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Cu2+-catalyzed oxidative degradation of thyroglobulin
Authors:Hyun-Jeong Lee  Dai-Eun Sok
Institution:College of Pharmacy, Chungnam National University, Yuseong-Ku, Taejon, 305-764, Korea
Abstract:Thyroglobulin (Tg) was subjected to metal-catalyzed oxidation, and the oxidative degradation was analyzed by SDS-polyacrylamide gel electrophoresis under reducing conditions. In contrast to no effect of hydrogen peroxide (H2O2) alone on the Tg degradation, the inclusion of Cu2+ (30 μM), in combination with 2 mM H2O2, caused a remarkable degradation of Tg, time- and concentration-dependent. The action of Cu2+ was not mimicked by Fe2+, suggesting that Tg may interact selectively with Cu2+. A similar degradation of Tg was also observed with Cu2+corbate system, and the concentration of Cu2+ (5–10 μM), in combination with ascorbate, required for the effective degradation was smaller than that of Cu2+ (10–30 μM) in combination with H2O2. In support of involvement of H2O2 in the Cu2+ corbate action, catalase expressed a complete protection. However, hydroxyl radical scavengers such as dimethylsulfoxide or mannitol failed to prevent the oxidation of Tg whereas phenolic compounds, which can interact with Cu2+, diminished the oxidative degradation, presumably consistent with the mechanism for Cu2+-catalyzed oxidation of protein. Moreover, the amount of carbonyl groups in Tg was increased as the concentration (3–100 μM) of Cu2+ was enhanced, while the formation of acid-soluble peptides was not remarkable in the presence of Cu2+ up to 200 μM. In further studies, Tg pretreated with heat or trichloroacetic acid seemed to be somewhat resistant to Cu2+-catalyzed oxidation, implying a possible involvement of protein conformation in the susceptibility to the oxidation. Based on these observations, it is proposed that Tg could be degraded non-enzymatically by Cu2+-catalyzed oxidation.
Keywords:Thyroglobulin  copper ions  hydrogen peroxide  ascorbic acid  oxidative degradation
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