Two α‐3‐d‐Galactosyltransferases in Rabbit Stomach Mucosa with Different Acceptor Substrate Specificities

Abstract

Homogenates of rabbit stomach mucosa were examined for enzymes catalysing the transfer of d-galactose from UDP-d-galactose to various low-molecular-weight acceptors of known structure. Treatment of the products with α and β-d-galactosidases revealed that d-galactose was transferred in both α and β-anomeric linkages. The β-d-galactosyltransferase used N-acetylglucosamine and compounds containing terminal nonreducing β-anomeric linkages. The β-d-galactosyltransferase used N-acetylglucosamine and compounds containing terminal nonreducing β-N-acetylglucosaminyl residues as acceptor substrates. The compounds accepting d-galactose in α-anomeric linkage had unsubstituted terminal non-reducing β-d-galactosyl units or a fucose substituent on the carbon-2 position of a subterminal β-d-galactosyl unit. Methylation analysis of the products formed with N-acetyllactosamine [β-d-Galp(1 → 4)d-GlcNAcp] and 2'fucosyllactose [α-l-Fucp(1 → 4)d-Glcp] revealed that d-galactose was transferred to the carbon-3 position of the β-d-galactosyl residue in both of these acceptor substrates. Competition experiments with the two substrates indicated that the transfer of d-galactose was catalysed in each case by a different α-3-d-galactosyltransferase. Differences were also observed in the solubility properties of the enzymes: the α-3-d-galactosyltransferase using acceptor substrates with unsubstituted β-d-galactosyl residues was more realdily soluble both in the presence and absence of detergents than the transferase using β-d-galactosyl residues substituted at carbon-2 with l-fucose. These findings demonstrate that rabbit stomach mucosa has two disfinct α-3-d-galactosyltransferases: one, which is more tightly membrane-bound, resembles the human B-gene-specified transferase in its acceptor specificity, and the second, which is a more soluble enzyme, transfers d-galactose to the same positional linkage in unsubstituted β-d-galactosyl residues.