Bovine UDP-N-acetylglucosamine:lysosomal-enzyme N-acetylglucosamine-1-phosphotransferase .2. Enzymatic characterization and identification of the catalytic subunit

The kinetic properties of UDP-N-acetylglucosamine: lysosomal-enzyme N-acetylglucosamine-1-phosphotransferase (GlcNAc-phosphotransferase) purified to homogeneity from lactating bovine mammary gland have been investigated. GlcNAc-phosphotransferase transferred GlcNAc 1-phosphate from UDP-GlcNAc to the synthetic acceptor alpha-methylmannoside, generating GlcNAc-1-phospho-6-mannose alpha-methyl, the structure of which was confirmed by mass spectroscopy, GlcNAc-phosphotransferase was active between pH 5.7 and 9.3, with optimal activity between pH 6.6 and 7.5, Activity was strictly dependent on Mg2+ or Mn2+. The K-m for Mn2+ was 185 mu M. The K-m for UDP-GlcNAc was 30 mu M, and that for alpha-methylmannoside was 63 mM. The enzyme was competitively inhibited by UDP-Glc, with a K-i of 733 mu M. The 166-kDa subunit was identified as the catalytic subunit by photoaffinity labeling with azido-[beta-P-32]UDP-Glc. Purified GlcNAc-phosphotransferase utilizes the lysosomal enzyme uteroferrin similar to 163-fold more effectively than the non-lysosomal glycoprotein ribonuclease B. Antibodies to GlcNAc-phosphotransferase blocked the transfer to cathepsin D, but not to alpha-methylmannoside, suggesting that protein-protein interactions are required for the efficient utilization of glycoprotein accepters, These results indicate that the purified bovine GlcNAc-phosphotransferase retains the specificity for lysosomal enzymes as accepters previously observed with crude preparations.