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Cucurbitacins in plant food

image of Cucurbitacins in plant food

Poisoning due to Cucurbitaceous vegetables seems to be linked to intake of immensely bitter vegetables. The bitter and toxic compounds in these vegetables are cucurbitacins, which are well known in wild varieties of these food plants and their related species. The cultivated forms, on the other hand, have during cultivation been selected for being free of the bitter and toxic compounds. Occasionally, cultivars of cucurbitaceous food plants (e.g. squash) back-mutate and regain the ability to produce toxic amounts of cucurbitacins. This review summarises the information available on cucurbitacins in food plants of the family Cucurbitaceae, with the aim to lay down background information required to evaluate the potential risk of being intoxicated by cucurbitacins as a part of the safety assessment of cucurbitaceous food plants, and especially in relation to genetically modified Cucurbitaceous plants.

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Biosynthesis

Balliano et al. (1983) have reviewed the biosynthesis of cucurbitacin glycosides from squalene-2,3-epoxide to the final cucurbitacin, pointing out the possible routes for biosynthesis (see Figure 4): “It has been suggested that cucurbitacins are biosynthesised from squalene-2,3-epoxide (1), through cyclization to the cat ion, 2, and subsequent transformation of a lanostane C-9 carbonium ion, 3, which could follow different routes (a, b, and c in Figure 4). Loss of the C-11 proton from 3 (route a, figure 4) could give parkeol (4) a tetracyclic triterpene found in Butyrospermum parkii. The migration of the C-10 methyl to C-9, induced by an electron deficiency at C-9, followed by hydrogen migration (H-5 → H-10) and elimination of a C-6 proton, will give the cucurbitane skeleton 5 (cucurbita- 5,24-dienol). Alternatively (route b), loss of a proton from the C-19 methyl group with closure of the 9ß,19-cyclopropane ring, could give cycloartenol (6), the supposed general precursor of phytosterols. Such an intermediate as 6, by opening of the 9ß,19-cyclopropane ring in concert with hydrogen migration and proton elimination, could lead to 5. The third possibility (route c) invokes the direct intermediary of cucurbita- 5,24-dienol (5), the simplest tetracyclic triterpene with a cucurbitane skeleton, present both in seed oil of gourd (Lagenaria leucantha var. Gourda) and in Bryonia dioica seedlings. Cucurbita-5,24- dienol (5) could be produced directly from 3 by multiple migration (Me-10 → Me-9, H-5 → H-10) together with the final elimination of the C-6 proton”.

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