Synthesis and Identification of decarbamoyloxySaxitoxins in Toxic Microalgae and their Reactions with the Oxygenase, SxtT, Reveal Saxitoxin Biosynthesis.

Saxitoxin (STX, 1) is a representative compound of paralytic shellfish toxins (PSTs) that are produced by marine dinoflagellates and freshwater cyanobacteria. Although several pathways have been proposed for the biosynthesis of STX, the order of ring and side chain hydroxylation, and formation of the tricyclic skeleton have not been well established. In this study, 12,12-dideoxy-decarbamoyloxySTX (dd-doSTX, 2), the most reduced STX analogue having the tricyclic skeleton, and its analogues, 12ß-deoxy-doSTX (12ß-d-doSTX, 3), 12α-deoxy-doSTX (12α-d-doSTX, 4), and doSTX (5), were synthesized, and these compounds were screened in the toxic microalgae using high-resolution LCMSMS. dd-doSTX (2) and 12ß-d-doSTX (3) were identified in the PSTs-producing dinoflagellates (Alexandrium catenella, A. pacificum, and/or Gymnodinium catenatum) and in the cyanobacterium Dolichospermum circinale (TA04). doSTX (5), previously isolated from the dinoflagellate G. catenatum, was also identified in D. circinale (TA04). Furthermore, the conversion of 2 to 3, and 4 to 5, by SxtT with VanB, a reported Rieske oxygenase and its redox partner in STX biosynthesis, was confirmed. These results support that 2 is a possible biosynthetic precursor of STX, and that ring and side-chain hydroxylations proceed after cyclization.

Assay for okadaic acid O-acyl transferase using HPLC-FLD.

Dinophysistoxin 1 (DTX1, 1) and okadaic acid (OA, 2), produced by the dinoflagellates Dinophysis spp. and Prorocentrum spp., are primary diarrhetic shellfish toxins (DSTs), which may cause gastric illness in people consuming such as bivalves. Both compounds convert to dinophysistoxin 3 (DTX3, 3; generic name for 1 and 2 with fatty acids conjugated at 7-OH) in bivalves. The enzyme okadaic acid O-acyl transferase (OOAT) is a membrane protein found in the microsomes of the digestive glands of bivalves. In this study, we established an in vitro enzymatic conversion reaction using 4-nitro-2,1,3-benzoxadiazole (NBD)-OA (4), an OA derivative conjugated with (R)-(-)-4-nitro-7-(3-aminopyrrolidin-1-yl)-2,1,3-benzoxadiazole (NBD-APy) on 1-CO2H, as a substrate. We detected the enzymatically produced 3, NBD-7-O-palmitoyl-OA (NBD-Pal-OA), using high-performance liquid chromatography-fluorescence detection. We believe that an OOAT assay using 4 will facilitate the fractionation and isolation of OOAT in the future.

State-Dependent Inhibition of Voltage-Gated Sodium Channels in Neuroblastoma Neuro-2A Cells by Arachidonic Acid from Halichondria okadai.

Voltage-gated sodium channels (Nav) are closely associated with epilepsy, cardiac and skeletal muscle diseases, and neuropathic pain. Several toxic compounds have been isolated from the marine sponge Halichondria okadai; however, toxic substances that modulate Nav are yet to be identified. This study aimed to identify Nav inhibitors from two snake venoms and H. okadai using mouse neuroblastoma Neuro-2A cells (N2A), which primarily express the specific Nav subtype Nav1.7, using whole-cell patch-clamp recordings. We successfully isolated arachidonic acid (AA, 1) from the hexane extract of H. okadai, and then the fatty acid-mediated modulation of Nav in N2A was investigated in detail for the first time. Octanoic acid (2), palmitic acid (3), and oleic acid (4) showed no inhibitory activity at 100 µM, whereas AA (1), dihomo-γ-linolenic acid (DGLA, 5), and eicosapentaenoic acid (EPA, 6) showed IC50 values of 6.1 ± 2.0, 58 ± 19, and 25 ± 4.0 µM, respectively (N = 4, mean ± SEM). Structure and activity relationships were investigated for the first time using two ω-3 polyunsaturated fatty acids (PUFAs), EPA (6) and eicosatetraenoic acid (ETA, 7), and two ω-6 PUFAs, AA (1) and DGLA (5), to determine their effects on a resting state, activated state, and inactivated state. Steady-state analysis showed that the half inactivation potential was largely hyperpolarized by 10 µM AA (1), while 50 µM DGLA (5), 50 µM EPA (6), and 10 µM ETA (7) led to a slight change. The percentages of the resting state block were 24 ± 1, 22 ± 1, 34 ± 4, and 38 ± 9% in the presence of AA (1), DGLA (5), EPA (6), and ETA (7), respectively, with EPA (6) and ETA (7) exhibiting a greater inhibition than both AA (1) and DGLA (5), and their inhibitions did not increase in the following depolarization pulses. None of the compounds exhibited the use-dependent block. The half recovery times from the inactivated state for the control, AA (1), DGLA (5), EPA (6), and ETA (7) were 7.67 ± 0.33, 34.3 ± 1.10, 15.5 ± 1.10, 10.7 ± 0.31, and 3.59 ± 0.18 ms, respectively, with AA (1) exhibiting a distinctively large effect. Overall, distributed binding to the resting and the inactivated states of Nav would be significant for the inhibition of Nav, which presumably depends on the active structure of each PUFA.

