Identification of an enzymatically formed glutathione conjugate of the cyanobacterial hepatotoxin microcystin-LR: the first step of detoxication

Cyanobacterial toxins have adverse effects on mammals, birds and fish and a re being increasingly recognised as a potent stress factor and health hazar d factor in aquatic ecosystems. Microcystins, cyclic heptapeptides and a ma in group of the cyanotoxins are mainly retained within the producer cells d uring cyanobacterial bloom development. However, these toxins are released into the surrounding medium by senescence and lysis of the blooms. Any toxi n present could then come into contact with a wide range of aquatic organis ms including phytoplankton grazers, invertebrates, fish and aquatic plants. Recent studies showed the conversion of microcystin in animal liver to a m ore polar compound in correlation with a depletion of the glutathione pool of the cell. The present study shows the existence of a microcystin-LR glut athione conjugate formed enzymatically via soluble glutathione S-transferas e in various aquatic organisms ranging from plants (Ceratophyllum demersum) , invertebrates (Dreissena polymorpha, Daphnia magna) up to fish eggs and f ish (Danio rerio). The main derived conjugate was characterized by matrix-a ssisted laser desorption/ionization time-of-flight mass spectrometry yieldi ng a mass of m/z 1302, which is equivalent to the mass assumed for a glutat hione microcystin-la conjugate. This conjugate appears to be the first step in the detoxication of a cyanobacterial toxin in aquatic organisms. (C) 19 98 Elsevier Science B.V. All sights reserved.