Identification of slow and fast-acting toxins in a highly ciguatoxic barracuda (Sphyraena barracuda) by HPLC/MS and radiolabelled ligand binding.

A barracuda implicated in ciguatera fish poisoning in Guadeloupe was estimated to have an overall flesh toxicity of 15 MUg/g using mouse bioassay. A lipid soluble extract was separated into two toxic fractions, FrA and FrB, on a LH20 Sephadex column eluted with dichloromethane/methanol (1:1). When intraperitoneal injected into mice, FrA provoked symptoms characteristic of slow-acting ciguatoxins, whereas FrB produced symptoms indicative of fast-acting toxins (FAT). High performance liquid chromatography/mass spectrometry/radio-ligand binding (HPLC/MS/RLB) analysis confirmed the two fractions were distinct, because only a weak overlap of some compounds was observed. HPLC/MS/RLB analysis revealed C-CTX-1 as the potent toxin present in FrA, and two coeluting active compounds at m/z 809.43 and 857.42 in FrB, all displaying the characteristic pattern of ion formation for hydroxy-polyethers. Other C-CTX congeners and putative hydroxy-polyether-like compounds were detected in both fractions, however, the RLB found them inactive. C-CTX-1 accounted for > 90% of total toxicity in this barracuda and was confirmed to be a competitive inhibitor of brevetoxin binding to voltage-sensitive sodium channels (VSSCs) with a potency two-times lower than P-CTX-1. However, FAT active on VSSCs and < 900 Da were suspected to contribute to the overall toxicity.     

Analysis of paralytic shellfish poisoning toxin congeners by a sodium channel receptor binding assay.

This study was carried out to characterize the detection and quantitation of several paralytic shellfish poisoning (PSP) toxin congeners using a receptor binding assay (RBA). This involved competitive binding of the toxin congeners against tritium-labeled STX for receptor sites on rat brain sodium channels. Competitive binding curves were described by a four-parameter logistic equation. Half-saturation values (EC(50)) ranged from 4.38 nM for STX to 142 nM for GTX5. Receptor binding affinity was in the order STX>GTX1/4>neoSTX>GTX2/3>dcSTX>GTX5, and this was similar to the order of mouse toxicity of these congeners. Predicted toxin concentrations from observed STXeq values and EC(50) ratios relative to STX were within 20% or better of the actual concentrations used in the assay. In contrast predicted toxin concentrations using mouse toxicity ratios relative to STX did not provide a good match to actual concentrations, except for GTX1/4. This study has shown that the rat brain sodium channel RBA will provide a reliable integration of total toxicity of various PSP toxin congeners present in a sample.     

Intrinsic potency of synthetically prepared brevetoxin cysteine metabolites BTX-B2 and desoxyBTX-B2.

In mammals and shellfish, brevetoxins produced by the dinoflagellate Karenia brevis are rapidly metabolized to cysteine conjugates. These metabolites identified by mass spectrometry are produced in abundance in mammals and are potentially major bioactive products for intoxication. They are also abundant metabolites in shellfish where they are, in contrast to mammals, retained for prolonged periods, posing a potential threat to shellfish consumers. In this work, we analyze the intrinsic potency of the semi-synthetic cysteine brevetoxin sulfoxide (BTX-B2) and the cysteine brevetoxin (desoxyBTX-B2), each confirmed for purity by LC-MS and NMR techniques, on receptor site 5 of the voltage-gated sodium channels (VGSCs) in brain, heart and skeletal muscle. We show that both brevetoxin conjugates compete with the tritiated reduced parent brevetoxin ([(3)H]PbTx-3) in rat brain membrane preparations and in HEK cells expressing skeletal muscle or cardiac VGSC, albeit, with 8-16-fold lower affinity than the PbTx-3. On neuroblastoma cell assays we show a 3-fold reduction in cytotoxic potency for BTX-B2 relative to PbTx-3, and an 8-fold reduction for desoxyBTX-B2. In conclusion, the major transformation product of brevetoxin observed in diverse species through cysteine adduction and oxidation leads to metabolites with reduced potency on brain, skeletal muscle and heart cells.     

Type B brevetoxins show tissue selectivity for voltage-gated sodium channels: comparison of brain, skeletal muscle and cardiac sodium channels.

Brevetoxins and ciguatoxins are two classes of phycotoxins which exert their toxic effect by binding to site-5 of voltage-gated sodium channels. Sodium channels, a family of at least 10 structurally different proteins, are responsible for the rising phase of the action potential in membranes of neuronal, cardiac and muscular excitable cells. This work is a comparative study of the binding properties and the cytotoxic effects of ciguatoxins and brevetoxins on human embryonic cells (HEK) stably expressing either the skeletal muscle (Na(v)1.4), or the cardiac (Na(v)1.5) sodium channel alpha-subunit isoforms. We report that type A (PbTx-1) and type B (PbTx-3 and PbTx-2) brevetoxins as well as ciguatoxins target both cardiac and muscle channels; type B brevetoxins show isoform selectivity, presenting a lower affinity for the heart than the skeletal muscle channel. The lower selectivity of type B brevetoxins for heart sodium channels may result from a more rigid backbone structure than is found in type A brevetoxins and ciguatoxins.     

Comparative concentrations of brevetoxins PbTx-2, PbTx-3, BTX-B1 and BTX-B5 in cockle, Austrovenus stutchburyi, greenshell mussel, Perna canaliculus, and Pacific oyster, Crassostrea gigas, involved neurotoxic shellfish poisoning in New Zealand.

Previously, we found brevetoxins PbTx-3, BTX-B5 and BTX-B1 in cockle, Austrovenus (A.) stutchburyi, PbTx-2, PbTx-3 and BTX-B1 in Pacific oyster, Crassostrea (C.) gigas and PbTx-3 and BTX-B1 in greenshell mussel, Perna (P.) canaliculus following outbreak of neurotoxic shellfish poisoning (NSP) in New Zealand by isolation and/or liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). In this study, procedures for quantitative determination of PbTx-2 and BTX-B5 were developed and those for PbTx-3 and BTX-B1 were further examined by LC-MS/MS. In mass spectrometry with an electrospray ionization interface operating in the positive or negative ion mode, the protonated ions [M+H]+ of PbTx-2 (m/z 895), [M+H]+ of PbTx-3 (m/z 897), [M-H]- of BTX-B5 (m/z 909), and [M-Na]- of BTX-B1 (m/z 1016) were generated abundantly, when 0.1% formic acid-acetonitrile was used as the mobile phase for column chromatography. The product ions of m/z 877, 725, 111 and 80 from PbTx-2, PbTx-3, BTX-B5 and BTX-B1 were identified, respectively, allowing unambiguous confirmation of these toxins by selective reaction monitoring LC-MS/MS analysis. High levels of PbTx-3 and BTX-B5 were detected in C. gigas, of PbTx-3, BTX-B1 and BTX-B5 in A. stutchburyi, and of PbTx-2, PbTx-3 and BTX-B5 in P. canaliculus by this LC-MS/MS method.