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The synthesis and biosynthesis of the complex saxitoxin (STX) structure have garnered significant interest. Previously, we hypothesized that the tricyclic skeleton of STX originates from the monocyclic precursor 11-hydroxy-IntC'2 during biosynthesis, although direct evidence has been lacking. In this study, we identified conditions to synthesize a proposed tricyclic biosynthetic intermediate, 12,12-dideoxy-decarbamoyloxySTX (dd-doSTX), along with its 6-epimer (6-epi-dd-doSTX) and a bicyclic compound, in a single step from di-Boc protected 11-hydroxy-IntC'2. The reaction mechanism involves successive aza-Michael addition of a guanidino amine to the conjugated olefin. Notably, both dd-doSTX and 6-epi-dd-doSTX were detected in a toxin-producing cyanobacterium, suggesting that the biosynthetic enzymes may generate these compounds via similar mechanisms., (© 2024 The Authors. Chemistry - An Asian Journal published by Wiley-VCH GmbH.)

Dinophysistoxin 1 (DTX1, 1) and okadaic acid (OA, 2), produced by the dinoflagellates Dinophysis spp. and Prorocentrum spp., are primary diarrhetic shellfish toxins (DSTs), which may cause gastric illness in people consuming such as bivalves. Both compounds convert to dinophysistoxin 3 (DTX3, 3; generic name for 1 and 2 with fatty acids conjugated at 7-OH) in bivalves. The enzyme okadaic acid O-acyl transferase (OOAT) is a membrane protein found in the microsomes of the digestive glands of bivalves. In this study, we established an in vitro enzymatic conversion reaction using 4-nitro-2,1,3-benzoxadiazole (NBD)-OA (4), an OA derivative conjugated with (R)-(-)-4-nitro-7-(3-aminopyrrolidin-1-yl)-2,1,3-benzoxadiazole (NBD-APy) on 1-CO2H, as a substrate. We detected the enzymatically produced 3, NBD-7-O-palmitoyl-OA (NBD-Pal-OA), using high-performance liquid chromatography-fluorescence detection. We believe that an OOAT assay using 4 will facilitate the fractionation and isolation of OOAT in the future., (© The Author(s) 2024. Published by Oxford University Press on behalf of Japan Society for Bioscience, Biotechnology, and Agrochemistry.)

Saxitoxin (STX, 1) is a representative compound of paralytic shellfish toxins (PSTs) that are produced by marine dinoflagellates and freshwater cyanobacteria. Although several pathways have been proposed for the biosynthesis of STX, the order of ring and side chain hydroxylation, and formation of the tricyclic skeleton have not been well established. In this study, 12,12-dideoxy-decarbamoyloxySTX (dd-doSTX, 2), the most reduced STX analogue having the tricyclic skeleton, and its analogues, 12β-deoxy-doSTX (12β-d-doSTX, 3), 12α-deoxy-doSTX (12α-d-doSTX, 4), and doSTX (5), were synthesized, and these compounds were screened in the toxic microalgae using high-resolution LCMSMS. dd-doSTX (2) and 12β-d-doSTX (3) were identified in the PSTs-producing dinoflagellates (Alexandrium catenella, A. pacificum, and/or Gymnodinium catenatum) and in the cyanobacterium Dolichospermum circinale (TA04). doSTX (5), previously isolated from the dinoflagellate G. catenatum, was also identified in D. circinale (TA04). Furthermore, the conversion of 2 to 3, and 4 to 5, by SxtT with VanB, a reported Rieske oxygenase and its redox partner in STX biosynthesis, was confirmed. These results support that 2 is a possible biosynthetic precursor of STX, and that ring and side-chain hydroxylations proceed after cyclization., (© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)