Aflatoxins are toxic and carcinogenic secondary metabolites produced primarily by Aspergillus flavus and A. parasiticus as well as A. nomius. One isolate of each of A. tamarii (Goto et al. 1996) and A. ochraceoroseus (Frisvad and Samson 1999) have been reported to produce aflatoxins (Klich et al. 2000). The aflatoxins consist of a group of at least 16 structurally related toxins (Goldblatt 1969) characterized so far. Amongst these, aflatoxins, B1, B2, G1 and G2 (AFB1, AFB2, AFG1 and AFG2) are the major toxins. Aflatoxins are polyketide-derived, bis-furan-containing dihydrofuranofuran and tetra-hydrofuran moieties (rings) fused with a substituted coumarin (Figure 1). A. flavus produces aflatoxins, B1 and B2; while A. parasiticus produces the four major aflatoxins, B1, B2, G1 and G2. The aflatoxins, M1 and M2, are modified forms of aflatoxin B1 found in bovine milk. Aflatoxin B1 (AFB1) is the most potent naturally occurring toxin and carcinogen known (Squire 1989). The toxicity of the major aflatoxins has been established in the following order: B1 > G1 > B2 > G2. Aflatoxin M1 is 10-fold less toxic than AFB1, but its presence in milk is of concern in human health (Cullen et al. 1987; Galvano et al. 1996; Van Egmond 1989b). Due to the toxic and carcinogenic properties of aflatoxins (Busby and Wogan 1981; Eaton and Groopman 1994), these compounds are the most thoroughly studied of all the mycotoxins, and significant research has been conducted on their biosynthetic pathway
and genetic control of its regulation (Bhatnagar et al. 2002; Chang et al. 1996; Payne and Brown 1998; Yu et al. 1995a).
Sterigmatocystin and dihydrosterigmatocystin (DHST) are produced by various Aspergilli including A. nidulans; several genera Bipolaris and Chaetomium, and Penicillium luteum are also reported to produce ST (Cole and Cox (1987)). Aspergillus nidulans is an industrial fungus and has been the model organism for the study of ST biosynthesis. ST and DHST are the penultimate precursors of AFB1, AFB2, AFG1 and AFG2, respectively (Figure 1; Betina 1989; CAST 1989; Chu 1991). Like aflatoxins, ST and DHST are hepatotoxic and carcinogenic mycotoxins. However, their carcinogenicity is far less than that of aflatoxin B1 in animal test (Mori and Kawai 1989; Van der Watt 1977). ST contaminates cereal grains (barley, rice and corn), coffee beans and cheese (Jelinek et al. 1989) and is considered a health hazard as well. Since ST and DHST are aflatoxin precursors, they share a common biochemical pathway, homologous genes and regulatory mechanism. For these reasons, aflatoxins and ST are discussed together.
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