Aflatoxins
The aflatoxins were discovered in the 1960s, when they were identified as toxic compounds of the fungus Aspergillus flavus, which is shown in Fig. 2.1 (11, 12).
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The aflatoxins were discovered in the 1960s, when they were identified as toxic compounds of the fungus Aspergillus flavus, which is shown in Fig. 2.1 (11, 12).
Fig. 2.1 Aspergillus flavus spores as seen under the light microscope under 600-fold magnification
This fungus was found in ground nut meal, which had been fed to different farm animals. Due to this contamination, 100.000 turkeys died in 1960 in Britain of the so-called “Turkey-X disease” (13). Later, the aflatoxins were also found in other Aspergillus species and in some Penicillium fungi. The name “aflatoxin” is an abbreviation of Aspergillus flavus toxins (14). Up to the present, the aflatoxins are among the most acutely toxic and carcinogenic compounds known (13). Although most countries in the world now have limitations for the maximum tolerated levels of aflatoxins in food, contamination by these compounds is still a problem (15). Aflatoxins are found regularly in different foods, especially the milk of cows, which gets intoxicated by affected animal feed (13, 15, 16). S. Bra¨se et al., The Chemistry of Mycotoxins, Progress in the Chemistry of Organic Natural Products, Vol. 97, DOI 10.1007/978-3-7091-1312-7_2, # Springer-Verlag Wien 2013
3
4
2 O
O E
D
O
O O
O
H C
A O
B O
O
O
O
H
O
H
O O
O
H
2 (aflatoxin B2) O
O
O
O
O
O
H
O
H
1 (aflatoxin B1) O
Aflatoxins
H
3 (aflatoxin G1)
O O
O
H
4 (aflatoxin G2)
Fig. 2.2 The aflatoxins B1, B2, G1, and G2 (1–4)
The most widely examined aflatoxin is aflatoxin B1 (1), which is also the most toxic, carcinogenic, and mutagenic aflatoxin among all that are presently known (17, 18). It was isolated together with aflatoxins B2 (2), G1 (3), and G2 (4), which are shown in Fig. 2.2 (19). Their structures were revealed by the group of B€ uchi in 1963 (B1 (1) and G1 (3)) and 1965 (B2 (2) and G2 (4)) (20, 21). This group also elucidated the absolute stereochemistry of aflatoxins in the B and G series by chemical degradation (22). Structurally, these compounds consist of five rings, having a furofuran moiety (rings B and C), an aromatic six-membered ring (A), a six-membered lactone ring (D), and either a five-membered pentanone or a six-membered lactone ring (E). While the aflatoxins B and G are major compounds of the fungus Aspergillus flavus, there are also minor aflatoxin constituents from this organism, e.g. hydroxylated derivatives of aflatoxin B1 (1) and B2 (2), the so-called “milk-toxins”, M1 (5) and M2 (6), which bear a hydroxy group at the junction of the two furan rings (19). They are called “milk toxins”, because they are metabolites of aflatoxin B1 (1) and B2 (2), formed when cows get fed with contaminated foodstuffs. The toxins are then contained in the cow’s milk. Other aflatoxins have a hydroxy group instead of
2 Aflatoxins
5
O
O
O
O O
O
HO
O
HO
O O
O
O
H
OH
OH
H
O
H
9 (aflatoxin RB1)
H O
R
O
O
O OH
O
H
H
O O
H
7 (aflatoxin R0) R = H 8 (aflatoxin H1) R = OH
OH
HO
O O
H
6 (aflatoxin M2)
OH
HO
O
O
5 (aflatoxin M1)
O
HO
O
10 (aflato
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