Cucurbitacin


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-abeo-10α-lanost-5-ene. They often occur as glycosides. They and their derivatives have been found in many plant families, in some mushrooms 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.

Biosynthesis

The biosynthesis of cucurbitacin C has been described. Zhang et al. 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 and Bt 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 gene. Nonbitterness of cultivated cucumber fruit is conferred by bt, an allele selected during domestication. Bi is a member of the oxidosqualene cyclase gene family. Phylogenetic analysis showed that Bi is the ortholog of cucurbitadienol synthase gene CPQ in squash

Variants

The cucurbitacins include:

Cucurbitacin A

There are several substances that can be seen as derving from cucurbita-5-ene skeleton by loss of one of the methyl groups attached to carbon 4; often with the adjacent ring becoming aromatic.

Other

Several other cucurbitacins have been found in plants.

Occurrence and bitter taste

Constituents of the colocynth fruit and leaves 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 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.