Zetekitoxin AB (ZTX AB) is a guanidine alkaloid neurotoxin found in the skin of Panamanian golden frog Atelopus zeteki. It was firstly reported in 1969 by Mosher and colleagues who originally designated it as atelopidtoxin and renamed zetekitoxin to indicate a unique occurrence in Atelopus zeteki Subsequent studies failed to reveal the structure of the major toxic component zetekitoxin AB (LD50 i.p. mouse, 11 μg/kg) or the minor component zetekitoxin C (LD50 i.p. mouse, 80 μg/kg). For decades it was regarded as one of the most potent neurotoxins known. Early research indicated that zetekitoxin AB might consist of two closely related compounds, hence the "AB" designation. In 2004, Yotsu-Yamashita and coworkers determined its chemical structure, revealing it to be a saxitoxin analog, and confirmed that it exists as a single compound.
Natural source Zetekitoxin AB was first extracted in 1969 from the skin of the Panamanian golden frog (Atelopus zeteki), together with the minor congener zetekitoxin C. The species remains the only known natural source of zetekitoxin AB. Like saxitoxin, zetekitoxin AB is not produced by Atelopus zeteki itself, but is instead acquired from exogenous bacteria associated with the frog's diet. However, Atelopus zeteki is classified as Critically Endangered, and individuals raised in captivity do not produce the toxin. To date, no method has been developed for the synthetic or alternative production of zetekitoxin AB. Only approximately 0.3 mg of the rare sample remained in the world, and this material was used for the structural determination of zetekitoxin AB in 2004.
Toxity and mechanism of action Zetekitoxin AB is a highly potent voltage-dependent sodium channel blocker and neurotoxin whose LD50 in mice is 11 μg/kg. Its main biological activity is the high-affinity blocking of NaV channels, leading to inhibition of neuromuscular conduction and cardiac function. As lack of samples available for testing, detailed mechanism of zetekitoxin AB is still noy clear so far. It is assumed that zetekitoxin AB has similar biochemical mechanism with other saxitoxin analogues: The positively charged guanidinium groups (1,2,3- and 7,8,9-guanidinium) of saxitoxin analogues serve as the primary anchors by inserting into the outer vestibule of the voltage-gated sodium channel (NaV) and forming extensive electrostatic interactions with the conserved acidic residues on the P2 helices, while the gem-diol moiety at C12 provides critical additional hydrogen bonds that further stabilize the toxin in the selectivity filter vestibule, together enabling the picomolar-affinity pore blockade of Na⁺ conductance. But Zetekitoxin AB represents significantly more potent, making it one of the most potent sodium channel blockers reported to date from natural products. In electrophysiological assays using Xenopus oocytes expressing mammalian NaV channels, zetekitoxin AB exhibited IC50 values of 280 ± 3 pM for the human heart channel (hH1A), 6.1 ± 0.4 pM for the rat brain IIa channel (rBr2A), and 65 ± 10 pM for the rat skeletal muscle channel (μ). Compared with saxitoxin under identical conditions, ZTX AB is ~580-fold more potent on the heart channel, ~160-fold on the brain channel, and ~63-fold on the skeletal muscle channel, confirming its profile as one of the most potent natural sodium-channel blockers known.
Structure assignment and suspicion Zetekitoxin AB was characterized by Yotsu-Yamashita and colleagues in 2004 as a unique guanidine alkaloid structurally related to saxitoxin. Its proposed structure features an unprecedented, highly strained 9-membered N-acylisoxazolidine-bridged macrocycle, a sulfonate group, and an N-hydroxycarbamate moiety.
The elucidation of its unique structure immediately captured the attention of the synthetic community, stimulating intense pursuit of its total synthesis. Beginning in 2009, Toshio Nishikawa, Kazuo Nagasawa, and Ryan E. Looper independently investigated the synthesis of N-acylisoxazolidine models. However, they found that the 13C NMR chemical shifts of the carbonyl carbons in these synthetic models appearing approximately 170 ppm (similar to those of typical amides) were significantly higher than that observed in Zetekitoxin AB (156.5 ppm), which has raised significant questions to the initial structural assignment, but can still be rationalized by significant decoupling of the isoxazolidine nitrogen non-bonding electrons from the carbonyl due to the strained 9-membered bridged ring system or complex shielding phenomenon imposed by the rigid three dimensional structure of ZTX which significantly effects this carbonyl.
Direct structural challenging
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