Copper modifies liver microsomal UDP-glucuronyltransferase activity through different and opposite mechanisms

Treatment of hepatic microsomes with Fe3+/ascorbate activates UDP-glucuronyltransferase (UGT), a phenomenon totally prevented and reversed by reducing agents. At μM concentrations, iron and copper ions catalyze the formation of ROS through Fenton and/or Haber–Weiss reactions. Unlike iron ions, indiscriminate binding of copper ions to thiol groups of proteins different from the specialized copper-binding proteins may occur. Thus, we hypothesize that incubation of hepatic microsomes with the Cu2+/ascorbate system will lead to both UGT oxidative activation and Cu2+-binding induced inhibition, simultaneously. We studied the effects of Cu2+ alone and in the presence of ascorbate on rat liver microsomal UGT activity. Our results show that the effects of both copper alone and in the presence of ascorbate were copper ion concentration- and incubation time-dependent. At very low Cu2+ (25 nM), this ion did not modify UGT activity. In the presence of ascorbate, however, UGT activity was increased. At higher copper concentrations (10 and 50 μM), this ion led to UGT activity inhibition. In the presence of ascorbate, 10 μM Cu2+ activated UGT at short incubation periods but inhibited this enzyme at longer incubation times; 50 μM Cu2+ only inhibited UGT activity. Thiol reducing agent 2,4-dithiothreitol prevented and reversed UGT activation while EDTA prevented both, UGT activation and inhibition. Our results are consistent with a model in which Cu2+-induced oxidation of UGT leads to the activation of the enzyme, while Cu2+-binding leads to its inhibition. We discuss physiological and pathological implications of these findings.