Isolation and Biological Activity of 9-epiTetrodotoxin and Isolation of Tb-242B, Possible Biosynthetic Shunt Products of Tetrodotoxin from Pufferfish.

Tetrodotoxin (TTX, 1) is a potent voltage-gated sodium channel blocker detected in certain marine and terrestrial organisms. We report here a new TTX analogue, 9-epiTTX (2), and a TTX-related compound, Tb-242B (4), isolated from the pufferfish Takifugu flavipterus and Dichotomyctere ocellatus, respectively. NMR analysis suggested that 2 exists as a mixture of hemilactal and 10,8-lactone forms, whereas other reported TTX analogues are commonly present as an equilibrium mixture of hemilactal and 10,7-lactone forms. Compound 2 and TTX were confirmed not to convert to each other by incubation under neutral and acidic conditions at 37 °C for 24 h. Compound 4 was identified as the 9-epimer of Tb-242A (3), previously reported as a possible biosynthetic precursor of TTX. Compound 4 was partially converted to 3 by incubation in a neutral buffer at 37 °C for 7 days, whereas 3 was not converted to 4 under this condition. Compound 2 was detected in several TTX-containing marine animals and a newt. Mice injected with 600 ng of 2 by intraperitoneal injection did not show any adverse symptoms, suggesting that the C-9 configuration in TTX is critical for its biological activity. Based on the structures, 2 and 4 were predicted to be shunt products for TTX biosynthesis.

Mass spectrometry-guided discovery of new analogs of bicyclic phosphotriester salinipostin and evaluation of their monoacylglycerol lipase inhibitory activity.

Natural products containing the highly unusual phosphotriester ring are known to be potent serine hydrolase inhibitors. The long-chain bicyclic enol-phosphotriester salinipostins (SPTs) from the marine actinomycete Salinispora have been identified as selective antimalarial agents. A potential regulatory function has been suggested for phosphotriesters based on their structural relationship with actinomycete signaling molecules and the prevalence of spt-like biosynthetic gene clusters across actinomycetes. In this study, we established a mass spectrometry-guided screening method for phosphotriesters focusing on their characteristic fragment ions. Applying this screening method to the SPT producer Salinispora tropica CNB-440, new SPT analogs (4-6) were discovered and their structures were elucidated by spectroscopic analyses. Previously known and herein-identified SPT analogs inhibited the activity of human monoacylglycerol lipase (MAGL), a key serine hydrolase in the endocannabinoid system, in the nanomolar range. Our method could be applied to the screening of phosphotriesters, potential serine hydrolase inhibitors and signaling molecules.

First Identification of 12ß-Deoxygonyautoxin 5 (12α-Gonyautoxinol 5) in the Cyanobacterium Dolichospermum circinale (TA04) and 12ß-Deoxysaxitoxin (12α-Saxitoxinol) in D. circinale (TA04) and the Dinoflagellate Alexandrium pacificum (Group IV) (120518KureAC).

