Trichothecenes constitute a large group of chemically related mycotoxins. They are produced by fungi of the genera Fusarium, Myrothecium, Trichoderma, Trichothecium, Cephalosporium, and Stachybotrys. Chemically, trichothecenes are a class of sesquiterpenes.
All trichothecenes share a cyclic terpene core but differ in the type of functional groups (R groups) attached to the carbon backbone. They are produced on many different grains such as wheat, oats, or maize by various Fusarium species including F. graminearum, F. sporotrichioides, F. poae, and F. equiseti.
Some moulds that produce trichothecene mycotoxins, such as Stachybotrys chartarum, can grow in damp indoor environments. It has been found that macrocyclic trichothecenes produced by S. chartarum can become airborne and thus contribute to health problems in humans.Detection of Airborne Stachybotrys chartarum Macrocyclic Trichothecene Mycotoxins in the Indoor Environment
Trichoderma cornu-damae (syn. Podostroma cornu-damae), a mushroom native to Asia, contains potentially fatal levels of satratoxin H.
Classification
General classification
Trichothecenes are a group of over 150 chemically related toxic mycotoxins. Each trichothecene displays a core structure consisting of a six-membered ring containing a single oxygen atom, flanked by two carbon rings. This core ring structure contains an epoxide bridging carbons 12 and 13, as well as a double bond between carbons 9 and 10. These two functional groups are primarily responsible for trichothecenes' ability to inhibit protein synthesis and incur general cytotoxic effects. Notably, this core structure is amphipathic, containing both polar and nonpolar parts. All trichothecenes are related through this common structure but are differentiated by the substitution pattern of oxygen-containing functional groups on carbons 3, 4, 7, 8, and 15. These functional groups govern the properties of an individual trichothecene and also serve as the basis for the most commonly used classification system for this family of toxins. This classification system breaks up the trichothecene family into four groups: Type A, B, C, and D.
Type A trichothecenes have hydroxyl or O-linked ester substitutions around the core ring structure. Common examples of these are neosolaniol with a hydroxyl substitution at carbon 8, and T-2 toxin with an ester substitution at carbon 8. Type B trichothecenes are classified by the presence of oxo-substitutions around the core ring structure. Common examples of these include nivalenol and trichothecene, which both have a ketone functional group at carbon 8. Type C trichothecenes have an additional epoxide bridging the carbons 7 and 8. A common example of this is crotocin, which also has an O-linked ester functional group at carbon 4. Type D trichothecenes have an additional macrocylic ring between carbon 4 and carbon 15. These rings can have varied additional functional groups. Common examples of these are roridin A and satratoxin H.
Although the distinct functional groups of these classification types give each trichothecene unique chemical properties, their classification type does not explicitly indicate their relative toxicity. While the type D group is thought to contain the most toxic trichothecenes, type A and B trichothecenes vary considerably in their toxicity.
Alternative classifications The classification system described above is the most commonly used to group molecules of the trichothecene family. However, a variety of alternative classification systems also exist for these complex molecules. Trichothecenes can also be generally described as simple or macrocyclic. Simple trichothecenes include types A, B, and C, whereas macrocyclic trichothecenes include Type D and are characterized by the presence of a carbon 4 – carbon 15 bridge. Additionally, J. F. Grove proposed a classification of trichothecenes into three groups that was also based upon the functional substitution patterns of the ring skeleton. Group 1 trichothecenes only have functional groups substituted on the third, fully saturated carbon ring. Group 2 trichothecenes contain additional functional groups on the core ring containing the 9, 10 carbon double bond. Finally, group 3 trichothecenes contain a ketone functional group at carbon 8; this is the same criteria for type B trichothecenes.
Advances in the field of evolutionary genetics have also led to the proposal of trichothecene classification systems based on the pathway of their biosynthesis. Genes responsible for the biosynthesis of a mycotoxin are typically located in clusters; in Fusariumi these are known as TRI genes. TRI genes are each responsible for producing an enzyme that carries out a specific step in the biosynthesis of trichothecenes. Mutations in these genes can lead to the production of variant trichothecenes and therefore these molecules could be grouped based on shared biosynthesis steps. For example, a shared step in the biosynthesis of trichothecenes is controlled by the gene TRI4. This enzyme product controls the addition of either three or four oxygen atoms to trichodiene to form either isotrichodiol or isotrichotriol respectively. A variety of trichothecenes can then be synthesized from either of these intermediates and they could therefore be classified as either t-type if synthesized from isotrichotriol or d-type if synthesized from isotrichodiol.
Mechanism of action
The toxicity of trichothecenes is primarily due to their action as protein synthesis inhibitors; this inhibition occurs at ribosomes during all three stages of protein synthesis: initiation, elongation, and termination. During initiation, trichothecenes can either inhibit the association of the two ribosomal subunits or inhibit the function of the mature ribosome by preventing the association of the first tRNA with the start codon. Inhibition at elongation most likely occurs due to trichothecenes preventing the function of peptidyl transferase, the enzyme which catalyzes the formation of new peptide bonds on the 60s ribosomal subunit. Inhibition during termination can also be the result of peptidyl transferase inhibition or the ability of trichothecenes to prevent the hydrolysis required at this final step.
