Aflatoxins are categorized as secondary metabolites, which are substances that aren’t necessary for growth. Aspergillus flavus (hence the name aflatoxin) and Aspergillus parasiticus, both of which are common in tropical and subtropical environments, produce the majority of aflatoxin. They grow on the root systems of groundnut (peanut) trees and on agricultural leftovers, which provides inoculum for the colonization of underground groundnut fruits.
Almost any peanut purchased from a grocery store can have A. flavus separated from the shells; it appears as grey or black patches inside the shells. The majority of strains do not, however, make aflatoxin, and even those that do need particular environmental conditions. Aflatoxin formation is particularly favoured by oil-rich crops like cottonseed and peanut fruits, which is in line with the discovery that lipids increase aflatoxin production in laboratory cultures.
Some of the most susceptible crops to aflatoxin are maize and groundnut. During the pre and post-harvest periods, both are frequently exposed to Aspergillus infections. As a result of climate change, there are now more frequent extreme weather events, hot, dry weather, and variable rainfall patterns, which promote the growth of fungi that produce aflatoxin, increasing the frequency and severity of contamination attacks. The main abiotic factors that increase the risk of Aspergillus infection in crops and reduce crop productivity are heat and drought.
Aflatoxin contamination results in economic losses for commodities like corn, cottonseed, peanuts, sorghum, wheat and rice as well as for processed foods and animal feeds. The FAO estimates that mycotoxins impact 25% of the world’s food crops each year. The average yearly cost of crop loss due to aflatoxins contamination for US farmers is above $100 million, with peanuts accounting for $26 million ($69.34/ha).
Types of Aflatoxin:
The Turkey X disease, a serious animal poisoning incidence that occurred in England in 1960, was the first time the A. flavus poisons were found to be responsible. A. parasiticus also produces aflatoxins B1, B2, G1, and G2, although A. flavus produces the majority of aflatoxins B1 and B2. Based on their relative mobility by thin-layer chromatography on silica gel and their blue (B) or green (G) fluorescence under ultraviolet light, these four major aflatoxins are given their respective names.
Cows metabolize aflatoxin B1 into aflatoxin M1, a hydroxylated derivative that is released in milk. The most harmful and potent carcinogen for both humans and other species, including nonhuman primates, birds, fish and rodents is aflatoxin B1. It is one of the four main forms of aflatoxins. A reduced immune response, starvation, bile duct growth, centrilobular necrosis, fatty liver infiltration, hepatic lesions, and even hepatomas can be the results of prolonged exposure. Aflatoxin B1 is also a potential immunosuppressive agent.
How are aflatoxins detected?
Because aflatoxins naturally glow green or blue when exposed to UV light, thin-layer chromatography can be used to detect them in food extracts. Aflatoxin types G (green-fluorescing) and B (blue), which differ in colour due to the ring structure of the molecule, can be distinguished from one another. Each of these compounds has a different level of toxicity depending on whether or not the terminal furan ring has a double bond.
Aflatoxins B2 and G2 are carcinogenic in comparison to aflatoxins B1 and G1, which lack the double link and are merely mildly toxic. Aflatoxin B1’s increased toxicity is also related to toxicity.
Enzyme Related to aflatoxin biosynthesis:
Aflatoxin-producing enzymes do have certain unique properties of their own; they are only active or generated when normal growth has stopped and differentiation has started. In some instances, they show relative specificity, which means that an enzyme catalyzes comparable processes with a number of structurally related metabolites, frequently leading to the formation of a metabolic grid as opposed to main metabolic enzymes which are typically totally specific.
The main precursors of the secondary metabolites are acetate “polyketides.” Following the lipid breakdown by lipases, the fatty acids are converted to acetyl-CoA via -oxidation, and the aflatoxins can subsequently be produced from acetyl-CoA via the polyketide pathway.
Role of Aflatoxin in Pathogenesis
The considerable inhibition of chlorophyll and carotenoid synthesis, as well as the decreased germination and seedling growth of lettuce, mung, mustard, gramme, cowpea and sesame, allowed researchers to conclude that aflatoxin had a phytotoxic effect. It has been hypothesized that aflatoxin prevents the production of chlorophyll, causing the affected plants to virescent or develop albinism.
AFB1’s phytotoxic effects on regenerating plantlets of Nicotiana tabacum in vitro cultures revealed that root and leaf mass and development were considerably reduced. Another factor influencing the decline in tillers in plants treated with high levels of aflatoxin was the accumulation of DNA damage in cells, which causes apoptosis. Additionally, research using electron microscopy showed that aflatoxins treatment of maize leaves chloroplasts inhibited the formation of grana.
At greater toxin concentrations, aberrant cytoplasmic components, membrane disintegration, particularly of the tonoplast, loss of ribosomes, organellar rupture and disappearance of the endoplasmic reticulum were all associated with the inhibition of root and shoot elongation.
Aflatoxin inhibits the function of chromatin-bound DNA-dependent polymerase. The absence of m-RNA was considered to inhibit protein synthesis, whereas aflatoxin binding to DNA during replication or DNA polymerase inhibition was supposed to hinder DNA synthesis. Aflatoxin has been demonstrated to cause apoptosis and to stop the cell cycle in growing cells. The only viable option when a cell’s DNA repair mechanisms are overworked due to significant damage is the apoptotic pathway.
Commodities deemed hazardous for consumption by humans can be included in animal feeds. A. flavus growth causes a decline in grain quality, rendering it unfit for marketing and eating. Due to fungus infection, alfa-root disease and seed and non-emerged seedling deterioration were seen in groundnuts. The health of people and animals as well as global trade are seriously threatened by aflatoxin contamination in grain. Acute aflatoxicosis results in death, however chronic aflatoxicosis leads to pathologic changes that remain longer, like cancer and immunosuppression.
The liver is the primary organ that is affected by aflatoxin B1, which can cause liver damage in nonhuman primates, poultry, and rodents. The human manifestation of acute aflatoxicosis is acute hepatitis.
1Hafiz M. Rizwan Mazhar, 2Muhammad Huzaifa Tanveer
1. Nuclear Institute for Agriculture and Biology Faisalabad
2. Department of Plant Pathology University of Agriculture, Faisalabad
