Aflatoxin

Not to be confused with Alpha toxin (disambiguation).

File:(–)-Aflatoxin B1 Structural Formulae V.1.svg

Chemical structure of (–)-aflatoxin B₁

File:(–)-Aflatoxin G1 Structural Formulae V.1.svg

Chemical structure of (–)-aflatoxin G₁

File:Aflatoxin b1 3d structure.png

3D structure of aflatoxin B₁

Aflatoxins are naturally occurring mycotoxins that are produced by many species of Aspergillus, a fungus, most notably Aspergillus flavus and Aspergillus parasiticus. Aflatoxins are toxic and among the most carcinogenic substances known. After entering the body, aflatoxins may be metabolized by the liver to a reactive epoxide intermediate or be hydroxylated and become the less harmful aflatoxin M₁.

Contamination conditions

File:Aspergillus.gif

Aspergillus fumigatus as seen under the electron microscope

Aflatoxin-producing members of Aspergillus are common and widespread in nature. They can colonize and contaminate grain before harvest or during storage. Host crops are particularly susceptible to infection by Aspergillus following prolonged exposure to a high humidity environment or damage from stressful conditions such as drought, a condition which lowers the barrier to entry.

The native habitat of Aspergillus is in soil, decaying vegetation, hay, and grains undergoing microbiological deterioration and it invades all types of organic substrates whenever conditions are favorable for its growth. Favorable conditions include high moisture content (at least 7%) and high temperature.

Crops which are frequently affected include cereals (maize, sorghum, pearl millet, rice, wheat), oilseeds (peanut, soybean, sunflower, cotton), spices (chilli peppers, black pepper, coriander, turmeric, ginger), and tree nuts (almond, pistachio, walnut, coconut, brazil nut).

The toxin can also be found in the milk of animals which are fed contaminated feed.

Virtually all sources of commercial peanut butter in the United States contain minute quantities of aflatoxin, but it is usually far below the US Food and Drug Administration's (FDA) recommended safe level.

The United States Food and Drug Administration (FDA) has established action levels for aflatoxin present in food or feed to protect human and animal health.

Levels must not exceed:

Ppb	Criterion
20	For corn and other grains intended for immature animals (including immature poultry) and for dairy animals, or when its destination is not known, and for animal feeds, other than corn or cottonseed meal
100	For corn and other grains intended for breeding beef cattle, breeding swine, or mature poultry
200	For corn and other grains intended for finishing swine of 100 pounds or greater
300	For corn and other grains intended for finishing (i.e., feedlot) beef cattle and for cottonseed meal intended for beef cattle, swine or poultry

Pathology

High-level aflatoxin exposure produces an acute hepatic necrosis, resulting later in cirrhosis, and/or carcinoma of the liver. Acute hepatic failure is made manifest by hemorrhage, edema, alteration in digestion, and absorption and/or metabolism of nutrients and mental changes and/or coma.

No animal species is immune to the acute toxic effects of aflatoxins including humans; however, humans have an extraordinarily high tolerance for aflatoxin exposure and rarely succumb to acute aflatoxicosis.

Chronic, subclinical exposure does not lead to symptoms as dramatic as acute aflatoxicosis. Children, however, are particularly affected by aflatoxin exposure which leads to stunted growth and delayed development. Chronic exposure also leads to a high risk of developing liver cancer, as aflatoxin metabolite can intercalate into DNA and alkylate the bases through its epoxide moiety. This is thought to cause mutations in the p53 gene, an important gene in preventing cell cycle progression when there are DNA mutations, or signaling apoptosis. The aflatoxin acts as a mutagen, not only mutating DNA randomly, but specifically mutating the p53 gene at base 249 to cause liver tumors.

Medical research indicates that a regular diet including apiaceous vegetables such as carrots, parsnips, celery and parsley, reduces the carcinogenic effects of aflatoxin.

Microbiology

Aflatoxins are still recognized as the most important mycotoxins. They are synthesized by only a few Aspergillus species of which A. flavus and A. parasiticus are the most problematic. The expression of aflatoxin-related diseases is influenced by factors such as age, nutrition, sex, species and the possibility of concurrent exposure to other toxins. The main target organ in mammals is the liver so aflatoxicosis is primarily a hepatic disease. Conditions increasing the likelihood of aflatoxicosis in humans include limited availability of food, environmental conditions that favour mould growth on foodstuffs, and lack of regulatory systems for aflatoxin monitoring and control.

