Novel oxidations of (+)-catechin by horseradish peroxidase and laccase

Abstract

Horseradish peroxidase (HRP; EC 1.11.1.7) catalyzed the H2O2-dependent oxidative coupling of (+)-catechin 1 to form three different biphenyl C-C dimers 2-4, whereas Rhus vernicifera laccase catalyzed the formation of two new catechin-hydroquinone adducts 5 and 6. Spectroscopic evidence showed that HRP dimers were linked through position 8 of the A-ring of one catechin moiety to C-5′ of ring B in 2 and 4 and to C-2 of ring C in 3. The unusual catechin dicarboxylic acid dimer 4 was obtained by ortho cleavage of the E-ring. Hydroquinone served as both a shuttle oxidant and a reactant by coupling at C-2′ and C-5′ of the catechin B-ring during laccase oxidations. HRP and laccase oxidation products were compared to D,L-α-tocopherol and (+)-catechin for their abilities to inhibit iron-induced lipid peroxidation in rat brain homogenates and Fe3+-ADP/NADPH in rat liver microsomes, as measured by the intensity of thiobarbituric acid reactive substance. All metabolites exhibited antilipid peroxidation with IC50 values ∼2-8 times higher than those of standard compounds. Characteristic reaction products may prove to be novel markers for (+)-catechin antioxidant reactions in living systems.