A competitive displacement assay to detect saxitoxin and tetrodotoxin

Emerging Lyngbya wollei toxins: A new high resolution mass spectrometry method to elucidate a potential environmental threat

Mass spectrometric methods for the quantitative and qualitative analyses of algal biotoxins are often complicated by co-eluting compounds that present analytically as interferences. This issue is particularly critical for organic polyamines, where co-eluting materials can suppress the formation of cations during electrospray ionization. Here we present an extraction procedure designed specifically to overcome matrix-derived ion suppression of algal toxins in samples of Lyngbya wollei, a filamentous benthic algae known to produce several saxitoxin analogues. Lyngbya wollei samples were collected from a large, persistent harmful algal bloom in Lake Wateree, SC. Six known Lyngbya wollei-specific toxins (LWT1-6) were successfully resolved and quantified against saxitoxin using hydrophilic interaction liquid chromatography coupled with triple quadrupole and quadrupole time-of-flight mass spectrometry. The parent ions [M²⁺ - H⁺]⁺ were observed for LWTs 1-6 and the [M]²⁺ ion was observed for LWT5. High resolution mass spectra and unique fragmentation ions were obtained for LWTs 1-6. A dilution factor of 50 resulted in a linear calibration of saxitoxin in the algae matrix. Ion suppression was resolved by sample dilution, which led to linear, positive correlations between peak area and mass of the extracted sample (R² > 0.96). Optimized sample extraction method and instrument parameters are presented.

Use of the receptor binding assay for determination of paralytic shellfish poisoning toxins in bivalve molluscs from Great Britain and the assessment of method performance in oysters

A receptor binding assay (RBA) for the determination of paralytic shellfish poisoning toxicity is formally validated through collaborative study and approved for regulatory monitoring use in the US for mussels and clams. However, to date, the method has not been tested on bivalve molluscs originating from European waters and no validation studies have been conducted for oysters, a shellfish species of great importance globally. This study firstly reports the work conducted to assess the performance of the assay in comparison with a regulatory chemical detection method for a range of shellfish species originating from Great Britain. Data obtained showed a complete absence of false negative RBA results, with a tendency to over-estimate PSP toxicity for some shellfish species in comparison with liquid chromatography with fluorescence detection. Secondly, the performance of the RBA was assessed for oysters, with the analysis of a dilution series of oyster matrix certified reference materials. Method trueness, sensitivity and precision were found to compare well with results reported previously for other species. In addition, the RBA analysis of untreated and demetallated oyster extracts, provided good evidence that the RBA is not suppressed in the presence of high concentrations of zinc as reported previously for the mouse bioassay. Consequently, there is strong evidence from this study, that the RBA would be suitable for determination of PSP toxicity in bivalve molluscs from GB, with acceptable method performance in oysters. Further validation studies would be required for other shellfish species of interest before the method can be considered suitable for implementation in Europe.

Guanidinium Toxins and Their Interactions with Voltage-Gated Sodium Ion Channels

Guanidinium toxins, such as saxitoxin (STX), tetrodotoxin (TTX) and their analogs, are naturally occurring alkaloids with divergent evolutionary origins and biogeographical distribution, but which share the common chemical feature of guanidinium moieties. These guanidinium groups confer high biological activity with high affinity and ion flux blockage capacity for voltage-gated sodium channels (NaV). Members of the STX group, known collectively as paralytic shellfish toxins (PSTs), are produced among three genera of marine dinoflagellates and about a dozen genera of primarily freshwater or brackish water cyanobacteria. In contrast, toxins of the TTX group occur mainly in macrozoa, particularly among puffer fish, several species of marine invertebrates and a few terrestrial amphibians. In the case of TTX and analogs, most evidence suggests that symbiotic bacteria are the origin of the toxins, although endogenous biosynthesis independent from bacteria has not been excluded. The evolutionary origin of the biosynthetic genes for STX and analogs in dinoflagellates and cyanobacteria remains elusive. These highly potent molecules have been the subject of intensive research since the latter half of the past century; first to study the mode of action of their toxigenicity, and later as tools to characterize the role and structure of NaV channels, and finally as therapeutics. Their pharmacological activities have provided encouragement for their use as therapeutants for ion channel-related pathologies, such as pain control. The functional role in aquatic and terrestrial ecosystems for both groups of toxins is unproven, although plausible mechanisms of ion channel regulation and chemical defense are often invoked. Molecular approaches and the development of improved detection methods will yield deeper understanding of their physiological and ecological roles. This knowledge will facilitate their further biotechnological exploitation and point the way towards development of pharmaceuticals and therapeutic applications.

