Aflatoxins

作者：admin  点击次数：32  发布时间：2025-04-02

Aflatoxin (AFT) is a metabolite of Aspergillus favus and Aspergillus nominus. All strains of Aspergillus favus can produce aflatoxin, but Aspergillus nominus is rare in my country. Aspergillus favus is a common fungus in grain and feed in my country. AFT is highly carcinogenic and therefore has received attention.
1. Chemical structure and properties
Aflatoxin is a class of compounds with similar structures. Their basic structures all have a bicyclic ring and coumarin (oxa-naphthoquinone). They all emit fluorescence under ultraviolet light. According to the fluorescence color and its structure, they are named B, B, G, GM, M2, P, Q, H, GM, etc. At present, more than 20 species have been isolated and identified, of which 6 are the most toxic. Their chemical structures are shown in Figure 2-1. The toxicity of aflatoxin is related to its structure. Those with double chains at the end of the bicyclic ring are highly toxic and carcinogenic. The order of toxicity of aflatoxin is as follows: B, > M, > G, > B, > M,. When observed under ultraviolet light, the fluorescent colors of these toxins can be seen: B, B, - blue, G - green, G - green-blue, M, - blue-purple, M - purple.
Aflatoxin is heat-resistant, with a cracking temperature of 280°C. It is insoluble in water and easy to dissolve in oil and some organic solvents. However, under alkaline conditions (with NaOH), the lactone ring of aflatoxin is destroyed to form sodium coumarin, which is soluble in water and washed away.
2. Toxin production conditions and food contamination
The temperature range for the growth and production of aflatoxin by Aspergillus flavus is 12~42°C, the optimal temperature for toxin production is 25~33°C, and the optimal A value is 0.93~0.98. When Aspergillus flavus grows on corn, rice, and wheat with a moisture content of 18.5%, it begins to produce AFT on the 3rd day. The toxin production reaches its peak on the 10th day, and then gradually decreases. When the fungus forms spores, the toxins produced by the mycelium are gradually discharged into the matrix. This hysteresis phenomenon of aflatoxin production means that if high-moisture grains are dried within 2 days and the moisture content of the grains drops below 13%, no toxins will be produced even if they are contaminated with aflatoxin. Different strains have very different toxin-producing abilities. In addition to the matrix, temperature, humidity, and air are necessary conditions for the growth, reproduction, and toxin production of aflatoxin. Although molds produce more toxins on natural matrices (such as corn, peanuts, rice, etc.) than on synthetic culture media, most studies on the toxin-producing conditions of aflatoxin and parasitic Aspergillus are conducted under artificial culture media.
AFT contamination can occur in a variety of foods, among which corn, peanuts, and cottonseed oil are the most susceptible to contamination, followed by rice, wheat, barley, beans, etc. In addition to grain and oil foods, my country also has dried fruit foods such as walnuts, almonds, and seeds; animal foods such as milk and dairy products, liver, dried salted fish, and dried peppers have also been reported to be contaminated with aflatoxin. Fermented products produced on a large scale, such as sauces and soy sauces, are generally free of contamination, but home-made fermented foods have been reported to contain aflatoxin. In the hot and humid areas of southern my country, some grains, oils and their products are also contaminated. However, except for individual samples in North China, Northeast China and Northwest China, they are generally not contaminated by aflatoxin.
3. Metabolic pathways and metabolites The main metabolic pathways of AFB, (abbreviated as B,) in the body are hydroxylation, demethylation and epoxidation reactions. AFM, (abbreviated as M,) is the hydroxylation product of AFB, catalyzed by liver microsomal enzymes. It was first found in the milk of cows and sheep. If sheep are fed feed containing AFB, M, and a small amount of B, will be found in the milk within 7 hours. After stopping feeding this feed for 5 days, M, no longer appears. AFQ: (abbreviated as Q,) is the metabolite of AFB after hydroxylation. Its hydroxyl group is on the carbon atom B of cyclopentane and has a strong yellow-green fluorescence. In the liver of monkeys, the conversion ratio of B, to Q, is relatively high, ranging from 19% to 51%. Some people use "C to label aflatoxin B, and the results show that monkey liver microsomes can convert 32% of B into Q. The conversion of aflatoxin B into Q may be a detoxification process.
Another metabolite of AFB is the epoxide of the double bond at the end of the difuran ring (aflatoxin B-2,3-cyclized product). Part of the epoxide can combine with glutathione S-transferase, uridine diphosphate-glucuronyl transferase or sulfotransferase to form a macromolecule, which is detoxified by hydrolysis catalyzed by cyclooxygenase; the other part combines with biological macromolecules DNA, RNA and protein to exert its toxic, carcinogenic and mutagenic effects. Some people believe that the double bonds on the difuran rings of B, G, and M are very easy to undergo epoxidation reactions, so they are very toxic; B and G, which do not have dicyclic double bonds, are less toxic. Many studies have also shown that the adducts formed by the metabolic activation products of aflatoxin B and DNA are organ-specific and dose-dependent, and are closely related to the sensitivity of animals to the carcinogenicity of aflatoxin B. Aflatoxin B-DNA The main role of adducts in the carcinogenic process is to activate proto-oncogenes. In recent years, quantitative analysis of aflatoxin B,-DNA adducts has become an indicator for detecting the toxic effects of aflatoxin B, and the sensitivity of using monoclonal antibodies to detect aflatoxin B,-DNA adducts is very high, which can be directly used to detect the level of human exposure to aflatoxin B and the damage to the human body.
Except for M, most of the metabolites of AFT are excreted from milk, and the rest can be excreted through urine, feces and exhaled CO. After animals ingest AFT, the content in the liver is the highest, which can be 5 to 15 times that of other organs and tissues. It can also be detected in the kidneys, adrenal glands, and there are very small amounts in the blood, but it is generally not detected in the muscles. If aflatoxin is not ingested continuously, it generally does not accumulate in the body. After a single intake of aflatoxin, most of it can be excreted through breathing, urine, feces, etc. in about a week.