Factors Influencing the Toxicity, Detoxification and Biotransformation of Paralytic Shellfish Toxins - Reviews of Environmental Contamination and Toxicology - page 16

and 3) to more toxic ones (e.g., STX and neoSTX) is associated with the reductive
elimination of the O 22 -sulfate and N1-hydroxyl groups (Kotaki et al. 1985 ; Shimizu
and Yoshioka 1981 ). Nevertheless, the reaction only occurs after other carbon
sources are exhausted (Jones et al. 1994 ). The reaction was faster under anaerobic
than under aerobic conditions (Kotaki et al. 1985 ).
Enzymatic transformation of more toxic PSTs (carbamoyl and carbamoyl- N
sulfated toxins) to less PSTs (decarbamoyl compounds) has been documented to
occur in green mussels (Choi et al. 2003 ), and in littleneck clams and scallops
(Noguchi et al. 1989 ). Decarbamoylase clearage of the carbamoyl group in carbamoyl
and carbamoyl- N sulphated toxins can result in the formation of signifi cantly less
toxic decarbamoyl toxins (Smith et al. 2001 ). Bacterial sulfotansferases also trans-
formed the more toxic PST, GTX 2/3 and GTX 1/4, into less toxic PSTs, C1/C2 and
C3/C4, respectively, via enzymatic oxidation (Lee et al. 1995 ). Biotransformation of
STX, GTX 2 and GTX 3 into GTX 5, C1 and C2, respectively, by sulfotransferase
activity has been described for the dinofl agellates G. catenatum and Alexandrium
catenella (Yoshida et al. 1998 ). Moreover, bacterial isolates from bivalve gut have been
shown to degrade GTX1 and GTX4, without the appearance of GTX2 and GXT3
(Smith et al. 2001 ). Pseudoalteromonas haloplanktis , isolated from the digestive
tracts of blue mussels, has been shown to play an important role in reducing the
toxicity of PST by 90% within 3 days (Donovan et al. 2009 ).
Non-enzymatic transformation (detoxifi cation by chemical agents) by ozonation
is commonly used to inactivate toxins from extracts of dinofl agellates and shellfi sh
tissues. Ozone treatment is more effective than chlorine treatment for depuration of
PSP in green mussels and clams. Ozone treatment completely detoxifi ed the toxins
in bivalves within 10 days, while chlorine treatment required 15 days to achieve
similar results. Ozone treatment is preferred, because it inactivates bacteria or virus,
oxidizes toxins, and does not change the taste and appearance of the shellfi sh
(Sharma et al. 2011 ). Nevertheless, ozone treatment does not completely remove
toxins that are incorporated into tissues. Detoxifi cation is infl uenced by the diet of
bivalves during the depuration period. For example, oysters that feed on a mixture
of Chlorella and chitosan demonstrated higher depuration effi ciency (decrease from
12 MU g −1 into 0.5 MU g −1 in 7 days) than those feeding only on Chlorella or
chitosan (decreased from 12 MU g −1 into 3.47 MU g −1 and 1.4 MU g −1 , respectively,
in 7 days) (Xie et al. 2013 ). This is because active feeding is likely to accelerate
gut evacuation rates and overall metabolism (degradation, excretion) of the toxins.
In addition, shellfi sh have the natural ability to chemically transform GTX1/4 to
GTX2/3 by reduction of the N1-OH group in GTX1 and GTX4 to an N1-H group
from conjugation with glutathione and cysteine (Oshima 1995 ).

7 Conclusions

Paralytic shellfi sh poisoning is a global problem that affects coastal communities
throughout the world. The causative toxins, saxitoxins and saxitoxin-related
compounds are potent neurotoxins that negatively affect the fi sheries industry