Potential Threats Posed by Tetrodotoxins in UK Waters: Examination of Detection Methodology Used in Their Control

Tetrodotoxin is a neurotoxin responsible for many human fatalities, most commonly following the consumption of pufferfish. Whilst the source of the toxin has not been conclusively proven, it is thought to be associated with various species of marine bacteria. Whilst the toxins are well studied in fish and gastropods, in recent years, there have been a number of reports of tetrodotoxin occurring in bivalve shellfish, including those harvested from the UK and other parts of Europe. This paper reviews evidence concerning the prevalence of tetrodotoxins in the UK together with methodologies currently available for testing. Biological, biomolecular and chemical methods are reviewed, including recommendations for further work. With the recent development of quantitative chromatographic methods for these and other hydrophilic toxins, as well as the commercial availability of rapid testing kits, there are a number of options available to ensure consumers are protected against this threat.

An overview on the marine neurotoxin, saxitoxin: genetics, molecular targets, methods of detection and ecological functions

Marine neurotoxins are natural products produced by phytoplankton and select species of invertebrates and fish. These compounds interact with voltage-gated sodium, potassium and calcium channels and modulate the flux of these ions into various cell types. This review provides a summary of marine neurotoxins, including their structures, molecular targets and pharmacologies. Saxitoxin and its derivatives, collectively referred to as paralytic shellfish toxins (PSTs), are unique among neurotoxins in that they are found in both marine and freshwater environments by organisms inhabiting two kingdoms of life. Prokaryotic cyanobacteria are responsible for PST production in freshwater systems, while eukaryotic dinoflagellates are the main producers in marine waters. Bioaccumulation by filter-feeding bivalves and fish and subsequent transfer through the food web results in the potentially fatal human illnesses, paralytic shellfish poisoning and saxitoxin pufferfish poisoning. These illnesses are a result of saxitoxin’s ability to bind to the voltage-gated sodium channel, blocking the passage of nerve impulses and leading to death via respiratory paralysis. Recent advances in saxitoxin research are discussed, including the molecular biology of toxin synthesis, new protein targets, association with metal-binding motifs and methods of detection. The eco-evolutionary role(s) PSTs may serve for phytoplankton species that produce them are also discussed.

Fluidic force discrimination assays: a new technology for tetrodotoxin detection

Tetrodotoxin (TTX) is a low molecular weight (approximately 319 Da) neurotoxin found in a number of animal species, including pufferfish. Protection from toxin tainted food stuffs requires rapid, sensitive, and specific diagnostic tests. An emerging technique for the detection of both proteins and nucleic acids is Fluidic Force Discrimination (FFD) assays. This simple and rapid method typically uses a sandwich immunoassay format labeled with micrometer-diameter beads and has the novel capability of removing nonspecifically attached beads under controlled, fluidic conditions. This technique allows for near real-time, multiplexed analysis at levels of detection that exceed many of the conventional transduction methods (e.g., ELISAs). In addition, the large linear dynamic range afforded by FFD should decrease the need to perform multiple sample dilutions, a common challenge for food testing. By applying FFD assays to an inhibition immunoassay platform specific for TTX and transduction via low magnification microscopy, levels of detection of approximately 15 ng/mL and linear dynamic ranges of 4 to 5 orders of magnitude were achieved. The results from these studies on the first small molecule FFD assay, along with the impact to detection of seafood toxins, will be discussed in this manuscript.

