Further characterization of the galactosyltransferases in chick cartilage

Abstract

The substrate specificity of the galactosyltransferase system in microsomal particles from embryonic chick cartilage has been investigated further. In addition to the previously described galactosyltransferases, involved in the biosynthesis of the chondroitin sulfate-protein linkage region, a third galactosyltransferase was detected in the particulate enzyme preparation, which transferred galactose from UDP-galactose to N-acetylglucosamine ( N-acetyllactosamine synthetase). Mixed substrate experiments as well as heat inactivation studies with galactose acceptors derived from chondroitin sulfate indicated that the catalytic center involved in the N-acetyllactosamine synthetase reaction is distinct from those catalyzing the synthesis of the two galactose moieties of chondroitin sulfate. The addition of α-lactalbumin to the enzyme stimulated the synthesis of lactose from glucose and UDP-galactose, in agreement with similar observations on the A subunit of lactose synthetase from milk as well as UDP-galactose; N- acetyl- d-glucosamine galactosyltransferases from other sources. Partial acid hydrolysis of [ 14C]galactose-labeled endogenous acceptor from the cartilage homogenate yielded a fragment with the chromatographic properties of N-acetyllactosamine, suggesting that a portion of the galactose incorporated into the endogenous acceptor may be transferred to N-acetylglucosamine residues and not exclusively to precursors of the chondroitin sulfate-protein complex.