Saxitoxin and its analogues, paralytic shellfish toxins (PSTs), are potent and specific voltage-gated sodium channel blockers. These toxins are produced by some species of freshwater cyanobacteria and marine dinoflagellates. We previously identified several biosynthetic intermediates of PSTs, as well as new analogues, from such organisms and proposed the biosynthetic and metabolic pathways of PSTs. In this study, 12ß-deoxygonyautoxin 5 (12α-gonyautoxinol 5 = gonyautoxin 5-12(R)-ol) was identified in the freshwater cyanobacterium, Dolichospermum circinale (TA04), and 12ß-deoxysaxitoxin (12α-saxitoxinol = saxitoxin-12(R)-ol) was identified in the same cyanobacterium and in the marine dinoflagellate Alexandrium pacificum (Group IV) (120518KureAC) for the first time from natural sources. The authentic standards of these compounds and 12α-deoxygonyautoxin 5 (12ß-gonyautoxinol 5 = gonyautoxin 5-12(S)-ol) were prepared by chemical derivatization from the major PSTs, C1/C2, produced in D. circinale (TA04). These standards were used to identify the deoxy analogues by comparing the retention times and MS/MS spectra using high-resolution LC-MS/MS. Biosynthetic or metabolic pathways for these analogues have also been proposed based on their structures. The identification of these compounds supports the α-oriented stereoselective oxidation at C12 in the biosynthetic pathway towards PSTs.

Structures of N-Hydroxy-Type Tetrodotoxin Analogues and Bicyclic Guanidinium Compounds Found in Toxic Newts.

The biosynthesis of tetrodotoxin (TTX, 1), a potent neurotoxin widely distributed in marine and terrestrial metazoans, remains unresolved. A significant issue has been identifying intermediates and shunt products associated with the biosynthetic pathway of TTX. We investigated TTX biosynthesis by screening and identifying new TTX-related compounds from Cynops ensicauda popei and Taricha granulosa. Mass spectrometry (MS)-guided screening identified two new N-hydroxy TTX analogues in newts: 1-hydroxy-8-epiTTX (2) and 1-hydroxy-8-epi-5,11-dideoxyTTX (3, previously reported as 1-hydroxy-5,11-dideoxyTTX). We prepared a new analogue, 8-epi-5,11-dideoxyTTX (4), from 3 via N-OH reduction and confirmed the presence of 4 in T. granulosa using hydrophilic interaction liquid chromatography (HILIC)-LCMS. The presence of 8-epi-type TTX analogues in both Cynops and Taricha supports a branched biosynthetic pathway of terrestrial TTX, which produces 6- and 8-epimers. In addition, new bicyclic guanidinium compounds Tgr-238 (5) and Tgr-240 (6) were identified as putative shunt products of our proposed TTX biosynthesis pathway. A structural analysis of Cep-228A (7), another bicyclic compound, was performed using NMR. Based on the structures of 5-7 and their analogues, we propose a model of the shunt and metabolic pathways of the terrestrial TTX biosynthesis.

Comparative transcriptomics as a guide to natural product discovery and biosynthetic gene cluster functionality.

Bacterial natural products remain an important source of new medicines. DNA sequencing has revealed that a majority of natural product biosynthetic gene clusters (BGCs) maintained in bacterial genomes have yet to be linked to the small molecules whose biosynthesis they encode. Efforts to discover the products of these orphan BGCs are driving the development of genome mining techniques based on the premise that many are transcriptionally silent during normal laboratory cultivation. Here, we employ comparative transcriptomics to assess BGC expression among four closely related strains of marine bacteria belonging to the genus Salinispora The results reveal that slightly more than half of the BGCs are expressed at levels that should facilitate product detection. By comparing the expression profiles of similar gene clusters in different strains, we identified regulatory genes whose inactivation appears linked to cluster silencing. The significance of these subtle differences between expressed and silent BGCs could not have been predicted a priori and was only revealed by comparative transcriptomics. Evidence for the conservation of silent clusters among a larger number of strains for which genome sequences are available suggests they may be under different regulatory control from the expressed forms or that silencing may represent an underappreciated mechanism of gene cluster evolution. Coupling gene expression and metabolomics data established a bioinformatic link between the salinipostins and their associated BGC, while genetic manipulation established the genetic basis for this series of compounds, which were previously unknown from Salinispora pacifica.

Isolation and Biological Activity of 8- Epitetrodotoxin and the Structure of a Possible Biosynthetic Shunt Product of Tetrodotoxin, Cep-226A, from the Newt Cynops ensicauda popei.

Tetrodotoxin (TTX, 1), a potent neurotoxin, has been found in various animal species in both marine and terrestrial environments. In this study, a new TTX analogue, 8- epiTTX (2), and a possible biosynthetic shunt compound of TTX, Cep-226A (3), were isolated from the newt Cynops ensicauda popei. The voltage-gated sodium ion channel (Nav) blocking activity of 2 and 6- epiTTX (4), a known analogue, were investigated by a colorimetric cell-based assay and compared with that of 1. The EC50 values for 2 and 4 were determined to be 110 ± 40 and 33 ± 11 nM, respectively, which were larger than that of 1 (1.9 ± 0.7 nM). The results indicated that the equatorial hydroxy group at C-8 in TTX significantly contributes to its Nav blocking activity, whereas the 6-epimer of TTX retains substantial activity, consistent with its previously reported toxicity in mice and binding affinity to rat brain membrane preparations. The presence of these epimers of TTX (2 and 4) and Cep-226A (3) in newts supports our hypothesis that TTX is derived from a monoterpene in terrestrial environments.

