| Research Abstract |
Recently, toxic blooms of cyanobacteria (blue-green algae) have been detected in freshwater lakes all over the world. The bloom-forming Microcystis and Anabaena produce hepatotoxins called micro cystins. Also, filamentous cyanobacteria sch as Oscillatoria and Nostoc produce toxins. However, structural and biological characterization of these toxins has not been clarified in detail. It has been apprehensive that these toxic filamentous cyanobacgteria may be implicated in human health and poisoning of fish. In this project, we investigated chemical structures and biological activities of novel toxins of Nostoc and Oscillatoria isolated from Japan and Europe.
A cyclic depsipeptide, nostocyclin, was isolated from cells of fulminates cyanobacterium, Nostoc sp.The cyclic peptide accounted for about 0.35% of the lyophilized cells. The structure of cyclic peptize was identified by FABMS,NMR and amino acid analysis, and established as Hpl-Ile-Hse-Thr-Hse-Ahp-Phe-N-Metyr-Val, The hydroxy group of Thr was esterified with the carboxy group of Val (Hpl, p-hydroxyphenyllactic acid ; Ahp, 3-amino-6-hydroxy-2-piperidone.). Three novel variants of microcystins were also isolated from Nosfoc sp.These structures were identified as [Asp3, ADMAdda5, Dhb7] microcystin RR,HtyR and LR.The identification of dehydrobutyric acid (Dhb) -containing microcystins was the first case in the world.
The Dhb-containing microcystins were also found in toxic blooms of Oscillatoria agadhii isolated from a lake in Scotland. However, the geometrical structure of the Dhb was different from those of Nostoc sp. By N0E and R0ESY experiments in NMR analysis, these structures were identified as [Asp3, (Z) -Dhb7] microcystin HtyR and LR.In contrast, the geometrical structure of the Dhb unit in microcystins from Nosfoc sp.were showed as E.This difference may be due to the biosynthesis pathway.
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