STABILITY OF MICROCYSTINS FROM CYANOBACTERIA - EFFECT OF LIGHT ON DECOMPOSITION AND ISOMERIZATION

Microcystins are potent hepatotoxins produced by cyanobacteria. Their geometrical isomers [6(Z)-Adda microcystin] do not essentially show hepatotoxicity and show weaker tumor-promoting activity than their parent toxins. The present study was undertaken to examine stability of microcystin during the analysis and purification and under photolysis conditions in connection with the detoxification. Microcystin LR was very stable because of limited decomposition and isomerizata onto its geometrical isomer during analysis and purification. While microcystins decomposed very limitedly by exposure with sunlight alone, the addition of pigments extracted from cyanobacteria accelerated their decompositions. Additionally, the isomerization of microcystins to 6(Z)-Adda microcystins and vice versa occurred under the same conditions. The decomposition and isomerization rates depended upon pigment concentration. The converted microcystin LR from its reactive geometrical isomer showed essentially the same toxicity as that of intact microcystin LR. Photolysis in the presence of pigment may be regarded as one of the detoxification processes for microcystins under field conditions.