Comparative toxicity of four microcystins of different hydrophobicities tothe protozoan, Tetrahymena pyriformis

Microcystins (MC) are a group of over 60 cyclic heptapeptide hepatotoxins p roduced by cyanobacteria. The 1-octanol/water partition coefficients (log P ) of MC-LR, -LY, -LW and -LF have been estimated by HPLC to be 2.16, 2.92, 3.46 and 3.56, respectively. Their in vivo toxicities to Tetrahymena pyrifo rmis was also investigated. Twenty-four hour LC50 values followed the order MC-LR > -LY > -LW approximate to -LF. The LC50 values of MC-LR and -LY wer e significantly reduced in the presence of 1% (v/v) dimethylsulphoxide, alt hough no significant effect occurred with MC-LW or -LF. Tetrahymena pyrifor mis respiration rates were inhibited by MC-LR in both a time- and dose-depe ndent manner. Increasing log P of the MC used caused a significantly greate r inhibition of respiration. Population growth rate and maximum culture den sity were inhibited by all MC variants in proportion to log P. Positive cor relations between all toxicological endpoints and log P occurred, with the most hydrophobic toxin, MC-LF, being 1.4 to 3.5 times more toxic than MC-LR . MC-LW had a similar toxicity to MC-LF, while MC-LY toxicity was intermedi ate between that of NIC-LR and -LF. Implications of this positive relations hip between in vivo toxicity and hydrophobicity for the toxicity of MC res aquatic organisms, and the potential for using log P as a descriptor in a q uantitative structure activity relationship for MC, are discussed.