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-acetylglucosamin e [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 act ivities were determined towards different N-acetylglucosamine concentration s. 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 mu M and 41 mu M towards UDP-Gal, respective ly. No significant inhibition of N-acetyllactosamine synthesis with alpha-l actalbumin was observed for beta-1,4-GalT V but the significant inhibition with alpha-lactalbumin was observed for beta-1,4-GalT I.