Aflatoxins

Description

Aflatoxins are a group of polyketide metabolites produced mainly by strains of the fungus Aspergillus flavus and the closely related species A. parasiticus. The most biologically active aflatoxin is B1; it is considered the most potent naturally occurring carcinogen known.

Agricultural Uses

Aflatoxin refers to a group of highly toxic polynuclear moulds (mycotoxins.) They are designated as B₁,B₂,G₁,G₂,M₁and M₂, and are produced chiefly by fungus Aspergillusflavus, which is a natural contaminant of fruits, vegetables and cereal grains. Aflatoxins are toxic to many animals including fish and birds. The B₂and G₁ strains are carcinogens.
Aflatoxins fluoresce strongly under UV light. They are soluble in methanol, acetone and chloroform, but only slightly soluble in water and hydrocarbon solvents. Prevention of mould growth is the most effective protection against aflatoxins. Removal or inactivation is possible by physical and chemical means (solvent refining or hand picking). FDA (Food and Drug Administration) does not allow any aflatoxin content in foods or feeds, although its presence up to 20 ppb does not attract penalty.

Enzyme inhibitor

These mycotoxins (FWB1 = 312.30 g/mol; CAS 1162-65-8; FWB2 = 314.29 g/mol; CAS 7220-81-7; FWG1 = 328.27 g/mol; CAS 1165-39-5; FWG2 = 330.29 g/mol; CAS 7241-98-77; FWM1 = 328.27 g/mol; CAS 6795-23-9; FWM2 = 330.29 g/mol; CAS 6885-57-0), produced by the fungus Aspergillus flavus, A. parasiticus, A. oryzae, and some Penicillium strains, are a set of closely related metabolites that are known carcinogens. The B and G designations in the name refer to the blue or green fluorescence, respectively, that are exhibited by the mycotoxins (aflotoxins B and G are 22 dihydro derivatives of B1 and G1 that are less toxic). The M series of aflatoxins are hydroxylated derivatives of the B series toxins isolated from the milk of cows that ingested aflatoxin-contaminated feed. Initially innocuous forms are bioactivated by members of the cytochrome p450 family into mutagenic and carcinogenic intermediates. Aflatoxin-B1, for example, is converted into aflatoxin B1-78,9 exo-epoxide, which is in turn converted into the 8,9-dihydroxy-8-N-guanyl-9-hydroxy aflatoxin B1 adduct. This adduct is metabolized into aflatoxin B1 formaminopyrimidine adduct. It is these adducts that prove to be strongly mutagenic and often carcinogenic. Chronic hepatitis B virus (HBV) infection and persistent AFB? exposure are major risk factors for developing hepatocellular carcinogenesis, suggesting synergy in the action of these agents. Nucleic Acid Interactions: Aflatoxins have dimensions and so-called bay region aromatic characteristics that allow them to intercalate between base-pairs in DNA and double-stranded segments of RNA. Major groove recognition of DNA by proteins also relies the variation in hydrogen-bond donor/acceptor content to makes DNA base-pairs distinguishable from one another, and aflatoxins may mimic major groove-binding proteins. Aflatoxins also inhibit RNA biosynthesis, transcription, and protein biosynthesis. CAUTION: Aflatoxin B1 is a highly potent mutagen and carcinogen. Avoid direct exposure to the skin or eyes as well as inhalation. Target (s) : RNA polymerase; DNase-I (deoxyribonuclease I), by Aflatoxin B2a > Aflatoxin G2a;; electron transport; nucleoside transport; oxidative phosphorylation, uncoupled; b-galactoside a-2,6- sialyltransferase, inhibited by Aflatoxins B1, B2a, and G1; glycoprotein sialyltransferase; acetylcholinesterase, inhibited by Aflatoxin B1; aminopeptidase B.

Metabolism

Under optimum conditions, aflatoxin can be produced by A. flavus within 24 to 36 hours; a significant amount can be produced in a few days. Increases in aflatoxins of 200 to more than 2,000 parts per billion (ppb) have been reported in three days when recently harvested field corn was stored at high moisture levels. Aflatoxins may increase in truckloads of contaminated corn at a rate of up to 6% per hour.

Toxicity evaluation

Aflatoxins are both toxic and carcinogenic. They have been shown to produce cancer in all species tested; epidemiologic evidence suggests that they are involved in human liver cancer in some parts of the world. In general, the toxicity of aflatoxins is ofmore concern than its carcinogenicity in domestic animals. Regular or occasional consumption by farm animals of aflatoxin-containing feed, of less than 100 ppb to several hundred parts per million (ppm), results in lowered feed consumption, poor feed conversion, reduced growth, and decreased production. Reduced growth and productivity may be accompanied by liver damage, hemorrhage, and suppression of natural immunity to parasites. Once damage has occurred, animals do not fully recover when returned to a toxinfree ration. Growers who produce homegrown feed must be aware of the local aflatoxin hazard and, if necessary, have grain checked for aflatoxins before making it into feed (7–11).