Microcystin-like toxins in different freshwater species of Oscillatoria

In January and September of 1989 and March 1990 blooms of Oscillatoria rubescens, Oscillatoria tenuis and Oscillatoria mougetii were found in Lake Simbirizzi and Lake Flumendosa in Sardinia, and in Lake San Puoto in the Lazio region of Italy. By using different extraction methods and HPLC analysis, two microcystin-like toxins (RR-like and YR-like), similar to some of the toxic compounds produced by the Cyanophycea Microcystis aeruginosa, were detected in these blooms.

Hepatocellular uptake of 3H-dihydromicrocystin-LR, a cyclic peptide toxin

The cellular uptake of microcystin-LR, a cyclic heptapeptide hepatotoxin from the cyanobacterium Microcystis aeruginosa, was studied by means of a radiolabelled derivative of the toxin. 3H-dihydromicrocystin-LR. The uptake of 3H-dihydromicrocystin-LR was shown to be specific for freshly isolated rat hepatocytes whereas the uptake in the human hepatocarcinoma cell line Hep G2 as well as the mouse fibroblast cell line NIH-3T3, and the human neuroblastoma cell line SH-SY5Y, was negligible. By means of a surface barostat technique it was shown that the membrane penetrating capacity (surface activity) of microcystin-LR was low, indicating that the toxin requires an active uptake mechanism. The hepatocellular uptake of microcystin-LR could be inhibited in the presence of bile acid transport inhibitors such as antamanide (5 microM), sulfobromophthalein (50 microM) and rifampicin (30 microM). The uptake was also reduced in a concentration dependent manner when the hepatocytes were incubated in the presence the bile salts cholate and taurocholate. A complete inhibition of the hepatocellular uptake was achieved by 100 microM of either bile salt. The overall results indicate that the uptake of microcystin-LR is through the multispecific transport system for bile acids. This mechanism of cell entry would explain the previously observed cell specificity and organotropism of microcystin-LR.

Cyanobacterial microcystin-LR is a potent and specific inhibitor of protein phosphatases 1 and 2A from both mammals and higher plants

The cyclic heptapeptide, microcystin-LR, inhibits protein phosphatases 1 (PP1) and 2A (PP2A) with Ki values below 0.1 nM. Protein phosphatase 2B is inhibited 1000-fold less potently, while six other phosphatases and eight protein kinases tested are unaffected. These results are strikingly similar to those obtained with the tumour promoter okadaic acid. We establish that okadaic acid prevents the binding of microcystin-LR to PP2A, and that protein inhibitors 1 and 2 prevent the binding of microcystin-LR to PP1. We discuss the possibility that inhibition of PP1 and PP2A accounts for the extreme toxicity of microcystin-LR, and indicate its potential value in the detection and analysis of protein kinases and phosphatases.

Structure of a hepatotoxic pentapeptide from the cyanobacterium Nodularia spumigena

The structure of a hepatotoxic peptide from the cyanobacterium Nodularia spumigena was determined using 1D and 2D proton nuclear magnetic resonance spectroscopy and fast atom bombardment mass spectrometry. The toxin was a cyclic pentapeptide (mol. wt 824.5) with the structure cyclo-(beta-methylisoAsp-Arg-Adda-isoGlu-N-methylde hydrobutyric acid) (Adda: 3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid).

Synthesis, organotropism and hepatocellular uptake of two tritium-labeled epimers of dihydromicrocystin-LR, a cyanobacterial peptide toxin analog

Two tritium-labeled epimers of dihydromicrocystin-LR, a derivative of the cyanobacterial peptide hepatotoxin microcystin-LR, were synthesized by reduction with sodium boro[3H]hydride and purified with reversed-phase liquid chromatography. The epimers were hepatotoxic in mice; the i.p. LD50 was 120-135 micrograms/kg. They were concentrated in the liver and to some extent in the intestine and the kidney after an i.v. injection. Freshly isolated rat hepatocytes showed a rapid uptake of both epimers. The cellular uptake of the epimers was almost complete within 5 min at concentrations 1 microM (0.5 microM dihydromicrocystin-LR + 0.5 microM microcystin-LR) and 4 microM (0.5 microM + 3.5 microM). The uptake of the earlier eluting epimer was about three times higher than that of the later eluting epimer.

Structure and toxicity of a peptide hepatotoxin from the cyanobacterium Oscillatoria agardhii

A peptide hepatotoxin was isolated by reversed phase liquid chromatography from the cyanobacterium Oscillatoria agardhii and characterized structurally and toxicologically. Amino acid analyses, proton nuclear magnetic resonance and fast atom bombardment mass spectrometry showed that the toxin is a cyclic heptapeptide (mol. wt 1023.5) with the structure cyclo-(Ala-Arg-Asp-Arg-Adda-Glu-N-methyldehydroAla) (Adda: 3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid). In mice the toxic effects were restricted mainly to the liver where the toxin induced massive hemorrhages and a disruption of the lobular and sinusoidal structure. The i.p. LD50 of the toxin was 250 micrograms/kg. The structural and toxic properties of this peptide are very close to those of microcystins, cyclic peptide toxins produced by the cyanobacterium Microcystis aeruginosa.