Activation by dolichol phosphate-mannose of the biosynthesis of N-acetylglucosaminylpyrophosphoryl polyprenols by the retina.

The N-acetylglucosaminyltransferase of the embryonic chick retina which catalyze the process dolichol phosphate + UDP-GlcNAc Mg2+ leads to GlcNAc-P-P-dolichol + (GlcNAc)2-P-P-dolichol were stimulated 7- to 15-fold by dolichol phosphate-mannose added exogenously to the incubation medium or generated in situ. The relative molar distribution as measured by gel filtration on Bio-Gel P-4 (-400) of the saccharides liberated by mild acid hydrolysis of the GlcNAc-lipids were (per cent GlcNAc-P-P-dolichol/per cent. (Glc-NAc)2-P-P-dolichol) 95:5 and 90:10 for the basal level of activity and after activation by dolichol phosphate-mannose, respectively. When the reactions were examined in the presence of increasing concentrations of UDP-GlcNAc, the Vmax of the process was increased 10-fold in the presence of dolichol phosphate-mannose over that in its absence. The apparent Km for UDP-GlcNAc, however, was increased 2- to 4-fold. Dolichol phosphate-mannose, not a substrate in these reactions, may function as an allosteric activator of the N-acetylglucosaminyltransferases by bringing about this effect on the Vmax. Cooperativity was not apparent when the reaction was examined over a wide range in the concentration of UDP-GlcNAc. These observations suggest that dolichol phosphate-mannose may participate not only as a substrate for alpha-mannosylation, but also as a regulator of the biosynthesis of the oligosaccharide-lipids involved in glycoprotein biosynthesis.