Activation of GlcNAc-P-P-Dolichol Synthesis by Mannosylphosphoryldolichol Is Stereospecific and Requires a Saturated α-Isoprene Unit

Exogenous mannosylphosphoryldolichol (Man-P-Dol) has previously been shown to stimulate UDP-GlcNAc:dolichyl phosphate N-acetylglucosamine 1-phosphate transferase (GPT1), the enzyme catalyzing the biosynthesis of N-acetylglucosaminylpyrophosphoryldolichol (GlcNAc-P-P-Dol). To define the structural specificity of the mannolipid-mediated activation of GPT1, the ability of a variety of mannosylphosphorylisoprenols to stimulate GlcNAc-lipid biosynthesis in microsomal preparations from retinas of the embryonic chick has been tested. For these comparisons several Man-P-isoprenols were synthesized enzymatically and chemically. The catalytic efficiency of activation expressed as the V_max/K_a. ratio was substantially higher for Man-P-Dol₉₅ than for mannosylphosphorylpolyprenol₉₅ (Man-P-Poly₉₅), demonstrating that the saturated α-isoprene unit of the dolichyl moiety influences the mannolipid-enzyme interaction. The degree of activation increased with chain length and hydrophobicity of the dolichyl moiety when Man-P-dolichols containing 2, 11, and 19 isoprene units were evaluated. A strict stereospecificity was exhibited as β-Man-P-Dol₉₅ provided a 100-fold greater stimulation than the corresponding α-stereoisomer. The recognition of the saturated α-isoprene unit, the influence of chain length, and the strict stereospecificity of the interaction between β-Man-P-Dol and GPT1 extend the description of the mannolipid-enzyme interaction and provide strong new evidence that Man-P-Dol levels can influence the rate of GlcNAc-P-P-Dol synthesis.