Of cucurbitacin

One may conclude from these studies that phytoalexins have a selective effect on herbivores. They inhibit feeding and growth on some, but seem to be innocuous to others. Given the coevolutionary origin of plant defenses, it is conceivable that some phytoalexins may even be kairomones for some insects. There is ample precedence to this dual role of several allelochemics (1). The example of cucurbitacins mentioned above is one of them. 

Within the cucurbitacins and withanolides it has been found that certain structural analogues possess agonistic rather than antagonistic activity hexanorcucurbitacin D (EC50 = 2.5 x 10 5M) and withaperuvin D (EC50 = 2.5 x 10 5M) [170], In the case of hexanorcucurbitacin D, this concurs with the hypothesis that antagonistic activity of cucurbitacins is associated with the presence of an a,(3-unsaturated ketone in the side-chain [205], which hexanorcucurbitacin D lacks. 

Figure 9 Structures of cucurbitacins that act as feeding stimulants.

Cucurbitacin is a bitter-tasting principle that can be isolated from members of the family Cucurbitaceae, such as cucumber Cucumis sativus) and melon (Cucumis melo L.). In particular, cucurbitacin149 and momordicine,150 which have an intensely bitter taste, are contained abundantly in Momordica charantia (bitter melon in English, go-yaa in Okinawa, Japan), which people enjoy due to its bitterness. There are more than 18 kinds of cucurbitacin, and among them cucurbitacin B is a typical component. It has been reported that cucurbitacin exhibits anticancer activity.1 1 In addition, it is used for the treatment of hepatic disease in traditional Chinese medicine. It is also found in some herbal teas. 

Alfhough bearing a sterane nucleus, fhe biosyntheses of cucurbitacins and dammaran-type saponins will not be reviewed here. Both groups belong to the triterpenoids, the chemistry, biosynthesis and biological activities of which have been described elsewhere (Charlwood and Banthorpe, 1991 ... 

Begonia tuberhybrida It was readily prepared by sodium metaperiodate oxidation of cucurbitacin D diacetate (53). 

Isocucurbitacin D (41) and 3-epi-isocucurbitacin D (42) are new tumour-inhibitory compounds from the leaves of Phormium tenax. They may be artefacts of isolation since they are both formed by isomerization of cucurbitacin D (43) on silica gel. Dihydroisocucurbitacin B (44) has been isolated from Marah... 

A partially purified enzyme preparation which catalyses the hydroxylation of the C-19 methyl group of cucurbitacins has been isolated from the ripe fruit of Cucurbita maxima. Cucurbitacins E and I act as feeding inhibitors for the flea beetle in the green parts of Iberis amara. ... 

Bryoniae radix Bryony root Bryonia alba L. and B. cretica ssp. dioica PLANCH. Cucurbitaceae, MD Tetracyclic triterpenes cucurbitadn gliicosides 1,L,E and dihydro-cucurbitacins E,B and aglycones Bryonia alba and B. dioica qualitatively similar contents of cucurbitacins Fig. 11,12... 

The total contents of cucurbitacins are generally lower in the extracts 1-4 than in Tayuae (5) and Bryoniae radix extracts (6-8). 

A second example of this same NCI procedure resulted in the rapid identification of cucurbitacin D from a novel source (55). The pattern of cytotoxic activity observed from a crude extract of Gonystylus keithii against renal, brain, and melanoma subpanels correlated strongly with the pattern observed earlier for purified cucurbitacins B, E, and I (Scheme 5). This bioprofile indication led to the expeditious fractionation and identification of cucurbitacin D as the anticancer active component in the extract. 

P) and C-25, while a carbonyl is typically present at C-ll and C-22. On the other hand, C-2 and C-3 may be indistinctly substituted by a hydroxyl or carbonyl group. Other typical substitutions include a possible double bond at C-l, sometimes a hydroxyl at C-24, and rarely a hydroxyl at C-9. Moreover, the C-25 hydroxyl is sometimes acetylated [2]. The basic skeleton of cucurbitacins is shown in Fig. (1). 

Fig. (2). Relevant chemical modification in the general structures of cucurbitacins...

Thus, once the appropriate orientation within the corresponding enzyme occurs, steroids, cucurbitacins, or other triterpenes may be formed. In the case of cucurbitacins, the squalene epoxide adopts a chair-boat-chair conformation, and the cyclization leads to a protostane cation, which is the immediate precursor of the cucurbitanes through a series of 1,2-proton and methyl group shifts, Fig. (4) [40]. 

Thin-layer chromatography (TLC) is a good method for detecting the presence of cucurbitacins in a plant extract. The best results to date have been achieved using high-performance TLC (HPTLC) plates [41] with silica gel and chloroform-methanol (95 10) (Table 1) or toluene-ethyl acetate (25 75) as mobile phases [3,11]. Reversed-phase (RP) HPTLC plates with methanol-water (7 3) have also been used to separate cucurbitacins [3]. The compounds are easily detected with vanillin-sulphuric acid or vanillin-phosphoric acid reagents and ultraviolet light (UV) at 365 nm [11]. 

