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

Treatment of hepatic microsomes with Fe(3+)/ascorbate activates UDP-glucuronyltransferase (UGT), a phenomenon totally prevented and reversed by reducing agents. At microM 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 Cu(2+)/ascorbate system will lead to both UGT oxidative activation and Cu(2+)-binding induced inhibition, simultaneously. We studied the effects of Cu(2+) 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 Cu(2+) (25nM), this ion did not modify UGT activity. In the presence of ascorbate, however, UGT activity was increased. At higher copper concentrations (10 and 50microM), this ion led to UGT activity inhibition. In the presence of ascorbate, 10microM Cu(2+) activated UGT at short incubation periods but inhibited this enzyme at longer incubation times; 50microM Cu(2+) 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 Cu(2+)-induced oxidation of UGT leads to the activation of the enzyme, while Cu(2+)-binding leads to its inhibition. We discuss physiological and pathological implications of these findings.