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 he patotoxicity and show weaker tumor-promoting activity than their paren t toxins. The present study was undertaken to examine stability of mic rocystin during the analysis and purification and under photolysis con ditions in connection with the detoxification. Microcystin LR was very stable because of limited decomposition and isomerizata onto its geom etrical isomer during analysis and purification. While microcystins de composed very limitedly by exposure with sunlight alone, the addition of pigments extracted from cyanobacteria accelerated their decompositi ons. Additionally, the isomerization of microcystins to 6(Z)-Adda micr ocystins and vice versa occurred under the same conditions. The decomp osition and isomerization rates depended upon pigment concentration. T he converted microcystin LR from its reactive geometrical isomer showe d essentially the same toxicity as that of intact microcystin LR. Phot olysis in the presence of pigment may be regarded as one of the detoxi fication processes for microcystins under field conditions.