Porcine A blood group-specific N-acetylgalactosaminyltransferase

Porcine A blood group-specific N-acetylgalactosaminyl-transferase required either Mn2+, Cd2+, or Zn2+ for activity and 2'-O-alpha-fucosylgalactosides as acceptor substrates. The presence of detergent stabilizes the enzyme but is not essential for catalysis. To obtain information about the kinetic mechanism of the transferase reaction, initial rate parameters have been determined using 2'-fucosyllactose or A--mucin as acceptors, and Mn2+ or Cd2+ as cosubstrates. 2'-Fucosyllactose is a competitive inhibitor with respect to A--mucin and a noncompetitive inhibitor with respect to UDP-N-acetylgalactosamine. UDP inhibits noncompetively with respect to acceptor; thus UDP-N-acetylgalactosamine or acceptor can bind to the transferase via an equilibrium random pathway. The transferase converts human O blood type erythrocytes of A blood types. After exhaustive glycosylation, 3 X 10(6) N-acetylgalactosaminyl residues were incorporated per cell. Gel electrophoretic analysis of the labeled erythrocyte membranes indicates that glycoproteins with apparents molecular weights from 30,000 to 100,000 have been glycosylated; glycolipids account for only 15% of the labeled material, although pure H-glycolipid is a good acceptor. The transferase, with its strict acceptor specificity, can thus be used as a tool to study the biosynthesis and function of glycolipids and glycoproteins.