Quantitation of paralytic shellfish toxins using mouse brain synaptoneurosomes

A membrane potential assay based on synaptoneurosomes prepared from mouse brain was evaluated further for its utility in estimating saxitoxin and related bioactives. Saxitoxin concentrations quantitated in mussel extracts by the synaptoneurosomal technique correlated well with spiked concentrations in these samples (r2 = 0.995; slope=1.048). Other experiments found that the synaptoneurosomal assay can detect saxitoxin-like bioactives in zooplankton samples and the concentrations measured were consistent with preliminary estimations of saxitoxin equivalents using the [3H] saxitoxin receptor binding technique. Veratrine, a mixture of alkaloids that activate sodium channels, had similar potential as a substitute for veratridine in the synaptoneurosomal assay. The results provide additional evidence that the mouse brain synaptoneurosomal membrane potential assay has excellent capability for quantitation of saxitoxin-like activity in shellfish tissues and may also be applied to zooplankton samples.

A fluorimetric method based on changes in membrane potential for screening paralytic shellfish toxins in mussels

To prevent the consumption of bivalves contaminated with paralytic shellfish poisoning (PSP), toxin levels in seafood products are estimated by using the official mouse bioassay. Because of the limitations of this bioassay other methods of monitoring toxins are clearly needed. We have developed a test to screen for PSP toxins based on its functional activity; the toxins bind to the voltage-gated Na+ channels and block their activity. The method is a fluorimetric assay that allows quantitation of the toxins by detecting changes in the membrane potential of human excitable cells. This assay gives an estimate of toxicity, since each toxin present in the sample binds to sodium channels with an affinity which is proportional to its intrinsic toxic potency. The detection limits for paralytic shellfish toxins were found to be 1 ng saxitoxin equivalents/ml compared to the regulatory limit threshold of 400 ng/ml (equivalent to 80 microg/100 g) used in most countries. Our results indicate that this fluorescent assay is a specific, very sensitive, rapid, and reliable method of monitoring PSP toxin levels in samples from seafood products and toxic algae.

Development and preliminary validation of a microtiter plate-based receptor binding assay for paralytic shellfish poisoning toxins

More than 20 countries have either established or proposed regulatory limits for one or more of the paralytic shellfish poisoning (PSP) toxins as they occur in seafood products. PSP toxin levels are generally estimated using the standard AOAC mouse bioassay, yet because of various limitations of this method [e.g. high variability (+/-20%), low sensitivity, limited sample throughput and use of live animals], there remains a need for alternative testing protocols. A sensitive and selective, high capacity assay was developed for the PSP toxins which exploits the highly specific interaction of these toxins with their biological receptor (i.e. voltage-dependent sodium channel) and is thus based on functional activity. This receptor binding assay provides a radioactive endpoint, and is performed in a microtiter filter plate format with results determined by standard liquid scintillation counting within 24 hr. The Ki for the assay is 3.66 +/- 0.86 nM saxitoxin, with a limit of detection of c. 5 ng saxitoxin/ml in a sample extract. Good quantitative agreement of the assay with both mouse bioassay and high-performance liquid chromatographic analysis of crude extracts of contaminated shellfish, as well as PSP toxin-producing algae, was observed. Our findings indicate that the receptor binding assay has a strong predictive value for toxicity determined by mouse bioassay, and that this approach warrants consideration as a rapid, reliable and cost-effective alternative to live animal testing for detection and estimation of PSP-related toxicity in seafood and toxic algae.

Identification of Caribbean ciguatoxins as the cause of an outbreak of fish poisoning among U.S. soldiers in Haiti

On 24 February 1995, six U.S. soldiers serving with the Multinational Force in Haiti became ill after eating a locally caught fish identified as the greater amberjack Seriola dumerili. The victims presented with nausea, vomiting, watery diarrhea and abdominal cramps 5-8 hr after consumption. Also present in some victims were numbness in the extremities or perioral region, bradycardia and scalp paresthesia. Patients were treated with i.v. hydration therapy and antiemetics. All recovered without sequelae over the course of 1-3 months. A portion of the cooked fish was obtained for analysis. A semipurified lipid extract was prepared according to standard methods and analyzed for the presence of Na+ channel site 5 binding activity using a brevetoxin receptor binding assay. By this assay, the fish sample contained the equivalent of approximately 20 ng Caribbean ciguatoxin/g flesh. The presence of the major Caribbean ciguatoxin (C-CTX-1) was confirmed by liquid chromatography-mass spectrometry. Using the receptor binding assay to monitor activity in TSK and PRP-1 column fractions, two minor toxins were detected in addition to C-CTX-1. One of these minor toxins was more polar, and the other less polar, than C-CTX-1. These data provide firm evidence that a family of C-CTX-1 is responsible for ciguatera in the Caribbean.

