Classification of Mycotoxins | Fungi

In this article we will discuss about the classification of mycotoxins:- A. Food Toxins B. Ergot Toxins C. Mushroom Toxins.

A. Food Toxins:

These toxins are mainly responsible for poisoning of foods. The mycotoxin production occurs in many plant products, especially cereals and oilseeds. Others like peanuts, rice, corn and cotton seeds also get con­taminated and store mycotoxins. The mycotoxins are produced by more than 100 mold species, mainly by Aspergillus, Penicillium and Fusarium.

According to World Health Organisation (WHO), 1969, the mycotoxins associated with human diseases are of four types. These are aflatoxins, ochratoxins, zearalenone, and trichothecenes.

(a) Aflatoxins:

Aflatoxins are potent toxic, carcinogenic, mutagenic, immuno-suppressive agents, produced as secondary metabolites by the fungi on variety of food products. The ana­toxin problem was first recognised in 1960, when there was severe outbreak of a disease referred as “Turkey X’ Disease” in U.K., in which over 100,000 turkey birds died.

Aflatoxins are the most potent carcinogens, known as the suspec­ted cause of liver cancer in human beings. The aflatoxins are produced in dried food and ground­nut meal, infested with Aspergillus flavus, A. para­siticus, A. fumigatus or Penicillum islandicum.

Aflatoxins normally refers to the group of difuranocoumarins and are classified in two broad groups according to their chemical struc­ture the difurocoumarocyclopentenone series and the difurocoumarolactone series. About 18 types of aflatoxins have been identified, the major members are aflatoxin B₁, B₂, G₁, G₂, while M₁, and M₂ are less significant.

Aflatoxins are soluble in methanol, chloroform, acetone, acetonitrile and fluoresce strongly in ultraviolet light (365 nm); B₁ and B₂ produce a blue fluo­rescence, while G₁ and G₂ produce green fluo­rescence.

Aflatoxins bind with DNA to prevent trans­cription, thereby protein synthesis is inhibited.

In addition to suspected carcinogenic nature, other clinical symptoms due to aflatoxin poisoning are jaundice, portal hypertension, rapidly developing ascites, etc.

Structure of Aflatoxins:

(i) B₁:

Molecular weight, 312; melting point 268-269°C and molecular formula C₁₇H₁₂O₆.

(ii) B₂:

It is dihydro derivative of B₁, a minor toxin with lower Rf value than B₁. Molecular weight, 314; melting point 286-289°C and molecular formula C₁₇H₁₄O₆.

(iii) G₁:

Molecular weight, 328; melting point 244-246°C and molecular formula C₁₇H₁₂O₇.

(iv) G₂:

It is the dihydro derivative of G,. Molecular weight, 330; melting point 237-240°C and molecular formula C₁₇H,₄O₆.

(v) M₁:

It is the hydroxy aflatoxin B, with molecular weight 328; melting point 299°C and molecular formula C₁₇H₁₂O₇.

(vi) M₂:

It is dihydroaflatoxin M, with molecular eight 330; melting point 293°C and molecular formula C₁₇H₁₄O₇.

(vii) Other Forms such as B_2a Molecular Weight 330, Melting Point 240°C, Formula:

C₁₇H₁₄O₇) and G_2a (Molecular weight 346, mel­ting point 190°C, formula: C₁₇H₁₄O₈) are the hydroxyl derivatives of aflatoxin B₂ and G₂.

(b) Ochratoxins:

Ochratoxins are closely related derivatives of isocoumarin, linked to an amino acid, L-β-phenylalanine. Of the nine types of ochratoxins, ochratoxin A is the most effective one. Ochratoxins are produced in maize, peanuts, beans etc., infested mainly with Penicillium viridicatum or Aspergillus ochraceus.

(c) Zearalenone:

Zearalenone is a phenolic resorcyclic acid lactone, produced in maize and also in other cereals, infested with different members of Fusarium such as F. moniliformae and F. graminearum. It shows esterogenic symp­toms in swine.

(d) Trichothecenes:

Trichothecenes possess a tetracyclic 12, 13-epoxytrichothecoene skele­ton. So far, more than 30 trichothecenes have been identified, of which T-2 toxin, nivalenol and deoxynivalenol, shows marked importance.

Trichothecenes are produced by different species belongs to the genera Fusarium, Myrothecium, Trichoderma, Stachybotrys and Cephalosporium. This group of toxins shows sub-epidermal haemorrhage, several local irritations and gene­ral necrosis.

B. Ergot Toxins:

These toxins are stored in the elongated sclerotia of Claviceps purpurea, the pathogen of ergot disease of rye. These are the poi­sonous alkaloids such as ergotamine, ergocrystinine, ergometrimine, ergonovin and ergocrystine.

Due to consumption of sclerotia along with rye straw, the domestic animals show the follow­ing symptoms:

(i) The hoofs, legs and tail become gan­grenous,

(ii) Abortion in cows.

Due to consumption of powdered sclerotia along with rye flour, following symptoms become pronounced in human being:

(i) Diarrhoea, abdominal pain and vomi­ting.

(ii) Effects on nerves and cause psychiatric disturbances, and

(iii) Decrease the diametre of terminal blood vessels.

C. Mushroom Toxins:

Several mushrooms produce toxins and by consuming such mush­rooms as food, different abnormalities become visible in human beings like diarrhoea and vomi­ting within 2-3 hrs of consumption. At a later stage they cause liver damage, kidney failure, complete unconsciousness and even death.

The following mushrooms are important in this respect:

(i) Amanita phalloides (the ‘death cup’). Out of 10 toxins so far reported, the phalloidin and α-amanitin are important. The phalloidin affects the cell membrane of liver cells and α-amanitin causes lesions in stomach and intestine cells.

(ii) Different species of Helvella such as H. infula, H. gigas, H. esculenta and H. underwoodii, produce gyromitrin, a fatal toxin.

(iii) Inocybe and Clitocybe produce toxin muscarine that causes several abnor­malities.