Okadaic acid

For in vitro studies there are a number of compounds available to block protein phosphatase activity. Phosphate buffers inactivate all of these enzymes. Several naturally occurring toxins are potent inhibitors of PPPs, e.g., okadaic acid or microcystin, and are frequently used tools. PPM and PTP family members are not affected by these toxins. Vanadate containing solutions are competitive inhibitors of PTPs, pervanadate is an irreversible inhibitor of PTPs. 

Figure 5.59 Molecular structures of the diarrhetic shellfish poisons (a) pectenotoxin-6 (PTX6) (b) okadaic acid (OA) (c) dinophysistoxin-1 (DTXl) (d) yessotoxin (YTX). Reprinted from J. Chromatogr., A, 943, Matrix effect and correction by standard addition in quantitative liquid chromatographic-mass spectrometric analysis of diarrhetic shellfish poisoning toxins , Ito, S. and Tsukada, K., 39-46, Copyright (2002), with permission from Elsevier Science.

Three classes of polyethers, okadaic acid derivatives, pectenotoxins, and yessotoxin were isolated from bivalves in connection with diarrhetic shellfish poisoning. The etiology of the toxins, toxicological properties, and determination methods are described. 

Lyngbyatoxin A Debromoaplysiatoxin Aplysiatoxin Bromoaplysiatoxin Dibromoaplysiatoxin Oscillatoxin A Anhydrodebromo-aplysiatoxin Palytoxin Okadaic acid TPA... 

Non-TPA Type Tumor Promoters Palytoxin and Okadaic Acid Class... 

In contrast to the TPA-type tumor promoters, palytoxin, thapsigargin, and okadaic acid are classified as non-TPA type tumor promoters, which do not bind to phorbol ester receptors, or activate protein kinase C in vitro (Table II) (6,25-27). In this chapter, thapsigargin is not discussed, because it is derived from terrestrial plants. 

We synthesized H-okadaic acid chemically and demonstrated its specific binding to the particulate and cytosolic fractions of mouse skin. The specific binding of H-okadaic acid to the particulate fraction was not inhibited by TP A, lyngbyatoxin... 

A, aplysiatoxin, or palytoxin. Therefore, we proposed that okadaic acid binds to its own receptor and induces various biological effects, including tumor promoting activity, through this receptor (27). We are now investigating the nature of its receptor. 

In addition to okadaic acid, dinophysistoxin-1 (i.e., 35-methylokadaic acid), 7-0-palmitoyl-okadaic acid, and pectenotoxin 2 are reported to be diarrhetic toxins from shellfish 34). Application of 1 xg of dinophysistoxin-1 to mouse ear caused as strong irritation as the same dose of okadaic acid. Interestingly, the potencies of these compounds in the irritant test on mouse ear correlated well with their potencies as diarrhetic shellfish poisons. Dinophysistoxin-1 induced ODC activity as strongly as okadaic acid. Recently, we found that dinophysistoxin-1 is also a new non-TPA type tumor promoter with as high activity as okadaic acid 35). 

We have shown that TPA-type tumor promoters, such as lyngbyatoxin A and aplysi-atoxins exert tumor promoting activities on mouse skin through different mechanisms from those of non-TPA type tumor promoters such as palytoxin, okadaic acid, and dinophysistoxin-1. 

A variety of toxins are capable of forming their own channels in cell membranes. For example, okadaic acid has been shown to be an ionophoric polyether (27,29). 

Surprisingly few studies have been performed with purified toxins. When added externally to the water, toxins of various origins were tested on the cope-pod Tigriopus californicus. The protein phosphatase inhibitor okadaic acid (17) from red tide dinoflagellates [22] and the neuronal depolarizing agent do-moic acid (10) from diatoms [40, 41] had different effects on the herbivores (Scheme 3). Micromolar concentrations of okadaic acid (17) acted both as toxin... 

Interestingly, okadaic acid diol ester (16) in the culture medium is oxidatively transformed into the more hydrophilic metabolites 18-20 by intact cells of the diatom Thalassiosira weissflogii [52,53]. This transformation is speculated to change the toxic metabolite with allelopathic activity into a less toxic transport form that can be better excreted by the diatom. 

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