Brevetoxin A PbTX

This chapter deals with single crystal x-ray diffraction as a tool to study marine natural product structures. A brief introduction to the technique is given, and the structure determination of PbTX-1 (brevetoxin A), the most potent of the neurotoxic shellfish poisons produced by Ptychodiscus brevis in the Gulf of Mexico, is presented as an example. The absolute configuration of the brevetoxins is established via the single crystal x-ray diffraction analysis of a chiral 1,2-dioxolane derivative of PbTX-2 (brevetoxin B). 

Figure 6. Effect of brevetoxins on tritiated PbTx-3 binding to rat brain synaptosomes. Incubations, in the presence of 50 fig synaptosomal protein and 16 nM tritiated PbTx-3 with increasing amounts of unlabeled PbTx-1 ( ), PbTx-2 ( ), PbTx-3 ( ), PbTx-5 (A), PbTx-6 ( ), or PbTx-7 (o) were for 1 hr at 4 C. Each point represents the mean of three triplicate determinations.

To further investigate the role of the liver in brevetoxin metabolism, PbTx-3 was studied in the isolated perfused rat liver model (27, 28). Radiolabeled PbTx-3 was added to the reservoir of a recirculating system and allowed to mix thoroughly with the perfusate. Steady-state conditions were reached within 20 min. At steady-state, 55-65% of the delivered PbTx-3 was metabolized and/or extracted by the liver 26% remained in the effluent perfusate. Under a constant liver perfusion rate of 4 ml/min, the measured clearance rate was 0.11 ml/min/g liver. The calculated extraction ratio of 0.55 was in excellent agreement with the in vivo data. Radioactivity in the bile accounted for 7% of the total radiolabel perfused through the liver. PbTx-3 was metabolized and eliminated into bile as parent toxin plus four more-polar metabolites (Figure 3). Preliminary results of samples stained with 4-(p-nitrobenzyl)-pyridine (29) indicated the most polar metabolite was an epoxide. 

Figure 2.1. Brevetoxins are based on two different structural backbones, based on what are perceived to be the two parent molecules, PbTx-2 (brevetoxin B) and PbTx-1 (brevetoxin A). All other known derivatives are based on alteration of the R-side chain, epoxidation across the double bond in the H-ring of PbTx-2, or derivatization at the C-37 hydroxyl in PbTx-2. PbTx-8, the chloromethyl ketone derivative of PbTx-2, is an artifact of chloroform extraction and subsequent phosgene conversion of PbTx-2. Common features include trans-fused polyether ring systems consisting of five- to nine-membered rings. denotes likely chemical artifact from extraction (Baden et al. 2005).

Figure 2.2. Five new brevetoxins, based on the PbTx-2 type backbone, have been purified and characterized PbTx-11, a toxin with a shortened side chain PbTx-12, the only natural ketone brevetoxin known PbTx-13 and PbTx-14, which are both believed to be extraction artifacts formed in the presence of methanol reaction with the very active exomethylene-conjugated aldehyde of PbTx-2 and PbTx-tbm, a brevetoxin PbTx-2 backbone without any side chain, a form that is prevalent in senescent cultures (Baden et al. 2005).

Rodriguez, F.A., Escobales, N., and Maldonado, C. 1994. Brevetoxin-3 (Pbtx-3) inhibits oxygen consumption and increases Na+ content in mouse-liver slices through atetrodotoxin-sensitive pathway. Toxicon 32,1385—1395. 

Rodriguez-Rodriguez, F. A., and Maldonado, C. 1996. Brevetoxin-3 (PbTx-3) on mouse liver slices a histological study. P R Health i J 15, 261-267. 

Washburn, B. S., Rein, K. S., Baden, D. G. et al., Brevetoxin-6 (PbTx-6), a nonaromatic marine neurotoxin, is a ligand of the aryl hydrocarbon receptor. Arch. Biochem. Biophys. 343, 149, 1997. 

It was not until 1981 that the first brevetoxin structure was determined (13), PbTX-2, the most plentiful of the brevetoxins, was purified to crystallinity and the structure determined by x-ray crystallography. This important first step was the result of a three-group collatoration among Clardy, Lin, and Nakanishi. The yields of the major components from 50 L of culture (ca. 5 x 10 cells) were 0.8 mg of PbTX-1, 5.0 mg of PbTX-2, and 0.4 mg of PbTX-8. The structure of PbTX-2,... 

The crystal structures of PbTX-1 dimethyl acetal, PbTX-1, and dihydro PbTX-1 provide a total of four independent pictures of the same brevetoxin skeleton. It is rare that this quantity of structural data is available for a natural product of this size. A comparison of torsional angles shows that all four molecules have approximately the same conformations in all rings, except, of course, for the aldehyde side chain and the E-ring in one of the independent molecules of PbTX-1. Least squares superposition fits among the four molecules gave the following average distances ... 

