FUNGAL METABOLISM AND FUNGAL PRODUCTS 139
The toxicity of all these compounds depends on the
presence of a double bond on the end furan ring
(marked * in Fig. 7.19). Aflatoxins B 1 and G 1 have this
double bond and they are carcinogens, whereas alfa-
toxins B 2 and G 2 lack the double bond and are only
weakly toxic. A similar difference is found in the milk
of cows that are fed on aflatoxin-contaminated feed.
Roughly 5% of the aflatoxin intake by a cow can be
found as aflatoxin M in the milk, and it occurs as either
aflatoxin M 1 or M 2 depending on whether the cows have
ingested aflatoxin B 1 or B 2.
When aflatoxins are absorbed from the gut they
are passed to the liver, where the aflatoxins with a
double bond (at position *) are metabolized to a
highly reactive but unstable form with an epoxide
bridge at the position of the double bond on the end
furan ring (Fig. 7.19, bottom left). This epoxide group
enables the toxin to bind to DNA and depurinate it,
causing damage to the genome which can lead to
hepatomas. Alternatively (Fig. 7.19, bottom right), the
epoxide can give rise to dihydrodihydroxyaflatoxin,
which is acutely toxic. All developed countries now
impose strict limits on the levels of aflatoxins and other
common mycotoxins in foods intended for human
consumption.Fig. 7.19Structures of some common aflatoxins. See text for details.
debris of groundnuts (peanuts), providing inoculum for
colonization of the subterranean groundnut fruits. A.
flavuscan be isolated from the shells of almost any
peanuts bought from a grocery store – it is seen as gray
or black patches inside the shells. However, most strains
do not produce aflatoxin, and even the strains that do
produce these compounds require specific conditions
for this (Chapter 8). Oil-rich crops such as peanut fruits
and cottonseed are especially favorable for aflatoxin
production, consistent with the finding that aflatoxin
production in laboratory culture is stimulated by lipids.
After breakdown of the lipids by lipases, the fatty
acids are metabolized to acetyl-CoA by β-oxidation
(Fig. 7.4), then the aflatoxins can be synthesized from
acetyl-CoA by the polyketide pathway (Fig. 7.13). The
review by Hicks et al. (2002) provides details of the
genetics and biosynthesis of these compounds.
Aflatoxins can be detected in extracts of contaminated
foods by thin-layer chromatography, because they show
natural green or blue fluorescence under UV irradiation.
This colour difference is related to the ring structure
of the molecule (Fig. 7.19) and has led to the distinc-
tion between aflatoxins of the G (green-fluorescing) and
B (blue) types. This is also linked to toxicity, because
aflatoxin B 1 is more toxic than aflatoxin G 1.