Identification of 12 Hepatotoxins from a Homer Lake Bloom of the Cyanobacteria Microcystis aeruginosa, Microcystis viridis, and Microcystis wesenbergii: Nine New Microcystins

Eleven minor components were isolated, together with microcystin-LR (LR, 1, Scheme I) as the principal toxin (ca. 90% of the toxic components), from Microcystis cyanobacteria (blue-green algae) collected from Homer Lake (Illinois) in the summer of 1988. The components were characterized by amino acid analysis and HRFABMS, FABMS/MS, ¹H NMR, and UV spectroscopic methods as microcystins-RR (2) and -YR (3) (Scheme I) and nine new microcystins. The structures of seven new microcystins were assigned as [DMAdda⁵]microcystin-LR (4), [Dha⁷]microcystin-LR (5), microcystin-FR (6), microcystin-AR (7), microcystin-M(O)R (8), [Mser⁷]microcystin-LR (9), and microcystin-WR (12). Compound 4 is the first microcystin containing 9-O-demethyl-Adda, while phenylalanine, N-methylserine, and tryptophan are also new variations in amino add components of microcystins. Compound 11 was deduced to be a (C₃H₇O) monoester of the α-carboxyl on the Glu unit of LR (1). New microcystin 11 caused no apparent toxic effects in mice dosed ip at 1 mg/kg, while the others had LD₅₀'s of 90–800 µg/kg.