- Khekadaengosides D and K from the fruits of Trichosanthes tricuspidata :57,58
- 11-Deoxocucurbitacin I, from Desfontainia spinosa :59
- Spinosides A and B, from Desfontainia spinosa :61,62
- 23,24-dihydro-11-Deoxocucurbitacin I from Desfontainia spinosa :60
- Cucurbitacin J from Iberis amara :69
- Cucurbitacin K, stereoisomer of cucurbitacin J,:2 from Iberis amara :70
- Cucurbitacin L, or 23,24-dihydrocucurbitacin I,:63 :1
- Cucurbitacin O from Brandegea bigelovii :73
- Cucurbitacin Q 2-O-glucoside, from Picrorhiza kurrooa :76
- 16-Deoxy-D-16-hexanorcucurbitacin O from Ecballium elaterium :77
- Deacetylpicracin from Picrorhiza scrophulariaeflora :78
- Deacetylpicracin 2-O-glucoside from Picrorhiza scrophulariaeflora :80
- 22-Deoxocucurbitacin O from Wilbrandia ebracteata :83
- Cucurbitacin P from Brandegea bigelovii :74
- Cucurbitacin Q from Brandegea bigelovii :75
There are several substances that can be seen as derving from cucurbita-5-ene skeleton by loss of one of the methyl groups (28 or 29) attached to carbon 4; often with the adjacent ring (ring A) becoming aromatic.:87–130
Several other cucurbitacins have been found in plants.:152–156,164–165
Occurrence and bitter taste
Constituents of the colocynth fruit and leaves (Citrullus colocynthis) include cucurbitacins. The 2-O-β-D-glucopyranosides of cucurbitacins K and L can be extracted with ethanol from fruits of Cucurbita pepo cv dayangua. Pentanorcucurbitacins A and B can be extracted with methanol from the stems of Momordica charantia. Cucurbitacins B and I, and derivatives of cucurbitacins B, D and E, can be extracted with methanol from dried tubers of Hemsleya endecaphylla.
Research and toxicity
Cucurbitacins are under basic research for their biological properties, including toxicity and potential pharmacological uses in development of drugs for inflammation, cancer, cardiovascular diseases, and diabetes, among others.
The toxicity associated with consumption of foods high in cucurbatincs is sometimes referred to as "toxic squash syndrome". In France in 2018, two women who ate soup made from bitter pumpkins became sick, involving nausea, vomiting, and diarrhea, and had hair loss weeks later. Another French study of poisoning from bitter squash consumption found similar acute illnesses and no deaths. The high concentration of toxin in the plants could result from cross-pollination with wild cucurbitaceae species, or from plant growth stress due to high temperature and drought.
Research on antitumor activity of cucurbitacin focuses on four main variants of this molecule. Cucurbitacins with the most prominent antitumor activity are B, D, E and I. Of these, cucurbitacin B and D are the most common in plants. The mechanisms by which it affects cancer cells are mainly inhibition of STAT3 signaling pathway, induction of apoptosis and cell cycle arrest. It also affects function of proteasome and inflammasome.
Pathologists found cucurbitacin in the stomach of a 79-year-old man who died in Baden-Württemberg, Germany, shortly after eating a casserole containing zucchini he had received from a neighbor.
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