Brevetoxin (PbTx), or brevetoxins, are a suite of cyclic polyether compounds produced naturally by a species of dinoflagellate known as Karenia brevis. Brevetoxins are neurotoxins that bind to voltage-gated sodium channels in nerve cells, leading to disruption of normal neurological processes and causing the illness clinically described as neurotoxic shellfish poisoning (NSP).
Although brevetoxins are most well-studied in K. brevis, they are also found in other species of Karenia and at least one large fish kill has been traced to brevetoxins in Chattonella.
Other Brevetoxins: Brevetoxin-5 (PbTx-5): like PbTx-2, but acetylated hydroxyl group in position 38. Brevetoxin-6 (PbTx-6): like PbTx-2, but double bond 27-28 is epoxidated.
Brevetoxin-B was synthesized in 1995 by K. C. Nicolaou and coworkers in 123 steps with 91% average yield (final yield ~9·10−6) and in 2004 in a total of 90 steps with an average 93% yield for each step (0.14% overall).
K. C. Nicolaou and coworkers reported their synthesis of Brevetoxin-1 in 1998. In 2009, Michael Crimmins and co-workers reported their synthesis of Brevetoxin-1 as well.
Biosynthesis
Brevetoxins share the common backbone structure of polyketides, but there are several methyl and oxygen groups that are not typical of a traditional polyketide synthesis. Labelling studies identifying the origin of the various carbon atoms have shown that the biosynthesis of brevetoxins greatly deviates from the polyketide synthetic pathway.
From the labelling experiments of Brevetoxin-B (BTX-B), a 50-carbon molecule, 16 carbon signals were enhanced by [1-C13] acetate, 30 signals were enhanced by [2-C13] acetate, and 4 carbon signals were enhanced by [methyl-C13] methionine. Furthermore, 14 intact acetate units were identified with a fifteenth two carbon unit with a weak possibility of being an acetate unit. It is clear based on the oxygen locations of BTX-B that this molecule could not be produced using a traditional polyketide synthetic pathway. Attention was turned to the citric acid cycle to solve the problem. Acetate can be used in the polyketide synthetic pathway or modified by the citric acid cycle. Intermediate products of this cycle can then be reintroduced to the polyketide synthetic pathway, resulting in the addition of atypical carbon units. Previous studies of the citric acid pathway revealed three and four carbon units that can potentially explain the atypical condensation and oxidation pattern seen in BTX-B. That being said, there is currently no explanation as to why this particular pattern is favored.
All these things considered, a proposed biosynthetic pathway for brevetoxin class compounds begins with a traditional polyketide synthesis that has the potential to incorporate larger carbon units originating from acetate modified by the citric acid cycle. After the carbon backbone is synthesized, oxidation produces the necessary epoxides that lead to the closure of the multi-ring system. It is unclear if methyl groups as seen in BTX-B are added after cyclization or during the modification of the polyketide metabolites, but it is clear that methyl groups can originate from sources outside of acetate such as S-adenosylmethionine.
Impact on human and animal health
Exposure
The potent polyether brevetoxins produced by K. brevis activate voltage-sensitive sodium channels. Specifically, brevetoxins bind to site 5 on the alpha-subunit of voltage sensitive sodium channels (VSSCs), which serve as key proteins in the structure of the cell membrane. The binding of brevetoxin to VSSCs produces three key effects: a lowering of the activation potential to more negative values, the persistent activation of the channel and therefore repetitive firing of nerves, and the inability to reverse this prolonged mean open state. This leads to a number of health problems in both humans and animals alike. The disruption of normal sodium channel function namely results in massive fish kills and the poisoning of marine mammals and other aquatic invertebrates, which in turn are a source of human health problems as well. For instance, pulmonary receptors associated with ligand-gated epithelial Na+ channels and cathepsin inhibition in macrophages have been reported to be affected by brevetoxin exposure.