Hypertension and identification of toxin in human urine and serum following a cluster of mussel-associated paralytic shellfish poisoning outbreaks

Following four outbreaks of paralytic shellfish poisoning on Kodiak Island, Alaska, during 1994, medical records of ill persons were reviewed and interviews were conducted. Urine and serum specimens were analyzed at three independent laboratories using four different saxitoxin binding assays. High-performance liquid chromatography was used to determine the presence of specific toxin congeners. Among 11 ill persons, three required mechanical ventilation and one died. Mean peak systolic and diastolic blood pressure measurements were 172 (range 128-247) and 102 (range 78-165) mmHg, respectively, and blood pressure measurements corresponded with ingested toxin dose. All four different laboratory methodologies detected toxin in serum at 2.8-47 nM during acute illness and toxin in urine at 65-372 nM after acute symptom resolution. The composition of specific paralytic shellfish poisons differed between mussels and human biological specimens, suggesting that human metabolism of toxins had occurred. The results of this study indicate that saxitoxin analogues may cause severe hypertension. In addition, we demonstrate that saxitoxins can be detected in human biological specimens, that nanomolar serum toxin levels may cause serious illness and that human metabolism of toxin may occur. Clearance of paralytic shellfish poisons from serum was evident within 24 hr and urine was identified as a major route of toxin excretion in humans.

Prophylaxis and treatment with a monoclonal antibody of tetrodotoxin poisoning in mice

The ability of a tetrodotoxin (TTX)-specific monoclonal antibody to confer passive protection against lethal TTX challenge was investigated. The monoclonal antibody, T20G10, has an estimated affinity for TTX of approximately 10(-9) M and is about 50-fold less reactive with anhydrotetrodotoxin and unreactive with tetrodonic acid by competitive immunoassay. T20G10 specifically inhibited TTX binding in an in vitro radioligand receptor binding assay, but had no effect on the binding of saxitoxin to the sodium channel on rat brain membranes. In prophylaxis studies, mice were administered T20G10 via the tail vein 30 min prior to i.p. TTX challenge (10 micrograms/kg). Under these conditions, 100 micrograms T20G10 protected 6/6 mice, whereas 3/6 mice were protected with 50 micrograms T20G10. Non-specific control monoclonal antibody did not protect against lethality. Therapy studies simulating oral intoxication were performed with mice given a lethal dose of TTX by gavage in a suspension of non-fat dry milk in phosphate-buffered saline. Death occurred within 25-35 min in 6/6 mice not treated with T20G10. However, 500 micrograms T20G10 administered via the tail vein 10-15 min after oral TTX exposure prevented death in 6/6 mice. Lower doses of mAb conferred less protection.

A monoclonal antibody-based immunoassay for detecting tetrodotoxin in biological samples

Spleen cells from mice hyperimmunized with a keyhole limpet hemocyanin-tetrodotoxin-formaldehyde conjugate were fused with murine P3X63Ag8.653 myeloma cells. A single hybridoma clone was identified that secretes an IgG1,k monoclonal antibody (MAb), designated T20G10, against tetrodotoxin (TTX), with an estimated affinity of 1.2 x 10(8) L/M. Competitive inhibition enzyme immunoassays (CIEIAs) for detecting TTX were developed using this MAb. A direct CIEIA using alkaline phosphatase-labeled MAb detected TTX with sensitivities at IC50 and IC20 of 6-7 ng/ml and 2-3 ng/ml, respectively. The accuracy of the direct CIEIA was comparable with the high-performance liquid chromatography (HPLC) and the mouse bioassay systems, but the direct CIEIA exhibited greater sensitivity. The direct CIEIA was also more cost effective, as it required less sample preparation, a shorter assay time, and reduced investment in equipment than either of the other assay systems.

Lack of specific saxitoxin binding to rat mast cells

We studied whether or not mast cells are endowed with specific sodium channels, by using tritiated saxitoxin which binds to site 1 of sodium channels on excitable tissues. Our results suggest that rat pleural and peritoneal mast cells lack specific sodium channels.

