Activity of a Rat‐Liver Glycoprotein: Sialyltransferase Utilizing Desialyzed Prothrombin as an Acceptor in Normal and Hypothrombinemic Animals

A glycoprotein : sialyltransferase which catalyzed the addition of N ‐acetyl[ 14 C]neuraminic acid to desialyzed glycoprotein precursors has been partially purified and characterized from rat liver, using desialyzed human prothrombin as an exogenous acceptor. The enzyme was extracted with 0.1% Triton X‐100 and was purified 31‐fold by gel chromatography. The sialyltransferase with desialyzed human prothrombin as acceptor was most active at 37 ° C, pH 7.0, 15 mM Mg 2+ , and its activity toward other asialoglycoproteins was similar with respect to optimum conditions. The enzyme was inhibited by 10 mM HgCl 2 , PbCl 2 and ZnCl 2 . Kinetic data were treated according to the model of Cleland for a bisubstrate reaction and the various constants were K b = 50 μM, K b = 31 μM, K ia = 10 μM, K ib = 5.8 μM, and V 1 = 100 nmol × h −1 × mg protein −1 , where a = CMP‐ N ‐acetyl[ 14 C]neuraminic acid and b = prothrombin. The activities with various desialyzed acceptors could not be separated by gel chromatography and may be due to a single sialyltransferase. Rats made hypothrombinemic by maintenance on a vitamin‐K‐deficient diet had normal or slightly elevated amounts of sialyltransferase activity using either desialyzed prothrombin or fetuin as glycoprotein acceptors for N ‐acetylneuraminic acid. Rat liver obtained from vitamin‐K‐deficient rats also had normal amounts of other glycosyltransferases, including galactosyl, glucosyl and N ‐acetylglucosaminyltransferase, as measured with endogenous or exogenous acceptors.