Anemone toxin

Figure 1. Most cx)mmon (consensus) sequences of the two types of sea anemone toxins. Bold letters represent residues which both toxin types have in common. Letters above each sequence are nonconservative substitutions, while letters below each sequence are conservative substitutions. A nonconservative substitution was defined as one in which (a) electronic charge changed, (b) a hydrogen-bonding group was introduced or removed, (c) the molecular size of the sidechain was changed by at least 50%, or (d) the secondary structure propensity was changed drastically from b to h or vice versa (Ref.

Figure 2. Relative toxicity (LD50 and LD qq) estimates for actiniid sea anemone toxins upon crabs (Carcimis maenas) and mice. Values for Anemonia sulcata (As) and Anthopleura xanthogrammica (Ax) toxins are from ref. 24 data for Condylactis gigantea and Phyl-lactis flosculifera toxins are unpublished (Kem). The arrows indicate that the real mouse LD q values for Cg II and Pf II must exceed the values indicated in the Ogure. Although insufficient data are presently available to quantitatively define a relationship between mammalian and crustacean toxicity, it seems that there is usually an inverse relationship, which may be approximately defined by the stipple zone.

Comparisons between these toxins allow delineation of the variability of each position in the sequence. For instance, the residues which are extremely invariant (conservative) for both types of sea anemone toxin are the half-cystines, certain glycyl residues which are expected to be involved in )9-turns, and only a few other residues - Asp 5 or 6, Arg 13 or 14, and Tryp 30 or 31 (the numbering depends upon the toxin type) — expected to be important for folding or receptor binding. Rather surprising is the variation in the residues which NMR studies (22,23) have shown are involved in formation of the four stranded )9-pleated sheet. 

Some chemical modification studies on the sea anemone toxins have unfortunately been less than rigorous in analyzing the reaction products. Consequently, results from many of these studies can only provide suggestions, rather than firm conclusions, regarding the importance of particular sidechains. Many such studies also have failed to determine if the secondary and tertiary structures of the toxin products were affected by chemical modification. 

Despite the vast amount of data on the pharmacological properties, very little about the conformation of the proteins has b n known until recent NMR studies. 2D-NMR results have provided detailed information about the secondary structure of several related anemone toxins. ATX I from Anemonia sulcata (3,4) and AP-A from Anthopleura xanthogrammica (5,6) have been studied by Gooley and Norton, and more recently Widmer et al. have further purified the A. sulcata toxins and obtained complete sequence specific assignments for ATX la (7). Our laboratory, on the other hand, has studied the structures of RpII and RpIII from RadiarUhiis paumotensis (8). 

Schweitz et al. purified four related toxins (Rpl, RpII, RpIII, and RpIV) from sea anemone Radianthus paumotensis (Rp) and studied their pharmacological properties (9). During the course of initial NMR studies, the reported sequence of RpII was found to have errors, and was redetermined (8). Subsequently Metrione et al. determined the sequence of RpIII as well (10). T e other two Rp toxin sequences are yet to be determined. Sequences of the Rp, and several other sea anemone toxins, are shown in Table I. We have used a two letter code to denote the species consistently and this notation differs from the earlier designations of Norton and Wiithrich groups. In our notation. As la and Ax I correspond to ATX la and AP-A, respectively. From alignment of the cystines in these sequences, it is clear that Rp toxins have three disulfide bonds, as do the other toxins. 

We limit the discussion to the level of secondary structures of sea anemone toxins since the tertiary structure of no other anemone toxins is known at present. Wid-mer et al. (7), on the basis of high resolution 2D-NMR results, have provided the most detailed information about the secondary structure of the A, sulcata toxin ATX la. Gooley and Norton (4-6) have partially assigned the toxins ATX I and AP-A, and compared their secondary structures. The NMR results are consistent and show that the anemone toxins all consist of a four strand antiparallel -sheet core and have no significant helical structure. 

In other chapters of this volume considerable attention is given to marine toxins whose cellular sites of action have been identified. For example, saxitoxin, brevetoxin, and sea anemone toxins are prototypes of toxic molecules whose chemical structure is known, and whose actions on ionic channels in the cell membrane have been elucidated. Recent additions to such toxins are the piscivorus cone... 

In the light of such considerations, it is possible to discuss toxins which have already been analyzed in terms of their sites of action. Such a discussion is best conducted by categorizing the various possible cellular sites at which a toxin might act. The most obvious sites are the membrane channels for ions, receptors for neurotransmitters, membrane pumps, and the membrane itself. Invertebrate toxins acting on membrane channels include the conotoxins (10) and several of the sea anemone toxins (97). 

It is ironic that possibly the first animal model of relevance to immunotoxicology was reported by Portier and Richet in 1902 [45] in an attempt to induce tolerance to a sea anemone toxin, they accidentally produced a shock reaction in dogs. Since this was not the protective effect they had hoped to produce (phylaxis for protection in Greek), they named the reaction anaphylaxis [46], The irony, of course, is that this serious reaction, mediated by IgE in humans, has proven to be notoriously difficult to predict based on animal studies. This is no trivial issue, since anaphylaxis is a serious, life-threatening reaction associated with exposure to drugs, foods, cosmetic ingredients, and other exogenous substances [47],... 

Bloomquist JR, Soderlund DM (1988) Pyrethroid insecticides and DDT modify alkaloid-dependent sodium channel activation and its enhancement by sea anemone toxin. Mol Pharmacol 33 543-550... 

Apart from AP-A, the best characterized of these polypeptides with respect to its biological activity is Anemonia sulcata toxin II (ATX II) [19]. This molecule is also cardioactive [28], as would be expected from its similarity to AP-A. Renaud et al. [29] have compared the activities of a number of sea anemone and scorpion toxins on isolated rat atria and found that anthopleurin-B (AP-B, also known as Ax II) had the highest potency and the greatest margin between the concentrations necessary for maximal inotropic activity and for provoking arrhythmias (0.3 versus 10 n . It was also found that sodium channels of rat cardiac cells in culture, which have a low affinity for tetrodotoxin (TTX), have a particularly high affinity for Type 1 anemone toxins [29], whereas Type 2 toxins [30] and scorpion toxins [31] had similar affinities for TTX-sensitive and TTX-insensitive channels in rat neuroblastoma cells and skeletal myotubes, respectively. 

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