Galactosyltransferase purified from rat milk is distinct from the human and bovine enzyme

N-Acetylglucosaminide beta 1, 4-galactosyltransferase (EC 2.4.1.22) was purified from rat milk by affinity chromatography on N-acetylglucosamine-Sepharose and alpha-lactalbumin-Sepharose columns. The purified enzyme migrated as three polypeptides of relative molecular weight 59,000, 54,000, and 27,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Antiserum raised against the 54K rat protein immunoprecipitated all three polypeptides, suggesting that they share common antigenic sites. The human milk galactosyltransferase, purified under similar conditions, was electrophoretically homogeneous on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, with relative molecular weight 54K, and was not immunoprecipitated by the antiserum to the 54K rat milk protein. In addition, Michaelis constants for the enzyme from rat and human milk differed. The apparent Michaelis constant for N-acetylglucosamine and uridine 5'-diphosphate-galactose were 4.8 and .3 mM, respectively, for the rat enzyme, and 1.8 and .028 mM, respectively, for the human enzyme.