Temporal Variation of the Profile and Concentrations of Paralytic Shellfish Toxins and Tetrodotoxin in the Scallop, Patinopecten yessoensis, Cultured in a Bay of East Japan.

Paralytic shellfish toxins (PSTs) are the major neurotoxic contaminants of edible bivalves in Japan. Tetrodotoxin (TTX) was recently detected in bivalve shellfish around the world, drawing widespread attention. In Japan, high levels of TTX were reported in the digestive gland of the scallop, Patinopecten yessoensis, in 1993; however, no new data have emerged since then. In this study, we simultaneously analyzed PSTs and TTX in scallops cultured in a bay of east Japan using hydrophilic interaction chromatography (HILIC)-MS/MS. These scallops were temporally collected from April to December 2017. The highest concentration of PSTs (182 µmol/kg, total congeners) in the hepatopancreas was detected in samples collected on May 23, lined to the cell density of the dinoflagellate, Alexandrium tamarense, in seawater around the scallops, whereas the highest concentration of TTX (421 nmol/kg) was detected in samples collected on August 22. Contrary to the previous report, temporal variation of the PSTs and TTX concentrations did not coincide. The highest concentration of TTX in the entire edible tissues was 7.3 µg/kg (23 nmol/kg) in samples obtained on August 22, which was lower than the European Food Safety Authority (EFSA)-proposed threshold, 44 µg TTX equivalents/kg shellfish meat. In addition, 12ß-deoxygonyautoxin 3 was firstly identified in scallops.

Spiro Bicyclic Guanidino Compounds from Pufferfish: Possible Biosynthetic Intermediates of Tetrodotoxin in Marine Environments.

Tetrodotoxin (TTX, 1) is a potent neurotoxin that is widely found in both terrestrial and marine animals; however, the biosynthetic pathway and genes for TTX have not yet been elucidated. Previously, we proposed that TTX originated from a monoterpene; this hypothesis was based on the structures of cyclic guanidino compounds that are commonly found in toxic newts. However, these compounds have not been detected in marine organisms. Instead, a series of deoxy analogues of TTX were found in toxic marine animals; thus, we further screened for TTX-related compounds in marine animals. Herein, we report seven novel spiro bicyclic guanidino compounds 2-8 that were isolated from the pufferfish Tetraodon biocellatus. In compounds 2-5 and 7-8, a six-membered cyclic guanidino amide is spiro-fused with 2,4-dimethyl cyclohexane, whereas in compound 6, the same cyclic guanidino amide is spiro-fused with 2,3,5-trimethylcyclopentane. Compounds 2-5 and 7-8 have the same carbon skeleton and relative configuration as TTX. Thus, we proposed that compounds 2-8 are biosynthetic intermediates of TTX in marine environments. TTX could be biosynthetically derived from compound 7 via intermediates 2-5 through several oxidations, amide hydrolysis, and formation of the hemiaminal and lactone found in 5,6,11-trideoxyTTX, the major TTX analogue, whereas compounds 6 and 8 might be shunt products. LC-MS analysis confirmed the wide distribution of compounds 2, 3, or both in TTX-containing marine animals, namely pufferfish, crab, octopus, and flatworm, but compounds 2-8 were not detected in newts.

Structural Elucidation of Trace Components Combining GC/MS, GC/IR, DFT-Calculation and Synthesis-Salinilactones, Unprecedented Bicyclic Lactones from Salinispora Bacteria.

The analysis of volatiles released by marine Salinispora bacteria uncovered a new class of natural compounds displaying an unusual bicyclic [3.1.0]-lactone skeleton. Although only sub-µg quantities of the compounds were available, the combination of analytical methods, computational spectroscopy, and synthesis allowed unambiguous structural identification of the compounds, called salinilactones, without the need for isolation. Orthogonal hyphenated methods, GC/MS and solid-phase GC/IR allowed to propose a small set of structures consistent with the data. A candidate structure was selected by comparison of DFT-calculated IR spectra and the experimental IR-spectrum. Synthesis confirmed the structure and absolute configuration of three bicyclic lactones, salinilactones A-C. The salinilactones are structurally closely related to the A-factor class of compounds, autoregulators from streptomycete bacteria. They exhibited inhibitory activity against Salinispora and Streptomyces strains.