Strain dependent production of ciguatoxin precursors (gambiertoxins) by Gambierdiscus toxicus (Dinophyceae) in culture

Thirteen strains of Gambierdiscus toxicus isolated from Queensland (Australia), Hawaii, French Polynesia and the Virgin Islands were mass cultured and extracted for ciguatoxin. A biodetrital sample containing wild G. toxicus collected from the Republic of Kiribati was also extracted for ciguatoxin. Ciguatoxin, as characterized from moray eels, was not detected in any of the strains examined. Two Queensland strains and the wild G. toxicus produced putative ciguatoxin precursors named gambiertoxins. These gambiertoxins were less polar than ciguatoxin but produced bioassay signs in mice and in-vitro responses in isolated guinea pig atria and vas deferens which were similar (but not identical) to those produced by ciguatoxin. The gambiertoxins from cultured cells were also shown to competitively inhibit the binding of [3H]brevetoxin-3 to rat brain membranes in a dose-dependent manner. The gambiertoxins were more potent than ciguatoxin (on a per mouse unit basis) at stimulating neural elements of guinea pig atria. The two culture strains produced similar amounts of gambiertoxins, even when grown in nutrient media made from different seawater containing different concentrations of nutrients. Changes in nutrient media did not induce the other strains of G. toxicus to produce gambiertoxins. The production of these ciguatoxin precursors appears to be limited to only certain genetic strains of G. toxicus, with the majority of strains not producing these toxins. We propose that ciguatera occurs when blooms of G. toxicus strains genetically capable of producing these ciguatoxin precursors enter the marine food chain. These toxins could then become oxidatively metabolized in fishes to the major polar ciguatoxin. Wild cells produced approximately 100-fold greater quantities of gambiertoxins per cell than did the two culture strains indicating that there is considerable potential for increased production of these ciguatoxin precursors from G. toxicus in culture.

Purification and characterization of ciguatoxins from moray eel (Lycodontis javanicus, Muraenidae)

Viscera (48.3 kg) from moray eels (Lycodontis javanicus) collected in a ciguatera endemic area were extracted and the ciguatoxins characterized. Three major ciguatoxins, CTX-1, CTX-2 and CTX-3, were isolated and purified to homogeneity on reverse phase high performance liquid chromatography. Several minor toxins were also detected. CTX-1 (490 micrograms) was comparable by both 1H nuclear magnetic resonance (1H NMR) and mass spectroscopy (MH+ m/z = 1111) to ciguatoxin isolated previously from moray eels. CTX-2 (280 micrograms) and CTX-3 (100 micrograms) were less polar ciguatoxins not previously characterized. CTX-2 and CTX-3 differed from CTX-1 by 16 mass units, suggesting that they were less oxygenated analogues. 1H NMR revealed that the hydroxyl at C54 in CTX-1 was absent in CTX-2 and CTX-3. An additional change in the chemistry of CTX-2 compared to CTX-1 and CTX-3 was also suggested on the basis of 1H NMR, indicating that CTX-2 may arise from a different precursor to CTX-1. CTX-3 is likely to be an intermediate in the oxidation of a gambiertoxin (sodium channel toxins from Gambierdiscus toxicus) to CTX-1. The i.p. LD50 values for CTX-1, CTX-2 and CTX-3 were 0.25, 2.3 and 0.9 micrograms/kg, respectively. The signs induced in mice by the ciguatoxins were similar, except that CTX-2 and CTX-3 induced hind-limb paralysis that was absent with CTX-1. Each ciguatoxin was potent orally. CTX-1, CTX-2 and CTX-3 competitively inhibited the binding of [3H]brevetoxin-3 to voltage-dependent sodium channels with relative potencies qualitatively (but not quantitatively) comparable to mouse lethality. This study reveals that the relatively small chemical differences between CTX-1, CTX-2 and CTX-3 give rise to significant structure-activity and pharmacokinetic differences.

