Cell Surface Glycolipid and Glycoprotein Glycosyltransferases of Normal and Transformed Cells

Normal and transformed mouse fibroblasts catalyze transfer of sialic acid, galactose, N-acetylgalactosamine, N-acetylglucosamine, glucose, and mannose from nucleotide sugar donors to glycolipids and glycoproteins. The enzyme activity is associated with intact cells. Kinetic parameters and optimal ion concentrations have been determined for the glycosyltransferase activities detected when whole cells are incubated with nucleotide sugars. Homogenization of cells either decreased or did not change the activity observed. Adding unlabeled sugars did not affect incorporations. Trypsin caused a 50% inhibition of observable activity only when present in concentrations which also caused significant cell destruction. Swiss SV40 transformed cells showed decreased sialic acid-transferring ability compared to the parent cell line. Swiss Py3T3 and SV3T3 cells had reduced ability to catalyze transfer of N-acetylgalactosamine to glycolipids compared with the normal cell line. Since these alterations have also been reported in homogenates of these cells, and in view of the large number of glycosyltransferase activities observed, the in vitro whole cell reactions probably detect the normal cellular systems which are in the process of synthesizing glycoproteins and glycolipids. Evidence supporting this conclusion was obtained from experiments in which glycolipid products synthesized in cells incubated in the presence of 3Hgalactose and UDP-14Cgalactose are compared.