Toxic Terpenoids

A variety of chemicals may be leached from the aerial portions of plants by rainwater or by fog-drip (16) including organic acids, sugars, amino acids, pectic substances, gibberellic acids, terpenoids, alkaloids, and phenolic compounds. Colton and Einhellig (17) suggested that leaf leachates of velvetleaf (Abutilon theophrasti) may be inhibitory to soybean (Glycine maxT We have recently discovered specialized hairs on the stems of velvetleaf plants which exude toxic chemicals. 

Volatile toxicants A great many volatile substances, including gaseous compounds and volatile terpenoids, are involved in allelopathy (46). 

The great diversity of terpenes helps to cormteract tolerance by herbivores. In all, terpenes are not very toxic to vertebrates. Many mammals ingest a significant amount of terpenoids with their diet. Monoterpenes from pine oil added to the diet reduces food intake in red deer, Cervuselaphus, calves (Elliot and Loudon, 1987). The brush-tailed possmn, Trichosurus vulpecula, detoxifies (-l-)-a-pinene to alcohol and carboxylic acid derivatives. 

In the cnidarians, the true medusae (Cubozoa) contain unusual toxic polypeptides in their nematocysts. The anthozoans are characteristic for the production of lower terpenoids similar to those from terrestrial plants (Pietra 1995). Bryozoans, in analogy with the sponges, are only productive in the sea, giving unusual metabolites, in particular novel macrolides (Table 9.1). 

The toxicity of the cotton terpenoid aldehydes to different larvae has been reported by several groups (227, 259, 270-275). These results are summarized in Table 2. Although there is variation in the test results among the different laboratories, it is obvious that gossypol and probably the other terpenoid aldehydes have a wide range of toxicity to many moth larvae. 

The biological activity and toxicity of terpenoids to herbivores has been discussed (56,84) and representatives of each major type of terpene are known to be active. Well known examples are sesquiterpene lactones, pyrethrins, and several classes of diter-penes and triterpenes. 

Specific antagonists for GABAa receptors include the alkaloid convulsants bicuculline (Fig. 30-25)699 and picrotoxin (Fig. 22-4) and the convulsant terpenoid compound thujone (Fig. 22-3), which is present in the wormwood plant Artemesia absinthium. Thujone is present in the liqueur absinthe, which was the national drink of France in the late 19th century but, because of its toxicity, has been illegal in most countries since -1915.719... 

Saponins consist of a terpenoid core (the aglycone), having oxygenated positions bound to sugar moieties (up to ten monosaccharidic units). In water they form colloidal solutions which foam on shaking and precipitate cholesterol. When saponins are near cell membranes, their interaction with cholesterol may create pore-like structures that eventually cause the membrane to burst. Hemolysis is an example of this phenomenon (i.e. the distraction of erythocyte membranes, but not hemoglobin). Occasionally, they cause hypersecretion, which could explain their expectorant activities and also their toxicity to fish. 

The terpenoids usually play a role in the growth or the defence of the organism that contains them. That is why some terpenoids are toxic, irritant or allergenic and some are repellent smelling (small terpenes)... 

Both cell cultures and animal studies have shown that many of the naturally occurring mono-, sesqui-, di-, sester-, and meroterpenoids as well as retinoids possess potentially chemopreventive activities. Terpenoids are minor but ubiquitous components of our diet, and have the advantage of being non-toxic or relatively non-toxic to humans. More mechanistic-oriented basic research is needed to elucidate the mechanisms of action. Studies of derivatives of these naturally occurring terpenoids are also necessary to elucidate the structure-activity relationship and to guide the development of novel chemopreventive agents. 

---