Differences in N-acetyllactosamine synthesis between beta-1,4-galactosyltransferases I and V.

Unlike classical beta-1,4-galactosyltransferase (beta-1,4-GalT I), beta-1,4-GalT V (formerly IV**) has little activity towards 1 mM N-acetylglucosamine [Sato et al. (1998) Proc Natl Acad Sci USA 95:472-477]. The human beta-1,4-GalTs I and V were expressed individually in Sf-9 cells by transfection of the full coding sequences, and their N-acetyllactosamine synthetase activities were determined towards different N-acetylglucosamine concentrations. Kinetic studies using the cell homogenates as an enzyme source revealed that beta-1,4-GalTs I and V possess Km values of 0.6 mM and 33 mM towards N-acetylglucosamine, and of 48 microM and 41 microM towards UDP-Gal, respectively. No significant inhibition of N-acetyllactosamine synthesis with alpha-lactalbumin was observed for beta-1,4-GalT V but the significant inhibition with alpha-lactalbumin was observed for beta-1,4-GalT I.