Covalent glutathione conjugation to cyanobacterial hepatotoxin microcystin LR by F344 rat cytosolic and microsomal glutathione S-transferases

Most investigators report that microcystins (MCs) bound covalently to SH compounds, such as L-cysteine and reduced glutathione (GSH). However, their studies were based on a high pH condition. In the present study, we investigated the reaction between microcystin LR (MCLR) and GSH in various pH conditions. As a result, we found that no MCLR conjugated with GSH in these conditions, and MCLR mixed with GSH showed different peaks of retention time compared with intact MCLR on the high performance liquid chromatography-mass spectrometry (LC-MS) chromatograms. Furthermore, we found the GSH conjugate of MCLR was detected in the glutathione S-transferase (GST) assay using F344 rat liver cytosol and microsomes. This indicates that the covalent GSH conjugation was caused only by an enzymatic activity. We conclude, therefore, that the reaction is caused by enzymatic action and is not due to the Michael reaction.