AFLATOXINS

Aflatoxins are the toxic metabolities of toxigenic fungi Aspergillus flavus and A. parasiticus. These moulds invade and grow on all sorts of stored feed ingredients and organic matters and produce mycotoxins under favourable aerobic conditions when sufficient moisture (>15%) and relative humidity (90-95%) and ambient temperature (24-25oC) are available, through toxin production to some extent can take place at lower or higher temperatures as well.  

Aflatoxins are produced in varying quantities in a variety of grains and nuts, pea nut is the most important one. Cotton see meal, cotton seed cake, maize corns, com-meals, silages, wheat, barley, oats, rice and other grains or cereal products are invaded by these fungal toxins.

            Several aflatoxin fractions have been isolated and differentiated from one another by their fluorescence under ultraviolet, RF values on thin layer chromatography and structural identification and synthesis. Aflatoxins which fluoresce blue under ultraviolet light are termed as BI and B2 while others which are dihydroderivatives of B₁ and B₂ and fluoresce green are designated as G₁ and G₂ Hydroxylated metabolites of B, and B2 aflatoxins are excreted in milk and are termed as MI and M₂. Human exposure occurs on consumption of aflatoxins from the mycotoxins- infested feed stuffs as well as meat and milk of animals consuming toxigenic fungi contaminated feeds.

            Out of the four main aflatoxins, aflatoxin B₁ (AFB₁) is most important because of its toxicity and high concentration in the contaminated feeds. B₂ and G₂ are present in very less concentrations. The order of toxicity is: B₁ > G₁ > B₂ > G₂. These toxins are relatively heat resistant and not soluble in water. The melting points for B_I, B2 ' G_I and G₂ aflatoxins are 269°, 238°, 245° and 239°C, respectively. Even the toxigenic fungi or toxins are not destroyed by milling of the grains.

            Aflatoxins are a group of polycyclic, unsaturated, substituted coumarin derivatives with highly reactive bifuran nucleus on one side and a pentenone (B toxin) or a six membered lactone (G - toxin) ring on the other side as shown in Fig. 8.1.

Presence of additional oxygen in the AFG compounds results in reduction of their activity by a factor of 2 while unsaturated compounds are 4-5 times more potent compared to dihydro derivatives. The structure of B toxin resembles closely with the structure of pyrrozolidone which has got basic carcinogenic activity. Aflatoxins are potent mutagens, carcinogens, teratogens and liver damaging agents.

Mechanism of toxicity: Majority of the aflatoxins ingested in feed are physically bound to ruminal contents and as little as 2-5% reach the intestines. Aflatoxin BI in excess of 100  mg/kg of feed is considered to be toxic to cattle. Aflatoxins are rapidly absorbed from the intestine and bound to blood proteins. Liver removes most of the toxins from the blood stream. AFB₁ is primarily metabolized by a microsomal cytochrome P-450 dependent mixed function oxidase system in the liver and other organs to AFB₁ -2,3 epoxide mainly, which is a highly reactive intermediate. Other metabolites of AFB₁ are Q₁, P₁, B₂ and aflatoxicol. Epoxides are chiefly detoxified by glutathione