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Paralytic Shellfish Poisoning

Along the Pacific coast of the United States, the most costly problem associated with Harmful Algal Blooms is Paralytic Shellfish Poisoning or PSP. PSP is not a recent problem but was described over 200 years ago by Captain George Vancouver during his exploration of the waters around Vancouver Island and Puget Sound in Washington State. During this exploration of what is now the Canadian province of British Columbia, Vancouver reported in his log that at least four crewmen became ill after eating mussels for breakfast. Use of various known remedies of the time did not work and as a result Vancouver lost one crewman (a certain John Carter) to PSP on June 15, 1793. As the name implies the poisoning syndrome results in death by paralysis, usually of the respiratory system.

For an in-depth review of PSP, visit the The next link/button will exit from NWFSC web site University of Alaska Fairbanks Alaska Sea Grant website. In addition, there is an excellent published report by Ray RaLonde on PSP entitled "Paralytic Shellfish Poisoning: The Alaska Problem" available at the The next link/button will exit from NWFSC web site Alaska Sea Grant Information and Publications website. Much of what is discussed in this report is also applicable to other areas where PSP is endemic.

The structure of the most potent component of the PSP toxins is called saxitoxin and referred to as STX. Its structure was determined about 30 years ago. Since then, it was found that PSP is caused not by just one toxin but a suite of toxins, based on the backbone structure of the saxitoxin molecule. All of these toxins are low molecular weight, water soluble, nitrogen containing compounds. In the structure below, replaceable constituents are indicated by R1, R2, R3, and R4. The moiety in blue is the carbamoyl moiety making saxitoxin a carbamate derivative. Most carbamates are quite toxic to humans. The replaceable groups can be hydrogens, hydroxyls, or sulfate groups. The various isomers that are created with these different groups attached to the basic STX molecule form a very potent toxic suite.

Carbamoyl Saxitoxins

R1

R2

R3

R4

 
STX

H

H

H

H

GTX II

H

OSO3-

H

H

GTX III

H

H

OSO3-

H

NEO

OH

H

H

H

GTX I

OH

OSO3-

H

H

GTX IV

OH

H

OSO3-

H

In the above table under Carbamoyl Saxitoxins, STX stands for saxitoxin, NEO stands for neosaxitoxin, and GTX refers to gonyautoxins (sometimes called 'gonytoxins').  These names are based on species names from which the toxins were isolated.  For example, saxitoxin was isolated from the Alaska butter clam (Saxidomus giganteus) while the generic gonyautoxins were named after the old name of Alexandrium catenella (old name: Gonyaulax catenella). Neosaxitoxin and saxitoxin are the most potent of these toxins. While the "Sulfamate toxins" are toxic, they are the least toxic of the PSP suite, some of these toxins are about 1/10 the toxicity of neosaxitoxin.

Sulfamate Saxitoxins

R1

R2

R3

R4

B1

H

H

H

SO3-

C1

H

OSO3-

H

SO3-

C2

H

H

OSO3-

SO3-

B2

OH

H

H

SO3-

C3

OH

OSO3-

H

SO3-

C4

OH

H

OSO3-

"

When the carbamoyl group is removed from the basic structure, we have the decarbamated toxins. These structures have only been recently elucidated. Their basic naming structure is based on the removal of the carbamoyl group (-OCONH2) and replacing it with a hydrogen (yielding a hydroxyl group (-OH),  hence decarbamoyl saxitoxin. For the basic structure of the decarbamoylated saxitoxns see the structure below:

 

Decarbamoyl Saxitoxins

R1

R2

R3

 
dc-STX

H

H

H

dc-GTX II

H

OSO3-

OSO3-

dc-GTX III

H

H

OSO3-

dc-NEO

OH

H

H

For more detailed information about these structures, their chemistry, and toxicology we direct the reader to the American Chemical Society's Monograph "Marine Toxins, Origin, Structure, and Molecular Pharmacology" edited by Sherwood Hall and Gary Strichartz, ACS Symposium Series 418, American Chemical Society, Washington DC, 1990. The reader is directed especially to Chapter 3, "The Saxitoxins" by Sherwood Hall, Gary Strichartz, E. Moczydolowski, A. Ravindran, and P.B. Reichardt (pp 29-65).