N-acetyl-D-galactosaminyltransferase in human serum and erythrocyte membranes.

This study demonstrates the presence of an N-acetyl-D-galactosaminyltransferase in human serum and in erythrocyte membranes. This enzyme catalyzes the transfer of N-acetyl-D-galactosamine from UDP-N-acetyl-D-galactosamine to a mucin receptor and 2'-fucosyllactose that have blood group H activity and may be responsible, therefore, for blood group A antigenicity. It was present in the serum of individuals with blood group A or AB but was absent from those with blood group B or O. The activity measured in the erythrocyte membrane was low and did not show clear-cut separation among donors of different blood groups. The specificity of this enzyme in serum was suggested by the ability of 2'-fucosyllactose to act as an acceptor, as well as desialyzed porcine submaxillary mucin, while lactose and desialized fetuin failed to accept N-acetyl-D-galactosamine. The catalytic properties of the N-acetyl-D-galactosaminyltransferase from serum and from erythrocyte membranes were similar.