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

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 ‐Gal p (1 → 4) d ‐GlcNA cp ] and 2'fucosyllactose [α‐ l ‐Fu cp (1 → 4) d ‐Glc p ] 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.