Human Serum Contains N-Acetyllactosamine: β 1-3 N-Acetylglucosaminyltransferase Activity

Abstract

Human serum was shown to contain N-acetyllactosamine: N-acetylglucosaminyltransferase activity. The reaction product was hydrolyzed by β-N-acetylglucosaminidase and released [¹⁴C]N-acetylglucosamine, indicating that the N-acetylglucosaminyl residue was β-linked to N-acetyllactosamine. Methylation and hydrolysis of the reaction product yielded 2, 4, 6-trimethyl [³H] galactose, indicating that the N-acetylglucosaminyl residue was introduced at position C-3 of the terminal galactose of N-acetyllactosamine. In our experiments, 2, 3, 4-trimethyl[³H]galactose was not detected. Substrate competition studies between N-acetyllactosamine and lactose showed that this enzyme also catalyzed the transfer of N-acetylglucosamine from UDP-N-acetylglucosamine to lactose. Since the K_m value for N-acetyllactosamine, which was 7.0mM, was approximately a fourth of that for lactose (29.8mM), N-acetyllactosamine was more effective than lactose as an acceptor.