Occurrence of two fucosyltransferase activities at the outer surface of rat lymphocytes

To demonstrate the existence of ectofucosyltransferase activities on the outer surface of rat lymphocytes, we measured fucosyltransferase activities on whole cells using procedures enabling us to exclude the possibility of misleading results due to precursor hydrolysis and intracellular utilization of the free fucose, and to take into account the contamination by intracellular enzymes freed by the small percentage of broken cells. The described ectofucosyltransferases are able to catalyze the transfer of fucosyl residues from GDP-fucose to the endogenous membrane acceptors but the transfer activity towards exogenous acceptors is restricted to low molecular weight compounds. Use of galactose and di-N-acetylchitobiose as exogenous acceptors and concomitant study of the specific inhibition by N-ethylmaleimide enabled us to detect both types of ectofucosyltransferases: a GDP-fucose: galactoside ectofucosyltransferase and a GDP-fucose: N-acetylglucosaminide ectofucosyltransferase.