Nuclear membrane-bound UDPglucuronosyltransferase of rat liver

Some properties of rat liver nuclear membrane-bound UDPglucuronosyltransferase were compared with those of the endoplasmic reticulum bound enzyme. The activity of nuclear membrane-bound UDPglucuronosyltransferase was stimulated only about 1.5-fold by Lubrol WX. Under the same conditions microsomal UDPglucuronosyltransferase was, as usual, highly activated (up to 10-fold), when 4-nitrophenol was the acceptor of glucuronic acid. Specific activities of the detergent-activated enzyme were similar in microsomal and nuclear membrane preparations, when the following aglycone substrates were used: 4-methylumbelliferone, 4-nitrophenol, 1-naphthol, phenolphthalein, and testosterone. Apparent Km values for UDP-glucuronic acid ranged between 0.15-0.25 mM for glucuronidation of 4-nitrophenol and 1-naphthol, by either Lubrol WX activated or non-activated, nuclear membrane-bound UDPglucuronosyltransferase. These values were comparable to those found for detergent activated microsomal enzyme. The results show a similarity in behavior of detergent-activated UDPglucuronosyltransferase regardless of subcellular membrane source and, therefore, suggest the association of the same glucuronosyltransferase with nuclear membrane and endoplasmic reticulum. A possible significance of the presence of high activity of this enzyme in nuclear membrane is discussed.