Cucurbitacin

Cucurbitacin refers to a group of highly oxidized tetracyclic triterpenoids produced by plants in the family Cucurbitaceae. These compounds are best known for their intensely bitter taste and are widely distributed in cucumber, melon, squash, gourd and related species. In plants they function primarily as defense chemicals against herbivores and pathogens.

Cucurbitacins occur in various tissues, including fruits, seeds, and roots, with concentrations influenced by species, cultivar, environmental conditions, and plant stress. Many varieties accumulate higher cucurbitacin levels in roots or stems; the bitterness of fruit is a key selective trait in breeding programs.

Chemically, cucurbitacins are derived from a cucurbitane skeleton (C30 pentacyclic triterpenoid). They include aglycones and glycosides; common members include cucurbitacin B, D, E, and I, among others. They feature multiple hydroxylations and, in some cases, epoxide, lactone or ester functionalities.

Biosynthesis proceeds from squalene to 2,3-oxidosqualene, cyclization to cucurbitadienol/cucurbitane skeleton, followed by oxidative tailoring by cytochrome P450 enzymes and glycosylation in some species. In humans, cucurbitacins are at least mildly toxic; ingestion can cause severe gastrointestinal distress and liver injury in high doses. They have been studied for cytotoxic and anti-inflammatory properties, and research into anticancer activity has been ongoing, though no cucurbitacins are approved as medicines. In agriculture and horticulture, low-cucurbitacin varieties are developed to avoid bitterness.