Spiro Bicyclic Guanidino Compounds from Pufferfish: Possible Biosynthetic Intermediates of Tetrodotoxin in Marine Environments

• Published in: Chemistry : a European journal Vol. 24; no. 28; pp. 7250 - 7258
• Main Authors: Ueyama, Nozomi, Sugimoto, Keita, Kudo, Yuta, Onodera, Ken‐ichi, Cho, Yuko, Konoki, Keiichi, Nishikawa, Toshio, Yotsu‐Yamashita, Mari
• Format: Journal Article
• Language: English
• Published: WEINHEIM Wiley 17.05.2018; Wiley Subscription Services, Inc
• Subjects: Animals; biosynthesis; Chemistry; Chemistry, Multidisciplinary; Crabs; Cyclohexane; Intermediates; Marine animals; Marine environment; Marine organisms; natural products; Offshore structures; Physical Sciences; Science & Technology; spiro guanidino compounds; structure elucidation; Terrestrial environments; Tetrodotoxin; ANALOGS; IDENTIFICATION; spiro guanidino compounds; NEWT; natural products; CHIRIQUITOXIN; ENZYME; SAXITOXIN; biosynthesis; FISH; structure elucidation; tetrodotoxin; STEREOCONTROLLED SYNTHESIS
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201801006

Tetrodotoxin (TTX, 1) is a potent neurotoxin that is widely found in both terrestrial and marine animals; however, the biosynthetic pathway and genes for TTX have not yet been elucidated. Previously, we proposed that TTX originated from a monoterpene; this hypothesis was based on the structures of cyclic guanidino compounds that are commonly found in toxic newts. However, these compounds have not been detected in marine organisms. Instead, a series of deoxy analogues of TTX were found in toxic marine animals; thus, we further screened for TTX‐related compounds in marine animals. Herein, we report seven novel spiro bicyclic guanidino compounds 2–8 that were isolated from the pufferfish Tetraodon biocellatus. In compounds 2–5 and 7–8, a six‐membered cyclic guanidino amide is spiro‐fused with 2,4‐dimethyl cyclohexane, whereas in compound 6, the same cyclic guanidino amide is spiro‐fused with 2,3,5‐trimethylcyclopentane. Compounds 2–5 and 7–8 have the same carbon skeleton and relative configuration as TTX. Thus, we proposed that compounds 2–8 are biosynthetic intermediates of TTX in marine environments. TTX could be biosynthetically derived from compound 7 via intermediates 2–5 through several oxidations, amide hydrolysis, and formation of the hemiaminal and lactone found in 5,6,11‐trideoxyTTX, the major TTX analogue, whereas compounds 6 and 8 might be shunt products. LC‐MS analysis confirmed the wide distribution of compounds 2, 3, or both in TTX‐containing marine animals, namely pufferfish, crab, octopus, and flatworm, but compounds 2–8 were not detected in newts. Pufferfish knows: Seven new tetrodotoxin‐related compounds, which are potential biosynthetic precursors of 5,6,11‐trideoxytetrodotoxin, are isolated from pufferfish, and their structures are determined. A biosynthetic pathway of tetrodotoxin in marine environments via these species is proposed, which involves oxidation steps, amide hydrolysis, and ring closures (see scheme).