Effect of l-fucose and d-glucose concentration on l-fucoprotein metabolism in human Hep G2 cells and changes in fucosyltransferase and α- l-fucosidase activity in liver of diabetic rats

Abstract

l-Fucose is a monosaccharide that is present at low concentrations in serum and is a normal constituent of glycoproteins. In some pathological conditions, such as cancer, rheumatoid arthritis, and diabetes, there is an abnormal fucosylation of acute phase serum proteins. Because most serum proteins are produced in the liver, we have examined l-fucose accumulation, metabolism, and secretion of l-fucose-containing proteins in human Hep G2 liver cells. Accumulation of l-fucose by Hep G2 cells approached 3.5 nmol/mg protein after a 48 h incubation. This accumulation appears similar to accumulation in other cells, which we have shown occurs via a specific transport protein. Exogenous l-fucose was incorporated into protein in both O- and N-linked glycosidic linkages. After a 48 h incubation, 61% of the accumulated l-fucose was incorporated into protein and secreted into the medium, whereas 39% of the l-fucose remaining in the cells was incorporated into integral membrane proteins. Utilizing reverse-phase high-performance liquid chromatographic separation of l-[5,6- 3H]fucose-containing proteins and detection by scintillation counting, we determined that two major fucoproteins and numerous minor fucoproteins were produced and secreted by normal Hep G2 cells. This elution profile was unchanged when glucose-conditioned cells were examined. By size-separating secreted proteins by nondenaturing HPLC we determined that the size of the two major fucoproteins were ≈60 and ≈100 kDa. In these studies we also examined the effect of diabetes on hepatic fucosyltransferase and serum α- l-fucosidase activity and found that the activity of these enzymes is increased by 40 and 100%, respectively in diabetic rats.