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Cucurbitacin
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Cucurbitacin

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Cucurbita-5-ene with standard carbon numbering.
3D structure of cucurbitacin E as found in the crystal structure.

Cucurbitacin is a class of biochemical compounds that some plants – notably members of the pumpkin and gourd family, Cucurbitaceae – 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.

Most cucurbitacins are tetracyclic except some have an extra ring due to formal cyclization between C16 and C24 as in cucurbitacin S and cucurbitacin T.

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 and zucchinis. In laboratory research, cucurbitacins have cytotoxic properties and are under study for their potential biological activity.

Biosynthesis

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) 

Variants

The cucurbitacins include:

Cucurbitacin A

Cucurbitacin A

Cucurbitacin B

Cucurbitacin B

Cucurbitacin C

Cucurbitacin D

Cucurbitacin D

Cucurbitacin E

Cucurbitacin F

Cucurbitacin G

Cucurbitacin H

Cucurbitacin I

Cucurbitacin I

Cucurbitacin J

Cucurbitacin K

Cucurbitacin L

Cucurbitacin O

Cucurbitacin P

Cucurbitacin Q

Cucurbitacin Q

Cucurbitacin R

Cucurbitacin S

Cucurbitacin T

28/29 Norcucurbitacins

There are several substances that can be seen as deriving 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.

Other

Several other cucurbitacins have been found in plants.

Occurrence and bitter taste

Fruit and flower of the Ecballium elaterium, also called the squirting cucumber or exploding cucumber containing cucurbitacin B

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.

Cucurbitacins impart a bitter taste in plant foods such as cucumber, zucchini, melon and pumpkin.

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 cucurbitacins 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.

See also


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