Caffeic acid, chlorogenic acid, and dihydrocaffeic acid metabolism: Glutathione conjugate formation

The antioxidant properties of the dietary dihydroxycinnamic acids [caffeic (CA), dihydrocaffeic (DHCA), and chlorogenic (CGA) acids] have been well st udied but little is known about their metabolism. In this article, evidence is presented showing that CA, DHCA, and CGA form quinoids and hydroxylated products when oxidized by peroxidase/H2O2 or tyrosinase/O-2. Mass spectrom etry analyses of the metabolites formed with peroxidase/H2O2/glutathione (G SH) revealed that mono- and bi-glutathione conjugates were formed for all t hree compounds except CGA, which formed a bi-glutathione conjugate only whe n GSH was present. In contrast, the metabolism of the dihydroxycinnamic aci ds by tyrosinase/O-2/ GSH resulted in the formation of only mono-glutathion e conjugates. In the absence of GSH, hydroxylated products and p-quinones o f CA or CGA were formed by peroxidase/H2O2. DHCA formed a hydroxylated addu ct (even though GSH was present), as well as the corresponding p-quinone an d dihydroesculetin, an intramolecular cyclization product. NADPH also suppo rted rat liver microsomal-catalyzed CA-, CGA-, and DHCA-glutathione conjuga te formation, which was prevented by benzylimidazole, a cytochrome P450 inh ibitor. Furthermore, the cytotoxicity of CA, CGA, and DHCA toward isolated rat hepatocytes was markedly enhanced by hydrogen peroxide or cumene hydrop eroxide-supported cytochrome P450 and was prevented by benzylimidazole. Cyt otoxicity was also markedly enhanced by dicumarol, an NADPH/oxidoreductase inhibitor. These results suggest that dihydroxycinnamic acids were metaboli cally activated by P450 peroxidase activity to form cytotoxic quinoid metab olites.