A new sarasinoside congener, sarasinoside M2, from a marine sponge collected in the Solomon Islands.

A new sarasinoside congener (sarasinoside M2) and known sarasinoside B1 were obtained from a marine sponge. Sarasinoside M2 was suggested to have the same aglycon as sarasinoside M although the internal glucose in its sugar moiety is replaced by xylose. Sarasinosides B1 and M2 showed moderate cytotoxicity (approximate IC50 5-18 µM) toward Neuro-2a and HepG2 cell lines.

Cyclic Guanidine Compounds from Toxic Newts Support the Hypothesis that Tetrodotoxin is Derived from a Monoterpene.

The biosynthesis of tetrodotoxin (TTX), a potent neurotoxin consisting of a 2,4-dioxaadamantane skeleton and a guanidine moiety, is an unsolved problem in natural product chemistry. Recently, the first C5-C10 directly bonded TTX analogue, 4,9-anhydro-10-hemiketal-5-deoxyTTX, was obtained from toxic newts and its carbon skeleton suggested a possible monoterpene origin. On the basis of this hypothesis, screening of predicted biosynthetic intermediates of TTX was performed using two MS-guided methods. Herein, five novel cyclic guanidine compounds from toxic newts are reported which commonly contain a cis-fused bicyclic structure including a six-membered cyclic guanidine. These structures could be biosynthetically derived from geranyl guanidine through oxidation, cyclization, and/or isomerization steps. LC-MS analysis confirmed the widespread distribution of the five novel compounds in toxic newt species. These results support the hypothesis that TTX is derived from a monoterpene.

Isolation of 6-deoxytetrodotoxin from the pufferfish, Takifugu pardalis, and a comparison of the effects of the C-6 and C-11 hydroxy groups of tetrodotoxin on its activity.

Identification of new tetrodotoxin (TTX, 1) analogues would be significant in the elucidation of its biosynthetic pathway and a study of its structure-activity relationships. In this study, a new TTX analogue, 6-deoxyTTX (2), was isolated from the ovary of the pufferfish, Takifugu pardalis, and the structure was determined using spectroscopic methods. Compound 2 was also identified in other marine animals, Nassarius snail and blue-ringed octopuses, using LC-MS. Furthermore, we investigated the voltage-gated sodium channel blocking activity of 2 by examination of the inhibitory activities to cytotoxicity induced by ouabain and veratridine in mouse neuroblastoma cells (Neuro-2a). The activities were then compared with those of 1, 11-deoxyTTX (3), and 6,11-dideoxyTTX (4). The EC50 value for 2 was estimated to be 6.5±2.2 nM, approximately 3-fold larger than that of 1 (2.1±0.6 nM) and approximately 20-fold smaller than that of 3. These results suggested that contribution of the C-6 hydroxy group to the activity is less than that of the C-11 hydroxy group.

C5-C10 directly bonded tetrodotoxin analogues: possible biosynthetic precursors of tetrodotoxin from newts.

The identification of novel tetrodotoxin (TTX, 1) analogues would significantly contribute to the elucidation of its biosynthetic pathway. In this study, the first C5-C10 directly bonded TTX analogues, 4,9-anhydro-10-hemiketal-5-deoxyTTX (2) and 4,9-anhydro-8-epi-10-hemiketal-5,6,11-trideoxyTTX (3), were found in the newt Cynops ensicauda popei by using a screening method involving HILIC-LC-MS/MS focused on the fragment ions of TTX analogues, and their structures were elucidated by spectroscopic methods. Compound 2 was detected in a wide range of newt species, and the 2 and TTX contents of 22 newt specimens were correlated (rs =0.88). Based on these results and its structural features, 2 was predicted to serve as a precursor of TTX that would be directly converted into 4,9-anhydroTTX (4) by Baeyer-Villiger-like oxidation or via 4,9-anhydro-5-deoxyTTX formed by cleavage of the C5-C10 bond. The bicyclic carbon skeletons of 2 and 3 suggested a possible monoterpene origin for TTX.

Evaluation of the performance of a machine learning based atrial fibrillation screening algorithm using an oscillometric blood pressure monitor.

