T 2 mycotoxin

History

Alimentary toxic aleukia (ATA), a disease which is caused by trichothecenes like T-2 mycotoxin, killed many thousands of USSR citizens in the Orenburg District in the 1940s. It was reported that the mortality rate was 10% of the entire population in that area. During the 1970s it was proposed that the consumption of contaminated food was the cause of this mass poisoning. Because of World War II, harvesting of grains was delayed and food was scarce in Russia. This resulted in the consumption of grain that was contaminated with Fusarium molds, which produce T-2 mycotoxin.

Although it is still controversial, it is suspected that T-2 mycotoxin has been used as a chemical warfare agent from the 1970s till the 1990s. Based on the descriptions of eyewitnesses and victims, T-2 mycotoxin was mostly delivered by low-flying aircraft that released a yellow oily liquid. Hence, this phenomenon is also named "yellow rain".

In 1982, the US Secretary of State Alexander Haig and his successor George P. Shultz accused the Soviet Union of using T-2 mycotoxin as a chemical weapon in Laos (1975–81), Kampuchea (1979–81), and Afghanistan (1979–81), where it allegedly caused thousands of casualties. Although several US chemical weapons experts have identified "yellow rain" samples from Laos as trichothecenes, other experts believe that this exposure was due to naturally occurring T-2 mycotoxin in contaminated foods. A second alternative theory was developed by Harvard biologist Matthew Meselson, who proposed that the "yellow rain" found in Southeast Asia originates from the excrement of jungle bees. The first indication for this theory came from finding high levels of pollen in the collected samples, giving the substance its yellow color. It was also found that jungle bees in this area fly collectively in great numbers, at altitudes too high to be easily seen, producing showers of feces that could have been mistaken for sprays from aircraft.

T-2 mycotoxin is also thought to be a cause of Gulf War Syndrome. US troops suffered from mycotoxosis-like symptoms after an Iraqi missile detonated in a US military camp in Saudi Arabia during Operation Desert Storm in the Persian Gulf War, in 1991. It has been shown that Iraq researched trichothecene mycotoxins, among other substances, and thus was capable of its possession and its usage in chemical warfare. Nevertheless, much of the key information from these incidents remains classified, leaving these matters still unresolved.

Chemical properties

This compound has a tetracyclic sesquiterpenoid 12,13-epoxytrichothene ring system, which relates it to the trichothecenes. These compound are generally very stable and are not degraded during storage/milling and cooking/processing of food. They do not degrade at high temperatures either. This compound has an epoxide ring, and several acetyl and hydroxyl groups on its side chains. These features are mainly responsible for the biological activity of the compound and make it highly toxic. T-2 mycotoxin is able to inhibit DNA and RNA synthesis in vivo and in vitro and can induce apoptosis. However, in vivo the compound rapidly metabolizes to HT-2 mycotoxin (a major metabolite).

Mechanism of action

The toxicity of T-2 toxin is due to its 12,13-epoxy ring. Epoxides are in general toxic compounds; these react with nucleophiles and then undergo further enzymatic reactions. The reactivity of epoxides can lead to reactions with endogenous compounds and cellular constituents like DNA bases and proteins. These reactions could be the reason for the noticed actions and effects of T-2 mycotoxin. The toxic compound influences the metabolism of membrane phospholipids, leads to an increase of liver lipid peroxidases and has an inhibiting effect on DNA and RNA synthesis. In addition it can bind to an integral part of the 60s ribosomal subunit, peptidyltransferase, thereby inhibiting protein synthesis. These effects are thought to be the explanation for T-2 toxin inducing apoptosis (cell death) in different tissues as the immune system, the gastrointestinal tissue and also fetal tissue. With regard to apoptosis there has been noticed that the level of the pro-apoptotic factor Bas (Bcl-2-associated X protein) was increased and the level of Bcl-xl, an anti-apoptotic factor, was decreased in human chrondocytes (cartilage cells). When exposed to T-2 mycotoxin. Furthermore, the level of Fas, an apoptosis-related cell-surface antigen and p53, a protein regulating the cell cycle, were increased.

