Regulation of microsomal enzymes by phospholipids: III. The role on -SH groups in the regulation of microsomal UDP-glucuronyltransferase

Abstract

The effect of -SH group reagents on the activity of UDPglucuronyltransferase from beef liver microsomes has been studied. Contrary to data reported in the literature, the activity of this enzyme is increased several-fold by treatment of beef liver microsomes with a variety of organic mercurials, and to a lesser extent by N-ethylmaleimide. Although the extent of activation of UDPglucuronyltransferase by organic mercurials depends on the nature of the mercurial, all organic mercurials seem to react with the same type of -SH group (Type 2-SH), a concentration of 0.5 to 0.8 m m being required for maximum activation. The reactivity of the Type 2-SH can be altered by prior treatment of microsomes with small amounts of detergent so that activation occurs at 0.1 m m mercurial. Activation by the organic mercurials can be reversed completely by dithiothreitol. N-ethylmaleimide reacts with a type of -SH group (Type 1) which is different from the one titrated by the organic mercurials. Organic mercurials, but not N-ethylmaleimide, render the enzyme unstable at 37 ° in the absence of substrates. Relatively high concentrations of organic mercurials react with a third type of -SH group (Type 3) to produce a deactivated form of UDPglycuronyltransferase. The reactivity of Type 3-SH is different with different organic mercurials, but with each one tested activity was restored by treatment with dithiothreitol. N-ethylmaleimide does not react with Type 3-SH group.