Aflatoxin

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Overview

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.[1] After entering the body, aflatoxins are metabolized by the liver to a reactive intermediate, aflatoxin M1, an epoxide. Aflatoxin is frequently misspelled as "aflotoxin" and "alfatoxin", which could be confused with alpha toxin.

Contamination conditions

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 (chile 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 contain minute quantities of aflatoxin,[2] but it is usually far below the US Food and Drug Administration's (FDA) recommended safe level.

The Food and Drug Administration (FDA) has established action levels for aflatoxin present in food or feed to protect human and animal health. [3]

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 manifested 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.[4] Chronic exposure also leads to a high risk of developing liver cancer, as the metabolite aflatoxin M1 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. The Aflatoxin acts as a DNA mutatory, not only mutating DNA randomly, but has a selection for mutating the p53 DNA specifically at base 249 to cause liver tumors (249 is an arginine residue, thus extremely important for interacting with DNA. Slight mutations affecting this charge relationship between the positive Arginine and negative DNA can severely hamper the tumor suppressing effects of p53 and its role in apoptosis).

Medical research indicates that a regular diet including apiaceous vegetables such as carrots, parsnips, celery and parsley, reduces the carcinogenic effects of aflatoxin.[5]

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.

Pets

Aflatoxin in dry dog food manufactured by Diamond Pet Foods was responsible for at least 23 dog deaths due to liver failure between Dec 2005 and early 2006. In an April 12, 2006 letter FedEx'd from the Department of Health and Human Resources to a manufacturing plant,[6][7] the FDA warned Gary Schell, president of Schell and Kampeter Inc. of Missouri that independent testing of three samples of incoming corn to their processing plant showed between 90 and 1851 ppb, while paperwork on three (of four samples) showed aflatoxins levels <20 ppb, and other sample was not recorded. The results of this letter are unknown.

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 can come from mother's milk).
  • Aflatoxin M2 : metabolite of aflatoxin B2 in milk of cattle fed on contaminated foods.[8]
  • Aflatoxicol.

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[9].

Manufacturers

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

  • Biopure Referenzsubstanzen GmbH part of Romer Labs Group [10] produces fully isotope labeled 13C Aflatoxin (Patented)
  • Sigma-Aldrich [11]
  • Fermentek, the only one that produces aflatoxin M2

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

See also

Notes

  1. Hudler, George. 1998. Magical Mushrooms, Mischievous Molds. Princeton, NJ: Princeton University Press
  2. quantity can range from 0ppb-20ppb for direct human consumption, although feedlot food for finishing beef cattle/swine/poultry can acceptably reach 300ppb; http://scientificteaching.wisc.edu/products/PeanutFiles/library/places/FoodDrugAdmin.htm
  3. Smith, Tara (June 2005). "A Focus on Aflatoxin Contamination". United States National Agricultural Library, Food Safety Research Information Office. Retrieved December 17, 2008.
  4. Abbas, Hamed K. (2005). Aflatoxin and Food Safety. CRC Press. ISBN 0824723031.
  5. University of Washington, Apiaceous vegetable constituents inhibit human cytochrome P-450 1A2 (hCYP1A2) activity and hCYP1A2-mediated mutagenicity of aflatoxin B1., 2006 Sep;44(9):1474-84. (PMID 16762476)
  6. http://www.fda.gov/foi/warning_letters/g5811d.pdf
  7. http://72.14.209.104/search?q=cache:tMz3hrvL8OgJ:www.fda.gov/foi/warning_letters/g5811d.htm
  8. Aflatoxin M2 product page from Fermentek
  9. Williams JH, Phillips TD, Jolly PE, Stiles JK, Jolly CM, Aggarwal D. Human aflatoxicosis in developing countries: a review of toxicology, exposure, potential health consequences, and interventions. Am J Clin Nutr 2004;80:1106-22. (PMID 15531656)
  10. Romer Labs - Mycotoxin Standards
  11. For example see: http://www.sigmaaldrich.com/catalog/search/ProductDetail/SIGMA/A9887

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