Reductive Amination for LC-MS Signal Enhancement and Confirmation of the Presence of Caribbean Ciguatoxin-1 in Fish

3-Epimers of Caribbean ciguatoxins in fish and algae

Ciguatera poisoning (CP) is endemic to several subtropical and tropical regions and is caused by the consumption of fish contaminated with ciguatoxins (CTXs). The recent discovery of Caribbean CTXs (C-CTXs) in Gambierdiscus spp. isolated from the Caribbean resulted in the identification of a precursor analogue, C-CTX5, that is reduced into C-CTX1. C-CTX5 has two reducible sites, a ketone at C-3 and hemiketal at C-56. Chemical reductions of C-CTX5 into C-CTX3/4 resulted in two peaks in the LC-HRMS chromatograms with a ratio that differed markedly from that observed in fish extracts and the reduction of C-CTX1 isolated from fish. Reduction of C-CTX5 should have produced four diastereoisomers of C-CTX3/4, prompting a more detailed study of the reduction products. LC-HRMS with a slow gradient was used to separate and detect the four stereoisomers of C-CTX3/4, and to determine the distribution of these analogues in naturally contaminated fish tissues and following chemical reduction of isolated analogues. The results showed that in naturally contaminated fish tissues C-CTX1/2 is a mixture of two diastereoisomers at C-3 and that C-CTX3/4 is a mixture of two pairs of diastereoisomers at C-3 and C-56. The data suggests that there is variability in the enzymatic reduction at C-3 and C-56 of C-CTXs in reef fish, leading to variations in the ratios of the four stereoisomers. Based on these findings, a naming convention for C-CTXs is proposed which aligns with that used for Pacific CTX congeners and will aid in the identification of the structure and stereochemistry of the different CTX analogues.

Ciguatera Fish Poisoning in the Caribbean Sea and Atlantic Ocean: Reconciling the Multiplicity of Ciguatoxins and Analytical Chemistry Approach for Public Health Safety

Ciguatera is a major circumtropical poisoning caused by the consumption of marine fish and invertebrates contaminated with ciguatoxins (CTXs): neurotoxins produced by endemic and benthic dinoflagellates which are biotransformed in the fish food-web. We provide a history of ciguatera research conducted over the past 70 years on ciguatoxins from the Pacific Ocean (P-CTXs) and Caribbean Sea (C-CTXs) and describe their main chemical, biochemical, and toxicological properties. Currently, there is no official method for the extraction and quantification of ciguatoxins, regardless their origin, mainly due to limited CTX-certified reference materials. In this review, the extraction and purification procedures of C-CTXs are investigated, considering specific objectives such as isolating reference materials, analysing fish toxin profiles, or ensuring food safety control. Certain in vitro assays may provide sufficient sensitivity to detect C-CTXs at sub-ppb levels in fish, but they do not allow for individual identification of CTXs. Recent advances in analysis using liquid chromatography coupled with low- or high-resolution mass spectrometry provide new opportunities to identify known C-CTXs, to gain structural insights into new analogues, and to quantify C-CTXs. Together, these methods reveal that ciguatera arises from a multiplicity of CTXs, although one major form (C-CTX-1) seems to dominate. However, questions arise regarding the abundance and instability of certain C-CTXs, which are further complicated by the wide array of CTX-producing dinoflagellates and fish vectors. Further research is needed to assess the toxic potential of the new C-CTX and their role in ciguatera fish poisoning. With the identification of C-CTXs in the coastal USA and Eastern Atlantic Ocean, the investigation of ciguatera fish poisoning is now a truly global effort.

Selective and Efficient Capture and Release of vic-Diol-Containing Pacific and Caribbean Ciguatoxins from Fish Extracts with a Boronate Affinity Polymer

Ciguatera poisoning can occur following the consumption of fish contaminated with trace levels of ciguatoxins (CTXs). These trace levels represent an analytical challenge for confirmation by LC-MS due to matrix interferences and the high instrument sensitivity required. Sample preparation procedures are laborious and require extensive cleanup procedures to address these issues. The application of a selective isolation technique employing boronate affinity polymers was therefore investigated for the capture of vic-diol-containing Caribbean and Pacific CTXs from fish extracts. A dispersive SPE procedure was developed where nearly complete binding of CTXs in fish extracts occurred with boric acid gel in less than 1 h. Release of the bound CTXs resulted in >95% recovery of C-CTX1/2, C-CTX3/4, CTX1B, 54-deoxyCTX1B, and 52-epi-54-deoxyCTX1B from the extracts. This selective extraction tool has the potential to greatly simplify both analytical sample preparation and preparative extraction and isolation of CTXs for structure elucidation and production of standards.