The substitution pattern of the ring core of trichothecenes influences the toxin's action as either an inhibitor of initiation or as an inhibitor of elongation/termination. Trichothecenes may also affect general cellular enzyme function as the 12,13-epoxy moiety is susceptible to nucleophilic attack by active-site thiol groups.
These toxic effects are seen most dramatically in actively proliferating cells such as in the gastrointestinal tract or the bone marrow.
In addition to its effects on protein sysnthesis, it is likely that the trichothecene T-2 toxin also directly affects oxidative metabolism causing mitochondrial dysfunction.
In most eukaryotes, such as humans, energy is primarily produced in mitochondria through the oxidation of carbon and the reduction of oxygen via the electron transport chain. A small percentage of electrons commonly leak from this process, typically 0.2-2% resulting in generation of reactive oxygen species and reactive nitrogen species. These chemical species are highly reactive and result in damage to DNA, proteins, and other proteins through a process called oxidative stress.
T-2 toxin rapidly increases the leakage of electrons from the electron transport system, which corresponds with increased reactive oxygen species and oxidative stress in affected cells. This oxidative stress ultimately triggerrs programmed cell-death, apoptosis, via the intrinsic pathway.
Additionally, trichothecenes such as T-2 have also been shown to increase the c-Jun N-Terminal Kinase signaling pathway in cells. Here, c-Jun N-Terminal Kinase is able to increase to phosphorylation of its target, c-Jun, into its active form. Activated c-jun acts as a transcription factor in the cell nucleus for proteins important for facilitating the downstream apoptotic pathway.
Toxicity The trichothecene mycotoxins are toxic to humans, other mammals, birds, fish, a variety of invertebrates, plants, and eukaryotic cells. The specific toxicity varies depending on the particular toxin and animal species, however the route of administration plays a significantly higher role in determining lethality. The effects of poisoning will depend on the concentration of exposure, length of time and way the person is exposed. Exposure to a concentrated solution or aerosolized toxin is more likely to cause severe effects, including death. Upon consumption, the toxin inhibits ribosomal protein, DNA and RNA synthesis, mitochondrial functions cell division while simultaneously activating a cellular stress response named ribotoxic stress response. The trichothecene mycotoxins can be absorbed though topical, oral, and inhalational routes.
Trichothecenes differ from most other potential weapon toxins since they can act through the skin, which is attributed to their amphipathic and lipophilic characteristics. The small amphipathic nature of trichothecenes allows them to easily cross cell membranes and interact with different organelles such as the mitochondria, endoplasmic reticulum (ER). and chloroplast The lipophilic nature of trichothecenes allow them to be easily absorbed through skin pulmonary mucosa, and gut. Direct dermal application or oral ingestion of trichothecene causes rapid irritation to the skin or intestinal mucosa. As a dermal irritant and blistering agent, it is alleged to be 400 times more intoxicating than sulfur mustard.
The response in the body to the mycotoxin, alimentary toxic aleukia, occurs several days after consumption, in four stages:
The first stage includes inflammation of the gastric and intestinal mucosa. The second stage is characterized by leukopenia, granulopenia, and progressive lymphocytosis. The third stage is characterized by the appearance of a red rash on the skin of the body, as well as hemorrhage of the skin and mucosa. If severe, aphonia and death by strangulation can occur. By the fourth stage, cells in the lymphoid organs and erythropoiesis in the bone marrow and spleen are depleted and immune response is down. Infection can be triggered by an injury as minor as a cut, scratch, or abrasion.
The following symptoms are exhibited:
Severe itching and redness of the skin, sores, shedding of the skin Distortion of any of the senses, loss of the ability to coordinate muscle movement Nausea, vomiting and diarrhea Nose and throat pain, discharge from the nose, itching and sneezing Coughing, difficulty breathing, wheezing, chest pain and spitting up blood Temporary bleeding disorders Elevated body temperature
Regulatory issues When it comes to animal and human food, type A trichothecenes (e.g. T-2 toxin, HT-2 toxin, diacetoxyscirpenol) are of special interest because they are more toxic than the other foodborne trichothecenes, e.g. type B group (e.g. deoxynivalenol, nivalenol, 3- and 15-acetyldeoxynivalenol). However, deoxynivalenol is of concern as it is the most prevalent trichothecene in Europe. The major effects of trichothecenes are reduced feed uptake, vomiting, and immuno-suppression.
Relatively few countries, primarily in the European Union, have recommended maximum limits for these mycotoxins in food and animal feed. However, trichothecenes are often tested for elsewhere, in order to prevent them from entering the food chain and to prevent losses in animal production.
History