Aspergillus species aflatoxins

Environmental reservoirs for the Aspergillus species (s. tab. 26.2) include damp cellars and old stone walls, soil in flowerpots (thus possibly in hospital rooms ) and mouldy food. The aflatoxins of the mould Aspergillus flavus (e. g. in mouldy nuts) are deemed to have a high carcinogenic potential. 

Human exposure to aflatoxins occurs mainly through growth of the Aspergillus species A.flavus and A. parasiticus. Whether exposure is predominantly to aflatoxin Bl, or to mixtures of various aflatoxins, depends upon the geographical distribution of the strains. A.flavus, which produces aflatoxins Bl and B2, occurs worldwide, while A. parasiticus, which produces aflatoxins Bl, B2, G1 and G2, occurs principally in the Americas and in Africa. 

YU, J., WOLOSHUK, C.P., BHATNAGAR, D., CLEVELAND, T.E., Cloning and characterization of avfA and omtB genes involved in aflatoxin biosynthesis in three Aspergillus species. Gene, 2000, 248, 157-167. 

C, (decomp.). Metabolite from Aspergillus species (A. versicolor, A. parasiticus, A. ustus, A. ni-dulans, A. multicolor), and phytopathogenic fungi (Cercospora smilacis, Dothistroma pini), and Mono-cillium nordinii. A. is an intermediate in aflatoxin biosynthesis and shows weak mutagenic activity in the Ames test. 

These compounds were found to be important mycotoxin constituents of Aspergillus species shortly after the discovery of the aflatoxins (see Chap. 2.). The ochratoxins are isocoumarin derivatives coupled with p-phenylalanine. The major representatives of this class of pentaketides are shown in Fig. 6.2. Ochratoxin B (324) is the dechloro analog of ochratoxin A (323), which is the most important member of this group with respect to its toxicity. The corresponding methyl and ethyl esters of ochratoxin A (323) and B (324) were found also in Aspergillus species and the ethyl ester of ochratoxin A (323) has been named ochratoxin C (325). Ochratoxin a (326) is a free carboxylic acid that represents the dihydroi-socoumarin nucleus of ochratoxin A (323). 

This group of pigments is mainly produced by genotypically similar Aspergillus species namely, A. versicolor, A. nidulans, and A. ustus. None of these species produces aflatoxins, hence indicating a metabolic block in aflatoxin biosynthesis. 

Aflatoxins are products of species of the genus Aspergillus, particularly A flavus, a common fungus found as a contaminant of grain, maize, peanuts, and so on. First implicated in poultry diseases such as Turkey-X disease, they were subsequently shown to cause cancer in experimental animals and, from epidemiological studies, in humans. Aflatoxin Bl, the most toxic of the aflatoxins, must be activated enzymatically to exert its carcinogenic effect. 

The most common source of aflatoxins is moldy food, particularly nuts, some cereal grains, and oil seeds. The most notorious of the aflatoxins is aflatoxin B1( for which the structural formula is shown in Figure 19.1. Produced by Aspergillus niger, it is a potent liver toxin and liver carcinogen in some species. It is metabolized in the liver to an epoxide (see Section 7.3). The product is electrophilic with a strong tendency to bond covalently to protein, DNA, and RNA. Other common aflatoxins produced by molds are those designated by the letters B2, G1( G2, and M,. 

Two groups of mycotoxins have been of concern in a CBW context, trichothecenes produced by Fusarium species and aflatoxins produced by Aspergillus... 

Aflatoxin and sterigmatocysti form Aspergillus (various species). 

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