A. flavus and A. parasiticus are weedy moulds that grow on a large number of substrates, particularly under high moisture conditions. Aflatoxins have been isolated from all major cereal crops, and from sources as diverse as peanut butter and marijuana. The staple commodities regularly contaminated with aflatoxins include cassava, chillies, corn, cotton seed, millet, peanuts, rice, sorghum, sunflower seeds, tree nuts, wheat, and a variety of spices intended for human or animal food use. When processed, aflatoxins get into the general food supply where they have been found in both pet and human foods as well as in feedstocks for agricultural animals. Aflatoxin transformation products are sometimes found in eggs, milk products and meat when animals are fed contaminated grains.

Detection in humans

There are two principal techniques that have been used most often to detect levels of aflatoxin in humans.

The first method is measuring the AFB₁-guanine adduct in the urine of subjects. Presence of this breakdown product indicates exposure to aflatoxin B1 in the past 24 hours. However, this technique only measures recent exposure, and due to the half-life of this metabolite, the level of AFB₁-guanine measured can vary from day to day, based on diet, and thus is not ideal for assessing long term exposure.

Another technique that has been used is a measurement of the AFB₁-albumin adduct level in the blood serum. This approach provides a more integrated measure of exposure over several weeks/months.

Animals

Aflatoxin can potentially lead to liver disease in dogs; however, not all dogs exposed to aflatoxin will develop liver disease. As with any toxic exposure, development of aflatoxicosis is a dose-related occurrence. Some dogs that develop liver disease will recover; those exposed to large doses for extended periods may not.

Low levels of aflatoxin exposure require continuous consumption for several weeks to months in order for signs of liver dysfunction to appear. Some articles have suggested the toxic level in dog food is 100-300 ppb and requires continuous exposure/consumption for a few weeks to months to develop aflatoxicosis. No information is available to suggest that recovered dogs will later succumb to an aflatoxin-induced disease.

There is no specific antidote for aflatoxicosis. Symptomatic and supportive care tailored to the severity of the liver disease may include intravenous fluids with dextrose, active vitamin K, B vitamins, and a restricted, but high-quality protein diet with adequate carbohydrate content.

As a precautionary measure, both human and pet food recalls have occurred, casting a wide safety net to prevent exposure to potentially unsafe food. Recalled food products are subsequently sampled and tested for aflatoxin.

On December 20, 2005, Diamond Pet Food discovered aflatoxin in a product manufactured at their facility in Gaston, South Carolina. Diamond voluntarily recalled in 23 states 19 products formulated with corn and manufactured in the Gaston facility. Testing of more than 2,700 finished product samples conducted by laboratories confirmed that only two date codes of two adult dog formulas with the "Best By" dates of April 3, April 4, April 5 and April 11 were potentially toxic.

Major types of aflatoxins and their metabolites

At least 13 different types of aflatoxin are produced in nature. Aflatoxin B₁ is considered the most toxic and is produced by both Aspergillus flavus and Aspergillus parasiticus. Aflatoxin G₁ and G₂ are produced exclusively by A. parasiticus. While the presence of Aspergillus in food products does not always indicate harmful levels of aflatoxin are also present, it does imply a significant risk in consumption

Aflatoxins M₁, M₂ were originally discovered in the milk of cows which fed on moldy grain. These compounds are products of a conversion process in the animal's liver. However, aflatoxin M₁ is present in the fermentation broth of Aspergillus parasiticus.

• Aflatoxin B₁ & B₂ : produced by Aspergillus flavus and A. parasiticus.
• Aflatoxin G₁ & G₂ : produced by Aspergillus parasiticus.
• Aflatoxin M₁ : metabolite of aflatoxin B₁ in humans and animals (exposure in ng levels can come from a mother's milk).
• Aflatoxin M₂ : metabolite of aflatoxin B₂ in milk of cattle fed on contaminated foods.
• Aflatoxicol.

