ACTIVATION AND STABILIZATION OF UDP-GLUCURONOSYLTRANSFERASE BY LYSOPHOSPHATIDYLCHOLINE

Interactions between purified UDP-glucuronyltransferase from 3-methylcholanthrene-treated rat liver microsomes (named GT-1) and lysophosphatidylcholine, which is essential for expression of GT-1 activity, were examined. Phospholipid-free GT-1, which could not express its full activity [Yokota et al. (1988) J. Biochem. 104, 531-536], was activated fully by addition of lysophosphatidylcholine (0.04 mM final concentration) into the assay medium. Lysophosphatidylcholine also protected GT-1 effectively against heat inactivation. Palmitoyllysophosphatidylcholine and stearoyllysophosphatidylcholine were most successful for the activation and stabilization of GT-1. On treatment of GT-1 with carboxy-peptidase Y, the transferase was inactivated immediately, but the treatment in the presence of lysophosphatidylcholine affected the activity only a little. Lysophosphatidylcholine was also found to protect GT-1 against cleavage by carboxypeptidase Y. On treatment of GT-1 with trypsin or aminopeptidase T, the activity was lost and GT-1 protein could be digested even when lysophosphatidylcholine was present. It is suggested that UDP-glucuronyltransferase forms an active and stable conformation, in which the carboxy-terminal region is protected against protease, with lysophosphatidylcholine.