NEW NODULARINS - A GENERAL-METHOD FOR STRUCTURE ASSIGNMENT

A general method has been developed for assigning the structures of nodularin, a potent hepatotoxin, tumor promoter, and protein phosphatase inhibitor, and minor components isolated from a cultured and a bloom sample of the cyanobacterium Nodularia spumigena. It consists of (1) FABMS analysis (determination of molecular weight and molecular formula), (2) H-1 NMR spectroscopy on the parent compound and chiral GC analysis of an acid hydrolyzate (identification and stereochemistry of amino acid components), (3) ozonolysis followed by NaBH4 reduction (conversion to a linear peptide), and (4) FABMS/CID/MS analyses of the linear peptide and the parent compound (sequence analysis). The method has been employed in assigning structures to three new nodularins (2-4) and can be applied to other cyclic peptides containing alpha,beta-dehydroamino acid unit(s), especially the related microcystins, cyclic heptapeptide hepatotoxins. Two nodularins, [DMAdda(3)]nodularin (2) and [(6Z)-Adda(3)] nodularin (3), were obtained from a bloom sample collected from Lake Ellesmere (New Zealand), and [D-Asp(1)] nodularin (4) was isolated from cultured cells (strain L-575). The LD50s of 2 and 4 were 150 and 75 mu g/kg (ip, mice), respectively, but 3 did not show apparent toxicity at 2.0 mg/kg.