In vitro and in situ inhibition of the sodium channel blocker saxitoxin by monoclonal antibodies

The sodium channel blocker saxitoxin (STX) was conjugated to keyhole limpet hemocyanin (KLH) and used to immunize BALB/c mice. Anti-STX antibodies were detected in serum by an enzyme-linked immunosorbent assay (ELISA) within a week or two after the first immunization. Spleens from immunized mice were fused with NS-1 myeloma cells and approximately 7000 resultant hybrids were screened by ELISA for reactivity to STX. Two stable hybrids were isolated, subcloned, and characterized. These hybrids, termed S1A5 and S3E.2, secreted specific anti-STX antibodies that did not recognize the closely related toxin tetrodotoxin (TDT), as determined by competition ELISA. The S1A5 monoclonal antibody (mAb) was of the IgMk class and S3E.2 of the IgG1k subclass with affinity constants (Ka values) of approximately 10(6) M-1. The protective ability of these antibodies was tested by a competitive displacement assay for [3H]STX binding on rat brain membranes. Purified S3E.2 strongly displaced [3H]STX binding, whereas S1A5 weakly inhibited [3H]STX binding to membranes. One nanomole of S3E.2 or S1A5 was able to bind 0.03 nmol or 0.005 nmol, respectively, of STX. The S3E.2 mAb offered partial protection against STX-induced reduction of peripheral nerve action potential in rat tibial nerve when administered in situ at concentrations 10- to 30-fold greater than STX. The S1A5 mAb, despite its ability to inhibit STX binding in vitro, was completely ineffectual in situ. These antibodies, particularly S3E.2, thus represent potentially useful reagents for neurobiologic research, detection of toxin contamination, and diagnosis of poisoning, and may provide protection against the toxicity of STX in vivo.

Protection against nerve toxicity by monoclonal antibodies to the sodium channel blocker tetrodotoxin

The sodium channel blocker, tetrodotoxin (TDT), was conjugated to keyhole limpet hemocyanin (KLH) and used to immunize BALB/c mice. Anti-TDT antibodies were detected in serum by ELISA and reached stable levels 4-5 wk after the first immunization. Spleens from immunized mice were fused with NS-1 mouse myeloma cells and approximately 9,329 resultant hybrids were screened by ELISA for reactivity to TDT. Two stable hybrids were isolated, subcloned, and characterized. These hybrids, termed TD13a1 and TD2C5, secreted specific anti-TDT antibodies that recognized TDT but not the related sodium channel blocker, saxitoxin (STX), as determined by competition ELISA. Both antibodies were of the IgG1k subclass with Ka's approaching 10(7) M-1. The inhibitory ability of these antibodies was tested by a competitive displacement assay for [3H]STX on rat brain membranes. Both antibodies strongly inhibited TDT binding to membranes. A nanomole of TD2C5 was able to bind approximately 1.8 nmol of TDT, whereas a comparable amount of TD13a1 bound half as much. Furthermore, TD2C5 was able to protect against TDT-induced reduction of peripheral nerve action potentials in rat tibial nerve when administered in situ. These antibodies thus represent potentially useful reagents for neurobiologic research, detection of toxin contamination and diagnosis of poisoning, and may provide protection against the toxicity of TDT in vivo.

Evidence for the presence of tetrodotoxin in a powder used in Haiti for zombification

A powder prepared by Haitian voodoo sorcerers for the making of zombis was extracted with acetic acid, the extract concentrated and applied to a small cation exchange column followed by elution with water and then acetic acid. The water and acetic acid eluents were analysed by gas chromatography-mass spectrometry and liquid chromatography-mass spectrometry. The analyses indicated the presence of an alkaline degradation product of tetrodotoxin, namely 2-amino-6-hydroxymethyl-8-hydroxyquinazoline, after base treatment, and of tetrodotoxin and an isomer on direct thermospray mass spectral activity.

Appearance and partial purification of a high molecular weight protein in crabs exposed to saxitoxin

This paper provides evidence for a protein component which appears to be involved in the seasonal resistance of small shore crabs, Hemigrapsus oregonesis and Hemigrapsus nudus to saxitoxin, a principle neurotoxin involved in paralytic shellfish poisoning (PSP). This unique protein complex was isolated and partially purified by ion exchange chromatography using DEAE-cellulose from visceral tissue extracts of resistant crabs. The complex was absent in control crabs that were sensitive to saxitoxin. In addition, the protein complex was induced in the crab after acute administration of low doses of saxitoxin. Results indicate that the protein complex is acidic in nature and has an apparent mol. wt of 145,000.