Substituent Effects in the Free Radical Reactions of Silybin: Radiation-induced Oxidation of the Flavonoid at Neutral pH

Author： György I. Antus S. Blázovics A. Földiák G.

Publisher： Informa Healthcare

ISSN： 1362-3095

Source： International Journal of Radiation Biology, Vol.61, Iss.5, 1992-05, pp. : 603-609

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Abstract

Silybin dihemisuccinate sodium salt, a flavonoid used in human therapy of liver dysfunction, has an inhibitory effect in vivo on radiation-induced deactivation of enzymes and peroxidation of membrane lipids in rat liver microsomes. The reactivity of silybin and its phenolic OH groups towards free radicals (&.macr;OH, N^.₃, (SCN)^{. −}₂, Cl₃ CO^.₂) in aqueous solution was studied by pulse radiolysis. Absorption spectra for the phenoxyl-type radicals were assigned using structurally similar models. The one-electron reduction potential for silybin at pH 7 (E⁰₇ = 0·76 V), determined using the p-methoxy-phenoxyl/phenolate redox couple as reference standard (E⁰₇ = 0·72 V, Lind et al. 1990), is related to the 3′-methoxy-4′-OH structure, the exclusive target for one-electron oxidation at pH7, while the 7-OH and 5-OH groups are prevented from oxidation by 4-keto substitution and intramolecular H-bonding, respectively. The free radical reactivity of silybin compares favourably with poly-OH-substituted flavonoids; however, the latter compounds have been reported to generate potentially toxic oxygen species at a biologically relevant pH.