Table 1. Analytical separation of cucurbitacins by means of HPTLC (SiOi) and CHCI3-CH3OH (95 10) [11]...

Purification can be accomplished with different chromatographic protocols, but in most cases, open-column chromatography is used. For example, during the separation process of cucurbitacins from Cucurbita andreana, Jayaprakasam et al. [12] employed a medium pressure liquid chromatography (MPLC) system, using silica gel and mixtures of chloroform-acetone as eluent. A similar protocol was used by Peters et al. [27] during the isolation of the cucurbitacins from Wilbrandia ebracteata, but in this case the elution was performed first with mixtures of petroleum ether / ethyl acetate and then with ethyl acetate / i-propanol. 

Audier and Das [48] published a specific report on the MS fragmentation of cucurbitacins which exemplified how the use of modem MS techniques leads to a better understanding of molecule fragmentation. In a similar vein, Johnson et al. [49] reported the diagnostic fragmentation pathways that occur in cucurbitacins isolated from Fevillea cordifolia after using electron impact (El)-, chemical ionization (Cl)-, and fast-atom bombardment (FAB)-MS. 

Jacobs et al. [1] used two-dimensional NMR spectroscopy to assign the H and 13C chemical shifts of cucurbitacin B, establishing these values as a benchmark for analyzing the spectra of other cucurbitacins isolated from the same source. In fact, the characteristic groups present in all cucurbitacins are easily detectable by means of NMR and typically include the presence of carbonyls at C-ll and C-22, which appear between 212-213 ppm, as well as hydroxyls at C-16(a) and C-20( 3), which appear at approximately 71-73 ppm and 77-79 ppm, respectively. Other characteristic patterns of substitution such as the presence of acetoxyl groups at C-2 and C-25 have also been observed. In this case, the acetylation of the hydroxyl at C-2 modifies both the C-l and C-3 values. Indeed, when a carbonyl is present, this latter change involves a major shift, namely from 213 to 205 ppm. In the case of a second double... 

Some species containing cucurbitacins are used in traditional medicine as anti-inflammatories in different pathologies. In general, they are used in topical applications as they have a certain level of toxicity when applied per os (p.o.). In one paper, Miro [2] compiled the data on this subject that had been published up to 1995. The review cited only the antiinflammatory activity of cucurbitacin B isolated from the juice of Ecballium elaterium, as well as its possible mechanism of action by means of a modification in leukotriene B4 (LTB4) production. 

Several of the compounds cited above, e.g. cucurbitacins B, D, and E, together with cucurbitacin I, Fig. (6), were isolated from the fruits of Cucurbita andreana and their activity against COX-1 and COX-2 enzymes was studied. All of the compounds studied exerted a moderate, but significant inhibition of the induced form COX-2, but none of them inhibited the constitutive form COX-1. The range of activity at 100 pg/ml was 32% (cucurbitacin B), 29% (cucurbitacin D), 35% (cucurbitacin E), and 27% (cucurbitacin I), all of which are clearly inferior to the values obtained with specific inhibitors of COX, such as ibuprofen, naproxen, or rofecoxib. Whereas cucurbitacin B was found to inhibit lipid peroxidation by 59% at 100 pg/ml, cucurbitacin I did so only by 23% at the same concentration while the other two compounds were inactive [12]. This mechanism may well be a complementary pathway of action of cucurbitacins in inflammation however, other mechanisms such as the inhibition of integrin-mediated cell adhesion [51] should also be given further consideration. 

Some of the pharmacological data reported for cucurbitacin B have been corroborated in different studies. In one paper, Yesilada et al. [52] reported on the isolation of cucurbitacin B from the fruit juice of Ecballium elaterium. They further showed how the compound significantly reduced the vascular permeability induced by acetic acid in mice, giving an effective dose-50 (ED50) of 6.1 mg/kg however, the... 

Dihydrocucurbitacin B exhibited a similar pattern of activity to that of cucurbitacin R except in the case of the carrageenan-test. In this assay, the former was clearly more active, but the toxicity against rat leukocytes was higher [43],... 

Almeida et al. [6] thus tested a purified fraction from the rhizome of Wilbrandia sp. and demonstrated not only that the fraction inhibited KB cells with an effective concentration-50 (EC50) of 12 pg/ml, but also that it reduced the relative tumor weight of rats bearing Walker 256 carcinosarcoma by 65% [6,59]. These effects were thought to be due to the presence of cucurbitacins WGi and WG2 in the fraction, but the pure compounds were not tested. 

---