Mechanism of covalent adduct formation of aucubin to proteins

The iridoid glucoside aucubin can irreversibly bind to proteins through the formation of its aglycone. In view of a possible involvement of these prot ein adducts in the toxicity of aucubin, we investigated the mechanism of bi nding of aucubin to proteins. [H-3]aucubin in itself did not result in bind ing to protein whereas it covalently bound to rat serum albumin as a functi on of exposure time and dose in the presence of beta-glucosidase. The rate and extent of protein binding were significantly increased in the presence of the imine-trapping agent sodium cyanide. Oral administration of [H-3]auc ubin to rats showed that the total radioactivity in plasma remained at a si milar level for up to 6 h once peak level was reached, suggesting that a co nsiderable amount of radioactivity might be covalently associated with plas ma proteins. The levels of radioactivity in the liver and kidney after oral dosing were higher than those after i.v. dosing. These results indicate th at the open-chain aglycone of aucubin can form an imine bond with a nucleop hilic site of the protein and these irreversible bindings may partially con tribute to its biological and toxic effects. (C) 2000 Elsevier Science Irel and Ltd. All rights reserved.