Heterogeneity of human milk beta(1-4)-D-galactosyltransferase

beta(1-4)-Galactosyltransferase from human milk (the A protein of lactose synthase) has been found to be heterogeneous when fractionated by affinity chromatography against insolubilized alpha-lactalbumin, using a linear gradient of decreasing N-acetylglucosamine concentration. Three forms were isolated. Molecular weights of the different species, as determined by sodium dodecylsulphate gel electrophoresis, were found to be 38 000, 43 000 and 50 000. The 38 000 and 50 000 species were studied for their catalytic ability to synthesize either lactose in the presence of alpha-lactalbumin, or N-acetyllactosamine in the presence and absence of the 'specifier' protein. Appreciable difference was observed between the two enzyme forms with respect to their catalysis of lactose synthesis with alpha-lactalbumins from various sources. Differences in the rate of production of N-acetyllactosamine in the presence of alpha-lactalbumin were also observed. For the lowest-molecular-weight species it was found that the inhibitory effect of alpha-lactalbumin upon N-acetyllactosamine synthesis becomes an activating effect at higher alpha-lactalbumin concentrations, while no such inversion was observed for the other species. The results suggest that the conformation at the site of association of the enzyme with the acceptor saccharide or alpha-lactalbumin has been changed, presumably by a pratial enzymic hydrolysis.