Marine Biotoxins: Tasteless and Odorless

ASP. Domoic acid (DA) produced by red algae Chondria spp. and diatoms Pseudonitzschia spp. is responsible for a human illness known as amnesia shellfish poisoning. Due to the structural similarity of DA and kainic acid, it is no surprise that DA has an antagonistic effect on the glutamate receptor of the nerve cell terminal. Acute symptoms at high levels of DA exposure include vomiting, abdominal cramps, diarrhea, headache, seizures, respiratory excretions, and confusion as well as coma and death in some cases. Apart from DA, their isomers are also reported to cause an ASP effect. However, DA isomers are less toxic than DA.

Because of the potential hazard to humans, a quick, sensitive, and specific method is needed to determine the presence of marine biotoxins in seafood.

DSP. These toxins are polyether compounds produced by dinoflagellates Dinophysis spp. and Prorocentrum spp. They are also found in many filter feeders such as mussels, clams, and oysters. Depending on their chemical structure, DSP toxins can be divided into three groups: okadaic acid (OA) and dinophysistoxins (DTXs), pectenotoxins (PTXs), and yessotoxins (YTXs). Typical symptoms of DSP toxins are diarrhea, nausea, vomiting, and abdominal pain. Unlike bacterial diarrhea, the symptoms of DSP toxins may be severe and begin anytime between 30 minutes and a few hours after eating contaminated shellfish. OA and DTXs were reported as tumor promoters resulting from the inhibition of protein phosphatase. Unfortunately, non-diarrheic effects were shown in PTXs and YTXs after both groups were placed into DSP toxins. The EU has established a new regulation to separate PTXs and YTXs from OA and DTXs.

AZP. Azaspiracids (AZAs) are another class of polyether toxins associated with AZP. The chemical structure of AZAs consists of the unique spiro-ring assemblies, a cyclic amine and carboxylic acid. Five analogs, AZA-1, AZA-2, AZA-3, AZA-4, and AZA-5, have been isolated from dinoflagellate Protoperidinium crassipes. AZA-1 is the most toxic to humans, followed by AZA-2 and AZA-3. AZA-4 and AZA-5 are oxidized metabolites of AZA-3. Acute symptoms from human consumption of AZP contaminated shellfish are similar to DSP, including nausea, vomiting, diarrhea, and stomach cramps.

CFP. This is caused by consuming a large variety of tropical and subtropical fish contaminated with ciguatoxins (CTXs) and maitotoxins (MTXs). These toxins are mainly produced by dinoflagella, Gambierdiscus toxicus. CTXs are concentrated in both the viscera (e.g., liver, intestines, and gonads) and muscle, whereas MTXs seem to be confined to the viscera. CFP symptoms are characterized by moderate to severe gastrointestinal symptoms (vomiting, diarrhea, and abdominal cramps), neurological signs (myalgia, paraesthesia, cold allodynia, and ataxia), pruritus and, less commonly, cardiovascular effects.

Analytical Methods and ­Regulations

(click for larger image)
Table 1: Threshold values for marine biotoxins.

Because of the potential hazard to humans, a quick, sensitive, and specific method is needed to determine the presence of marine biotoxins in seafood. The mouse bioassay has been used as the official method in many countries since the 1980s, although the method has poor accuracy, low sample rates, and is time and labor consuming. In addition, the mouse bioassay is not able to differentiate and quantify between individual toxins. With animal ethics in mind, it is highly desirable to develop new analytical methods. Enzyme-linked immunosorbent assay (ELISA) and chromatographic techniques have been focused in order to comply with the international regulations. ELISA kits have nevertheless reported many false positive results. Therefore, ELISA is recommended only for screening tests. In 2011, the EU established Commission Regulation (EU) No 15/2011 implementing the liquid chromatography coupled to tandem mass spectrometry (LC/MS/MS) technique as the confirmative method. At present, development of a method to identify marine biotoxins using LC/MS/MS remains slow because there are no certified standards covering all parent and metabolized compounds.