Studies on Galactosyltransferase

The reaction catalyzed by galactosyltransferase has been studied kinetically at pH 8.0 using glucose as the galactosyl group acceptor in the presence of a fixed concentration of α-lactalbumin. The initial velocity patterns and the patterns for the dead-end inhibition by UDP-glucose were similar to those obtained with N-acetylglucosamine as a substrate in the absence of α-lactalbumin (in an accompanying paper). l -Arabinose acted as a dead-end inhibitor giving rise to inhibitions which were competitive with respect to glucose and uncompetitive in relation to both Mn2+ and UDP-galactose. These results have been interpreted to indicate that there is an ordered addition of Mn2+, UDP-galactose, and glucose to the enzyme, as well as an ordered release of lactose and UDP. It is postulated that Mn2+ does not dissociate from the enzyme-manganese complex during the catalytic reaction. Product inhibition by lactose was not observed. Comparisons have been made of the values for the kinetic constants associated with Mn2+ and UDP-galactose in the absence and presence of α-lactalbumin.