Identification of a α-NeuAc-(2→3)-β-d-galactopyranosyl N-acetyl-β-d-galactosaminyltransferase in human kidney

Abstract Microsomal preparations from human kidney were found to contain enzymic activity capable to transfer N -acetylgalactosamine from UDP- N -acetylgalactosamine to native bovine fetuin. The acceptor structures on the fetuin molecules were identified as N - as well as O -linked glycans with a markedly higher incorporation into the N -linked carbohydrate chains. Analysis of the alkali-labile transferase products by thin-layer chromatography indicated that the enzyme is able to synthesize structures having mobilities identical with those found on glycophorin from Cad erythrocytes. Mild acid treatment and enzymic hydrolysis with N -acetylhexosaminidase from jack beans of the N -linked transferase products suggested that β- d -Gal p NAc-(1→4)-[α-NeuAc-(2→3)]-β- d -Gal p -(1-ar structures were formed by the enzymic reaction on both N - and O -linked acceptors. The enzyme might, therefore, be involved in the biosynthesis of Sd a (and Cad) antigenic structures. By use of various oligosaccharides, glycopeptides, and glycolipids having well characterized carbohydrate sequences, the acceptor-substrate specificity of the N -acetylgalactosaminyltransferase was determined. The enzyme generally recognized α-NeuAc-(2→3)-β- d -Gal groups as acceptors, but in a certain conformation. Thus, tri- and tetra-saccharide alditols, native human glycophorin A, and GM 3 were not acceptor substrates although they carry the potential disaccharide acceptor unit. When these structures were presented as sialyl-(2→3)-lactose or as a tryptic peptide from glycophorin A, they were shown to be rather good acceptor substrates for the N -acetyl-β- d -galactosaminyltransferase from human kidney.