INHIBITION OF GAMMA-IRRADIATION-INDUCED OXIDATION OF CHLOROGENATE BY 1,3-DICAFFEOYLQUINATE IN XANTHIUM-OCCIDENTALE

The endogeneous inhibition of the radiation-induced conversion of chlo rogenate into its o-quinone in Xanthium occidentale leaves has been st udied. In addition to chlorogenate, this plant contains 1,3-dicaffeoyl quinate (cynarin). On exposure to gamma-radiation, the concentrations of both substances in aqueous solution were reduced by an indirect mec hanism, i.e. a distinct number of molecules were transformed by a give n dose of radiation irrespective of concentration. In the presence of cynarin, chlorogenate was protected from radiation-induced oxidation, and the degree of protection increased exponentially with increased cy narin amount; the protection was almost complete at a concentration ex ceeding ca five times that of chlorogenate. Similarly, chlorogenate re duced the conversion of cynarin, but this protection proceeded only li nearly with increased concentration. The high degree of protection aff orded by cynarin may be ascribed to the presence in the molecule of tw o oxidizable (oxidant-scavenging) caffeoyl groups, as compared with ch lorogenate which has only one. Since the physiological concentrations of cynarin and chlorogenate were estimated to be 8.7 and 2.1 mM, respe ctively, it is very probable that in X. occidentale cells only a sligh t conversion of chlorogenate can occur as a result of irradiation.