Isolation and Biological Activity of 9- epi Tetrodotoxin and Isolation of Tb-242B, Possible Biosynthetic Shunt Products of Tetrodotoxin from Pufferfish.

Tetrodotoxin (TTX, ) is a potent voltage-gated sodium channel blocker detected in certain marine and terrestrial organisms. We report here a new TTX analogue, 9-TTX (), and a TTX-related compound, Tb-242B (), isolated from the pufferfish and , respectively. NMR analysis suggested that exists as a mixture of hemilactal and 10,8-lactone forms, whereas other reported TTX analogues are commonly present as an equilibrium mixture of hemilactal and 10,7-lactone forms. Compound and TTX were confirmed not to convert to each other by incubation under neutral and acidic conditions at 37 °C for 24 h. Compound was identified as the 9-epimer of Tb-242A (), previously reported as a possible biosynthetic precursor of TTX. Compound was partially converted to by incubation in a neutral buffer at 37 °C for 7 days, whereas was not converted to under this condition. Compound was detected in several TTX-containing marine animals and a newt. Mice injected with 600 ng of by intraperitoneal injection did not show any adverse symptoms, suggesting that the C-9 configuration in TTX is critical for its biological activity. Based on the structures, and were predicted to be shunt products for TTX biosynthesis.