Blood pressure monitors (BPMs) with atrial fibrillation (AFib) detection function can be used to detect AFib early. However, conventional algorithms require multiple BP measurements. Here, the feasibility of a machine-learning-based approach for AFib detection through single BP measurement was evaluated. First, a custom AdaBoost-based software, which analyzes the pulse-to-pulse interval (PPI) pattern and classifies it based on AFib detection, was created. Then, its classification performance was validated. For the validation study, PPI and standard 12-lead electrocardiogram (ECG) datasets were collected from 79 and 92 Japanese participants with and without AFib, respectively. PPI data were obtained using two different BPMs. All ECG results were interpreted by cardiologists. The custom software output for the PPI dataset and ECG interpreted results was compared, and the sensitivity and specificity were calculated. A sensitivity and specificity for PPI from main device were 97.5% (95% confidence interval [CI] 91.2-99.3%) and 98.9 (95% CI 94.1-99.8), respectively. No significant differences in sensitivity and specificity were observed in the subgroup analysis between different devices, age groups, and arm size groups. These results reflect the high accuracy and robustness of this AFib algorithm using a single BP measurement and supports its use for widespread AFib screening.

Synthesis of Saxitoxin Biosynthetic Intermediates: Reveal the Mechanism for Formation of its Tricyclic Skeleton in Biosynthesis.

The synthesis and biosynthesis of the complex saxitoxin (STX) structure have garnered significant interest. Previously, we hypothesized that the tricyclic skeleton of STX originates from the monocyclic precursor 11-hydroxy-IntC'2 during biosynthesis, although direct evidence has been lacking. In this study, we identified conditions to synthesize a proposed tricyclic biosynthetic intermediate, 12,12-dideoxy-decarbamoyloxySTX (dd-doSTX), along with its 6-epimer (6-epi-dd-doSTX) and a bicyclic compound, in a single step from di-Boc protected 11-hydroxy-IntC'2. The reaction mechanism involves successive aza-Michael addition of a guanidino amine to the conjugated olefin. Notably, both dd-doSTX and 6-epi-dd-doSTX were detected in a toxin-producing cyanobacterium, suggesting that the biosynthetic enzymes may generate these compounds via similar mechanisms.

First identification of 5,11-dideoxytetrodotoxin in marine animals, and characterization of major fragment ions of tetrodotoxin and its analogs by high resolution ESI-MS/MS.

Even though tetrodotoxin (TTX) is a widespread toxin in marine and terrestrial organisms, very little is known about the biosynthetic pathway used to produce it. By describing chemical structures of natural analogs of TTX, we can start to identify some of the precursors that might be important for TTX biosynthesis. In the present study, an analog of TTX, 5,11-dideoxyTTX, was identified for the first time in natural sources, the ovary of the pufferfish and the pharynx of a flatworm (planocerid sp. 1), by comparison with totally synthesized (-)-5,11-dideoxyTTX, using high resolution ESI-LC-MS. Based on the presence of 5,11-dideoxyTTX together with a series of known deoxy analogs, 5,6, 11-trideoxyTTX, 6,11-dideoxyTTX, 11-deoxyTTX, and 5-deoxyTTX, in these animals, we predicted two routes of stepwise oxidation pathways in the late stages of biosynthesis of TTX. Furthermore, high resolution masses of the major fragment ions of TTX, 6,11-dideoxyTTX, and 5,6,11-trideoxyTTX were also measured, and their molecular formulas and structures were predicted to compare them with each other. Although both TTX and 5,6,11-trideoxyTTX give major fragment ions that are very close, m/z 162.0660 and 162.1020, respectively, they are distinguishable and predicted to be different molecular formulas. These data will be useful for identification of TTXs using high resolution LC-MS/MS.

The quite low cross-reactivity of Kawatsu's anti-tetrodotoxin monoclonal antibody to 5,6,11-trideoxytetrodotoxin, 11-nortetrodotoxin-6(S)-ol, and 11-oxotetrodotoxin, the major tetrodotoxin analogues in pufferfish.

The monoclonal antibody against tetrodotoxin (TTX), prepared by Kawatsu et al. (1997), has been used in several TTX-related studies. Herein, we confirmed the quite low cross-reactivity of this antibody to three major TTX analogues in pufferfish using competitive ELISA: 5,6,11-trideoxyTTX (<2.2%), 11-norTTX-6(S)-ol (<0.3%), and 11-oxoTTX (<1.5%), with reactivity against TTX being 100%. We further confirmed that the presence of these analogues did not cause a marked overestimation of TTX in pufferfish extracts using competitive ELISA.