Figure 5. Comparison of specific displacement of 10 nM tritiated saxitoxin (A) or 10 nM tritiated PbTx-3 ( ) by unlabeled competitor saxitoxin or brevetoxin, respectively, in rat brain synaptosomes. IC q in each case is 5-10 nM.

Several brevetoxins have been examined for their respective abilities to competitively displace tritiated brevetoxin PbTx-3 from its specific site of action in brain synaptosomes. Analysis of IC q values revealed no marked differences in the displacing abilities between any of the type-1 toxins, and similarly there was no apparent difference between displacing abilities of PbTx-1 or -7, both type-2 toxins. Although some specific details require correlation, a gross comparison indicates that sodium channels in brain are similar in the systems examined. In the system studied most extensively, the rat brain synaptosome, t-test analysis revealed no significant differences between PbTx-2 and PbTx-3 IC q, or between PbTx-1 and PbTx-7 IC q, but statistically significant differences were found between the two classes (P<0.01) (5). If the Cheng-Prusoff equation (15) is applied ... 

K, which necessitates use of high specific activity radioactive toxin (at or above 10 Ci/mmol). Work in progress indicates that all type-1 brevetoxins inhibit tritiated PbTx-3 binding in a purely competitive manner, whereas the type-2 brevetoxins inhibit in a mixed competition manner at higher concentrations (Figure 7). 

The comparison of fish bioassays (6) with the calculated effective doses indicate that the two most potent brevetoxins, PbTx-1 and PbTx-7, also are most effective at displacing tritiated probe from its specific site of action. The considerably lower potency of brevetoxins PbTx-5 and PbTx-6 in the rat system suggest that these two toxins may bind with lesser affinity to site 5. In a general sense, this is indicated in Table II, and is summmarized in Table III. 

Methods of detection, metabolism, and pathophysiology of the brevetoxins, PbTx-2 and PbTx-3, are summarized. Infrared spectroscopy and innovative chromatographic techniques were examined as methods for detection and structural analysis. Toxicokinetic and metabolic studies for in vivo and in vitro systems demonstrated hepatic metabolism and biliary excretion. An in vivo model of brevetoxin intoxication was developed in conscious tethered rats. Intravenous administration of toxin resulted in a precipitous decrease in body temperature and respiratory rate, as well as signs suggesting central nervous system involvement. A polyclonal antiserum against the brevetoxin polyether backbone was prepared a radioimmunoassay was developed with a sub-nanogram detection limit. This antiserum, when administered prophylactically, protected rats against the toxic effects of brevetoxin. 

Research in this area advanced in the 1970 s as several groups reported the isolation of potent toxins from P. brevis cell cultures (2-7). To date, the structures of at least eight active neurotoxins have been elucidated (PbTx-1 through PbTx-8) (8). Early studies of toxic fractions indicated diverse pathophysiological effects in vivo as well as in a number of nerve and muscle tissue preparations (reviewed in 9-11). The site of action of two major brevetoxins, PbTx-2 and PbTx-3, has been shown to be the voltage-sensitive sodium channel (8,12). These compounds bind to a specific receptor site on the channel complex where they cause persistent activation, increased Na flux, and subsequent depolarization of excitable cells at resting... 

Chromatography. A number of HPLC and TLC methods have been developed for separation and isolation of the brevetoxins. HPLC methods use both C18 reversed-phase and normal-phase silica gel columns (8, 14, 15). Gradient or iso-cratic elutions are employed and detection usually relies upon ultraviolet (UV) absorption in the 208-215-nm range. Both brevetoxin backbone structures possess a UV absorption maximum at 208 nm, corresponding to the enal moeity (16,17). In addition, the PbTx-1 backbone has an absorption shoulder at 215 nm corresponding to the 7-lactone structure. While UV detection is generally sufficient for isolation and purification, it is not sensitive (>1 ppm) enough to detect trace levels of toxins or metabolites. Excellent separations are achieved by silica gel TLC (14, 15, 18-20). Sensitivity (>1 ppm) remains a problem, but flexibility and ease of use continue to make TLC a popular technique. 

Of particular interest in brevetoxin research are the diagnosis of intoxication and identification of brevetoxins and their metabolites in biological fluids. We are investigating the distribution and fate of radiolabeled PbTx-3 in rats. Three model systems were used to study the toxicokinetics and metabolism of PbTx-3 1) rats injected intravenously with a bolus dose of toxin, 2) isolated rat livers perfused with toxin, and 3) isolated rat hepatocytes exposed to the toxin in vitro. 

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