Recently, Microcystis aeruginosa has been demonstrated to be a valuable source of unique biologically active peptides. In the course of our studies of protease inhibitors from microalgae, freshwater blue-green alga M aeruginosa (NIES-98) showed potent serine protease inhibitory activities. Here we report the isolation and structure elucidation of aeruginosin 98-A(1), B(2) and C(3) which inhibit trypsin, and aeruginoguanidine 98-A(4), B(5) and C(6) which shows weak cytotoxic activities. Microcystis aeruginosa (NIES-98) was obtained from the NIES-collection (Microbial Culture Collection, the National Institute for Environmental Studies, Japan). The 80% methanol extract of freeze-dried alga was partitioned between water and diethyl ether. The aqueous layer that showed potent trypsin inhibitory activity was further extracted with n-butanol and subjected to ODS column chromatography, followed by ODS HPLC to yield 1-6. Molecular formula of 1 was decided as C_<29>H_<45>N_6O_9ClS from HRFABMS and NMR data. Amino acid analysis of the acid hydrolyzate gave allo-Ile and an unknown imino acid. ^1H-^1H COSY, HMQC and HMBC data showed the presence of Chloro Hpla (3-chloro-4-hydroxyphenyllactic acid), allo-Ile, Choi sulfate (2-carboxy-6-hydroxyoctahydroindole sulfate) and agmatin. Interpretation of HMBC spectra revealed the structure of 1. The stereochemistry of allo-Ile was determined to be D-form by the chiral GC analysis. The relative stereochemistry of Choi sulfate was elucidated by NOESY spectra. The structures of 2 and 3 only differ from 1 in the substitution of the benzene ring of Chloro Hpla. The IC_<50> of 1, 2 and 3 against trypsin was 0.55, 0.6 and 3.93μg/mL, respectively. Molecular formula of 5 was decided as C_<34>H_<59>N_9O_<14>S_3 from HRFABMS and NMR data. ^1H-^1H COSY, HOHAHA, HMQC and HMBC data showed the presence of Hphpa trisulfate (1-(4-hydroxy-3-hydroxymethyl)phenyl-1 -hyciroxy-2-propylamine 4',3', 1-tri- O-sulfate), N-MeArg and NmgArg (Na^α-methyl-N^ω-geranylarginine). Interpretation of HMBC and NOESY spectra revealed the structure of 5. The structures of 4 and 6 were elucidated as analogs of 5, having the different side chains of N-MeArg. These compounds showed weak cytotoxity against P388 leukemia cells.
