Trichothecenes

The trichothecenes are a group of sesquiterpene toxins produced by several fungi of the genera Fusarium, Myrothecium, Trichothecium, and Trichoderma, which are parasitic on cereals such as maize, wheat, rye, barley, and rice. About 150 different structures have been identified, with some of these being isolated from plants of the genus Baccharis (Compositae/Asteraceae), where a symbiotic plant-fungus relationship may account for their production. Examples of trichothecene structures commonly encountered as food contaminants include deoxynivalenol (DON) (Figure 5.39), and diacetoxyscirpenol (DAS), T-2 toxin, and verrucarin A (Figure 5.40). The double bond and the epoxide group in the basic tri-chothecene skeleton are essential for toxicity, and the number of oxygen substituents and ester functions also contribute. Macrocyclic ester functions as seen in verrucarin A tend to produce the most toxic examples. Although these compounds are more toxic when injected, oral toxicity is relatively high, and lethal amounts can easily be consumed because of the nature of the host plants. They are sufficiently toxic to warrant routine analysis of foodstuffs such as wheat and flour, and also flour-derived products, e.g. bread, since they survive processing and the high temperatures used in baking. DON levels above 1 ppm are considered hazardous for human consumption. It is relevant to note that when mammals ingest these compounds, a degree of de-epoxidation can occur, ascribed to gut microflora, thus providing some detoxification by removing a structural feature necessary for toxicity.

As their main mechanism of action, these compounds inhibit protein biosynthesis by binding to the ribosome and inhibiting peptidyl transferase activity (see page 407). They also inhibit DNA biosynthesis. A major human condition known to be caused by trichothecenes is alimentary toxic aleukia (ATA), characterized by destruction of the skin, haemorrhaging, inflammation, sepsis, a decrease in red and white blood corpuscles, bone marrow atrophy, and a high mortality rate. A severe outbreak of ATA was recorded in the former Soviet Union shortly after the Second World War when food shortages necessitated the consumption of grain that had overwintered in the field. This had become badly contaminated with Fusarium sporotrichioides and hence T-2 toxin. It is estimated that tens of thousands died as a result.

Many trichothecene derivatives have been tested as potential anticancer agents but have proved too toxic for clinical use.

diacetoxyscirpenol (DAS)

OAc diacetoxyscirpenol (DAS)

T-2 toxin

verrucarin A

Figure 5.40

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