Metabolism

T-2 toxin is metabolized after ingestion. In vivo studies showed that the most occurring reactions are ester hydrolysis and hydroxylation of the isovaleryl group. Deepoxidation and glucuronide conjugation do also occur. Ht-2 is the main metabolite. For the hydroxylation, the cytochrome p450 enzyme complex is suggested to be involved. T-2 triol and T-2 tetraol are most likely to be formed via acetylcholine esterases. Some of the metabolic reactions of the mycotoxin are performed by the microflora in the gut. The formed metabolites in these reactions are species- and pH-dependent. The ester cleavages are however performed by the mammal itself and not by the microflora. In red blood cells T-2 mycotoxin is metabolized to neosolaniol and in white blood cells to HT-2 via hydrolysis catalyzed by carboxylesterases.T-2 toxin has a very short half-life.

Synthesis

T-2 mycotoxin is produced naturally by Fusarium fungi of which the most important species are: F. sporotrichioides, F. langsethiae, F. acuminatum and F. poae. These fungi are found in grains such as barley, wheat and oats. The production of this compound for research and commercial purposes is generally accomplished by cultivating some strain of T-2 mycotoxin producing fungi on agar plates. On these agar plates the fungi appear powdery and can yield substantial amounts of T-2 mycotoxin. For the isolation of the compound high pressure liquid chromatography is commonly used (HPLC). No methods for chemically synthesizing T-2 mycotoxin have been established as of yet.

Exposure

Humans and animals are generally exposed to T-2 mycotoxins through food. Certain grains can contain the toxin which makes it a threat to human health and an economic burden. Unlike most biological toxins T-2 mycotoxin can be absorbed through intact skin. The compound can be delivered via food, water, droplets, aerosols and smoke from various dispersal systems. This makes it a potential biological weapon, however large amounts of the compound are required for a lethal dose. T-2 mycotoxin has an LD50 of approximately 1 milligram per killogram of body weight

The EFSA estimates that the mean exposure of T-2 in the EU lies between 12 and 43 ng/kg bw/day. This range is below the TDI of 100 ng/ kg body weight for the sum of HT-2 and T-2 toxins which is used by the EFSA.

Toxic effects

T-2 is highly toxic when inhaled. Acute toxic symptoms include vomiting, diarrhea, skin irritation, itching, rash, blisters, bleeding and dyspnea. If the individual is exposed to T-2 over a longer period alimentary toxic aleukia (ATA) develops.

At first the patient experiences a burning sensation in the mouth, throat and stomach. After a few days the person will suffer from an acute gastroenteritis that will last for 3 to 9 days. Within 9 weeks the bone marrow will slowly degenerate. Also the skin starts bleeding and the total number of leukocytes decreases. Problems with the nervous system can occur.

In the end the following symptoms might occur: a high fever, petechial haemorrhage, necrosis of muscles and skin, bacterial infections of the necrotic tissue, enlarged lymph nodes. There is the possibility of asphyxiation because of laryngeal oedema and stenosis of the glottis. The lack of oxygen is then the cause of death. Otherwise the patient will die of bronchial pneumonia and lung bleeding.

Effects on animals

T-2 mycotoxin is also toxic to animals. The compound is known for having lethal and sub-lethal effects on farm animals. It is often found in contaminated cereal grains that are fed to these animals. Most of the toxic effects are shared between humans and animals. After exposing zebra fish embryos to a concentration of 20 µmol/L or higher malformation and mortality rates increased. The malformations included tail deformities, cardiovascular defects and changes in behavior in early stages of life. This is the result of an increase in the amount of epoxides, which causes cell apoptosis. Other studies have shown that T-2-toxin causes lipid peroxidation in rats after feeding it to them.

The compound also seems to reduce the fertility of ewes and heifers. Research has shown that a high dose of T-2 delays the ovulation due to a delayed follicle maturation. This possibly retards the following luteinisation, which makes it impossible for female animals to conceive.

T-2 also has an effect on the fertility of bulls. In 1998 it was discovered that moldy hay influenced the quality of semen of bulls. Analysis of the moldy hay showed that T-2 was present. The compound decreased motility and testosterone levels and increased the amount of morphological abnormalities within the sperm cells.

The liver is another target for the mycotoxin. It is one of the first organs where the compound passes through after ingestion. Here it causes a reduced expression of CYP1A proteins in rabbits, pigs and rats. CYP3A activity decreases in pigs too. These enzymes help metabolize drugs that pass through the liver. Decrease in the activity could lead to an increase of unmetabolized drugs in the plasma. This can have a dangerous effect on an animal's health.

All of the mentioned effects happen when T-2 is ingested in high doses. Animals are able to metabolize the compound with enzymes from the CYP3A family, just like humans.

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