Oxidative conjugation of chlorogenic acid with glutathione: Structural characterization of addition products and a new nitrite-promoted pathway

Chlorogenic acid (1), a cancer chemopreventive agent widely found in fruits, tea and coffee, undergoes efficient conjugation with glutathione (GSH), in the presence of horseradish peroxidase/H2O2 or tyrosinase at pH 7.4, to yield three main adducts that have been isolated and identified as 2-S-glutathionyichlorogenic acid (3), 2,5-di-S-glutathionylchlorogenic acid (4) and 2,5,6-tri-S-glutathionychlorogenic to ro genic acid (5) by extensive NMR analysis. The same pattern of products could be obtained by reaction of with GSH in the presence of nitrite ions in acetate buffer at pH 4. Mechanistic experiments suggested that oxidative conjugation reactions proceed by sequential nucleophilic attack of GSH on ortho-quinone intermediates. Overall, these results provide the first complete spectral characterization of the adducts generated by biomimetic oxidation of I in the presence of GSH, and disclose a new possible nitrite-mediated conjugation pathway of 1 with GSH at acidic pH of physiological relevance. (C) 2003 Elsevier Ltd. All rights reserved.