EFFECT OF INHIBITORS OF N-LINKED OLIGOSACCHARIDE PROCESSING ON THE HIGH-AFFINITY TRANSPORT OF D-ASPARTATE BY C6 GLIOMA-CELLS

The effect of several inhibitors of oligosaccharide-processing on the high-affinity transport of D-aspartate was investigated in C6 glioma cells. Swainsonine, an inhibitor of mannosidase II, had no effect on the uptake of the amino acid. Castanospermine (100-mu-g/ml) and 1-deoxynojirimycin (1 mM), inhibitors of glucosidases, and 1-deoxymannojirimycin (1 mM), an inhibitor of mannosidase I, reduced the rate of transport by 35-45%. All inhibitory compounds decreased the V(max) for transport without affecting the K(m) which suggests that inhibition of oligosaccharide trimming reduces the number of competent transporters on the surface of the plasma membrane. Returning the cells to a drugfree medium for 24 h, following a 24 h exposure, resulted in complete recovery of uptake. Treatment of cells with neuraminidase from V. cholerae also decreased the V(max) for transport by about 20%. The results suggest that: (i) a partial complex carbohydrate chain on the high-affinity transporter for acidic amino acid transmitters is sufficient for activity and (ii) sialic acid residues may be necessary for normal operation of the transporter.