Paralytic Shellfish Toxins: Saxitoxin and Analogues

Paralytic shellfish toxins (PSTs) are a kind of natural neurotoxic alkaloids, of which saxitoxin (STX) is the most studied PST since 1957. 57 Saxitonxins analogues have been found. PSTs is mainly produced at harmful concentrations during harmful algal bloom events (HAB). In the past few decades, HAB has become more frequent and more intense. PSTs can be widely characterized as hydrophilic or hydrophobic and can be divided into subunits according to substituent side chains such as carbamate, sulfate, hydroxyl, hydroxybenzoate or acetate. Then, each part will produce varying degrees of toxicity.

In marine environments, PSTs are primarily produced by the eukaryotic dinoflagellates, belonging to the genera Alexandrium, Gymnodinium and Pyrodinium. In freshwater environments the PSTs are produced by prokaryotic cyanobacteria belonging to the genera Anabaena, Cylindrospermopsis, Aphanizomenon, Planktothrix and Lyngbya. Cyanobacterial PST produces blooms and pollutes water for drinking and entertainment. In the past, high levels of toxins have been detected in freshwater resources in Australia, Brazil, the United States, Mexico, Germany, and China.

Saxitoxin Structure

Saxitonxin (STX) is one of the most effective natural neurotoxins known, and ingestion of about 1 mg of toxin in a single serving of contaminated shellfish can be fatal. Saxitonxin is an alkaloid with a molecular formula of C₁₀H₁₇N₇O₄ (molecular weight = 299). It consists of a tricyclic system of 3-propyl-dihydropurine. STX is a large class of marine natural products containing guanidine, and its high polarity is caused by the existence of two guanidine groups. Since its initial discovery, 57 naturally occurring saxitonxin analogues, collectively referred to as PSTs, have been identified in many organisms. A novel group of PSTs with a hydrophobic side chain were identified within the cyanobacterium Lyngbya wollei and are characterized by the presence of an acetate at C13 (LWTX 1–3,5,6) and a carbinol at C12 (LWTX 2,3,5) in place of a hydrated ketone.

The strangest saxitonxin isolates found so far was isolated from the Panamanian golden frog Atelopus zeteki and named Zetekitoxin AB. Zetekitoxin AB has been proved to be a kind of PST containing unique 1-line 2-oxazolidine ring fused lactam. It has strong binding affinity to brain, heart, and muscle Na⁺ channels, and its toxicity to each channel is about 580,160 and 63 times higher than that of saxitonxin, respectively. The continuous discovery of new and more diverse saxitonxin analogues is a challenge to the identification and monitoring of PST.

Figure 1. The structure of saxitonxin and its analogues
(Source: Wiese M, et al. 2010)

Saxitoxin Toxicity

PSTs poisoning can lead to a serious and sometimes fatal disease, paralytic shellfish poisoning (PSP). This disease occurs when PST reversibly binds the voltage-gated Na⁺ channel at an equal molar ratio. This is regulated by the interaction between the positively charged guanidine group of saxitonxin (STX) and the negatively charged carboxyl group at position 1 of the Na⁺ channel, thus blocking the channel. saxitonxin block sodium channels on nerve cells, thereby inhibiting the transmission of nerve impulses. Subsequently, stimulation of muscles is inhibited, including those associated with breathing, leading to respiratory paralysis and eventual death from respiratory failure. The threat of PSPs is not only a major cause for public health concern, but it is also detrimental to the economy. Outbreaks of PSTs often result in the death of marine life and livestock, the closure of contaminated fisheries, and the ongoing expenditures required to maintain and operate monitoring programs, all of which add up to a significant economic burden around the world.

Figure 2. The proposed transmembrane arrangement of the α-subunit of Na⁺ channels
(Source: Wiese M, et al. 2010)

The most common PST is hydrophilic. In some studies, it has been found that the introduction of hydrophobic side chains into the structure of saxitoxin analogues can reduce the binding to receptors on sodium channels, thus reducing toxicity. saxitoxin is also lipophilic, so it can be adsorbed on fat, perhaps related to bioaccumulation.

Figure 3. The toxicity of saxitoxin is compared to other toxins
(Source: Christensen VG, et al. 2020)

Pharmaceutical Potential of The Paralytic Shellfish Toxins

In recent years, there has been a renewed interest in PSTs as a therapeutic agent or as a drug leader. Biological activity studies and molecular simulations of a series of PSTs may also lead to the design of unnatural analogues with improved pharmaceutical properties. For example, ω-conotoxin MVIIA analogs act by targeting N-type voltage-sensitive calcium channels for the treatment of chronic pain caused by spinal cord injury.

Saxitoxin also has great pharmaceutical potential because it can induce anesthesia by interacting with the first site of the voltage-gated Na⁺ channel, but its systemic toxicity prevents it from entering clinical trials. The use of a controlled release system in male SD rats has proved that saxitoxin can be used as a slow-release, long-term anesthetic. The liposome preparation of saxitoxin alone or in combination with dexamethasone can block the sciatic nerve of rats for a long time without muscle toxicity, cytotoxicity or neurotoxicity and no related inflammation. Therefore, the liposome preparation of saxitoxin is expected to be used for slow and local targeted release and to prolong the time of anesthesia.

Gene identification of PST biosynthesis pathways from different biological sources improves our molecular understanding of the specific mechanism of PST synthesis. This will eventually promote the study of the pharmaceutical potential of PST as a sodium channel blocker by producing new analogues or increasing the availability of analogues that are otherwise biosynthesized at low concentrations. Bioengineering can also further enhance the structural diversity of bioactive small molecules by using enzymes for chemical synthesis in vitro and combinatorial biosynthesis in vivo.

STX Antibodies

STX Antigens