Biosynthetic Pathway of Aflatoxin B1

Aflatoxin B1 is derived from both fatty acid synthase (FAS) and polyketide synthase (PKS). The biosynthesis begins with the synthesis of hexanoate by the FAS, which then becomes the starter unit for the iterative type 1 PKS. The PKS adds seven malonyl-CoA extenders to the hexanoate to form the C20 polyacetate compound. These polyacetates are relatively unstable, thus this compound is stabilized by the enzyme norsolorinic acid synthase (E1) until all of the acetate subunits have been added. At which point norsolorinic acid synthase folds the polyacetate in a particular way to induce cyclization to form the anthraquinone norsolorinic acid. Norsolorinic acid synthase (E1) then catalyzes the reduction of the ketone on norsolorinic acid side chain to yield averantin. Averantin is converted to averufin via a two different enzymes, a hydroxylase (E2) and an alcohol dehydrogenase (E3). This will oxygenate and cyclize averantin's side chain to from the ketal in averufin. As one can tell by the function of the enzyme norsolorinic acid synthase, it contains both a complex of iterative type 1 PKS, yeast-like type 1 FAS and supplies NADPH for the reduction of norsolorinic acid.

From this point on the biosynthetic pathway of aflatoxin B1 becomes much more complicated with several major skeletal changes. Most of the enzymes have not been characterized and there may be several more intermediates that are still unknown. However, what is known is that averufin is oxidized by a P450-oxidase, AvfA (E4), in a Baeyer-Villiger oxidation. This opens the ether rings and upon rearrangement versiconal acetate is formed. Now an esterase, EstA (E5), catalyzes the hydrolysis of the acetyl forming the primary alcohol in versiconal. The acetal in versicolorin A is formed from the cyclization of the side chain in versiconal, which is catalyzed by VERB synthase (E6), and then VerB, a desaturase (E7), reduces versicolorin B to form the dihydrobisfuran.

There are two more enzymes that catalyze the conversion of versicolorin A to demetyhlsterigmatocystin, AflN, an oxidase (E8) and AflM, a reductase (E9). These enzymes utilize both molecular oxygen and two NADPH's to dehydrate one of the hydroxyl groups on the anthraquinone and open the quinine with the molecular oxygen. Upon forming the aldehyde in the ring opening step it is oxidized to form the carboxylic acid and subsequently a decarboxylation event occurs to close the ring forming the six member ether ring system seen in demethylsterigmatocystin. The next two steps in the biosynthetic pathway is the methylation by s-adenosylmethionine (SAM) of the two hydroxyl groups on the xanthone part of demethysterigmatocystin by two different methytransferases, OmtB (E10) and OmtA (E11). This yields O-methylsterigmatocystin. In the final steps there is an oxidative cleavage of the aromatic ring and loss of one carbon in O-methylsterigmatocystin, which is catalyzed by OrdA, an oxidoreductase (E12). Then a final recyclization occurs to form aflatoxin B1.

Interaction with the hepatitis B virus

Studies have shown that concurrent infection with the hepatitis B virus (HBV) during aflatoxin exposure increases the risk of hepatocellular carcinoma (HCC). As HBV interferes with the ability of hepatocytes to metabolize aflatoxins, an aflatoxin M₁-DNA conjugate exists for a longer period of time in the liver, increasing the probability of damage to tumor suppressor genes such as p53. This effect is synergistic with the resulting damage far greater than just the sum of aflatoxin and HBV. (Williams, 2004)

Decreasing HBV infection levels through vaccination is an effective and simple approach that can be taken to reduce these harmful synergistic effects, thus decreasing the impact of chronic aflatoxin exposure. This strategy may prove to be highly effective–many regions of the world which have high aflatoxin rates, such as western Africa and China, also have high HBV infection rates.

Manufacturers

As of May 2008, there are only three primary manufacturers (as distinguished from re-packers and re-sellers) of pure aflatoxins:

• Romer Labs produce patented fully isotope labeled 13C aflatoxins.
• Sigma-Aldrich
• Fermentek, the only one that produces aflatoxin M₂

Customers use these compounds as an internal standard when monitoring foodstuffs for aflatoxin contaminants.

See also

• Aflatoxin total synthesis
• Foodborne illness

External links

• Detailed listing and information on all Aspergillus mycotoxins
• Detailed information about mycotoxins
• Aflatoxin.info
• Aspergillusflavus.org
• Diamond Pet Food Recall

ar:أفلاتوكسين zh:黃麴毒素