Toxic secondary metabolites

Mycotoxins are toxic secondary metabolites produced by fungi growing within or on foods. They can be a serious threat to human and animal health (Nagler el al., 2001). Table 11.4 details mycotoxins associated with soft drinks and fruit juice manufacture and raw materials. Patulin is the most common mycotoxin associated with fruit juice, particular ly apple juice (Pitt Hocking, 1997). It commonly occurs if juice is produced from stored apples. Mould growth in infected apples increases with time, raising levels of patulin. The use of windfall apples for juice is also a factor. Avoidance of windfall apples, filtration of juice and pressing quickly after harvest are all methods to reduce the incidence of patulin in juice. Patulin can be destroyed by fermentation to cider or by the addition of ascorbic acid (Marth, 1992). Within Europe, the European Union has set a limit of 50 ig/kg for patulin in both apple juice and cider. A recent survey of apple products in Chile found that 28% of samples of juice and concentrate exceeded this limit (Canas Aranda, 1996). 

Metabolites from cyanobacteria are generally of amino acid or polyketide origin and frequently show potent biological activity. The series of dolastatin metabolites, exemplified by dolastatin-10 (Structure 2.18), are linear peptides which show potent cytotoxic activity and are of clinical interest as anti-tumour agents. Originally isolated in very low yield from the Indian Ocean sea hare Dolabella auricularia, dolastatins are now known to be cyanobacterial products.43,44 The discovery of a microbial source for these pharmaceutically important compounds will facilitate study of their biosynthesis and could potentially lead to the production of structural analogues by provision of modified biosynthetic precursors to the cultivar. As discussed below and in Section VI, toxic secondary metabolites from cyanobacteria have often been implicated in the chemical defenses of sea hares.45"17... 

Other large organic molecules may favourably interact with natural zeolite or clay surfaces. Of interest is the ability of these materials, e.g., clinoptilolite- or montmorillonite-rich rocks, to adsorb on their hydrophilic, negatively charged surfaces complex substances, such as aflatoxins, which arc toxic secondary metabolites of several agricultural products, containing polar functional groups [70,71J. Adsorption, which has been proven either in-vitro or in-vivo, is effective and amounts to some hundred pg per g of adsorber. 

Sonnenbichler, J., J. Dietrich, and H. Peipp Secondary fungal metabolites and their biological activities. V. Investigations concerning the induction of the biosynthesis of toxic secondary metabolites in Basidiomycetes. Biol. Chem. Hoppe-Seyler, 375, 71 (1994). 

Bark beetles oxidise toxic monoterpenoid hydrocarbons in their respiratory air, which originate from the resin of the tree they colonise, and thereby circumvent this defense barrier of the tree. Converting these compounds into less toxic secondary metabolites serves another purpose, by providing pheromones to attract insects of the same species, of both sexes. Therefore, an oxygen function is often introduced species-specifically and stereoselectively. European spruce bark beetles oxidise (-)-a-pinene to (S)-cis-verbenol, whereas pine beetles produce (R)-trans-verheno. In the California fivespined engraver Ips paraconfu-sus), myrcene is converted into (S)-ipsdienol and (S)-ipsenol. But Ips pint and Ips paraconfusus are also able to synthesise their pheromones de novo via the classical mevalonate biosynthetic pathway. 

The effect of many toxic secondary metabolites is probably to prevent or limit feeding rather than to loll the herbivore... 

Mycotoxins in the following sections are not sorted by their producers, but according to their structures related to toxic effects. It is obvious that some toxic secondary metabolites are produced by several types of filamentous fungi (Table 12.21). 

Sirikantaramas, S., Yamazaki, M., Saito, K., 2008. Mechanisms of resistance to self-prodnced toxic secondary metabolites in plants. Phytochem. Rev. 7,467-477. 

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