Glycosyltransferases of the human cervical epithelium. II. Characterization of a CMP-N-acetylneuraminate: galactosyl-glycoprotein sialyltransferase.

GMP-N-Acetylneuraminate: galactosyl-glycoprotein sialytransferase (CMP-N-acetylneuraminate: D-galactosyl-glycoprotein N-acetylneuraminyltransferase, EC 2.4.99.1) activity was identified in the human cervical epithelium. The enzyme has a pH optimum of 6.0, a temperature optimum of 28 degrees C, and demonstrates a partial requirement for Triton X-100. Michaelis constants for asialofetuin and CMP-N-acetyl[14C]neuraminic acid are 0.64 . 10(-5) M (expressed as the concentration of terminal galactose residues) and 2.05 . 10(-5) M, respectively. Sialytransferase demonstrated minimal affinity for the low molecular weight acceptors tested, and may have a requirement for a glycoprotein acceptor having a terminal N-acetyllactosamine (Gal beta (1 leads to 4)GlcNAc) type structure. Cytidine nucleotides are potent inhibitors of the sialyltransferase reaction; CMP acts as a competitive inhibitor.