Detection and identification of metabolites of microcystins formed in vivo in mouse and rat livers

The hepatic metabolism of microcystins (MCs), potent cyclic peptide hepatotoxins produced by cyanobacteria, was studied by ip injection in mice and rats. An immunoaffinity purification method using an anti-MC-LR monoclonal antibody showed a remarkable effect on the removal of contaminants in the hepatic cytosol and enabled us to analyze MCs and their metabolites by HPLC and Frit-FAB LC/MS. At 3, 6, and 24 h post-injection of MC-RR, a small percent of the applied dose was detected in all of the mouse livers together with several metabolites. Among them, GSH and Cys conjugates of MC-RR were identified at 3 and 24 h, respectively, by comparison with the chemically prepared standards, indicating that the thiols of GSH and Cys nucleophilically bound to the Mdha moiety of MCs. Another metabolite was presumed to be formed by both epoxidation followed by hydrolysis and sulfate conjugation in the Adda moiety and GSH conjugation in the Mdha moiety. In rat livers, MC-LR showed almost the same behavior as that of MC-RR in mouse livers. These results suggest that the conjugation of GSH with MCs may play a role in the metabolic pathway leading to detoxification of MCs.