Specificities of enzymes of glycosylphosphatidylinositol biosynthesis in trypanosoma brucei and HeLa cells

A series of synthetic analogues Of D-GlcNalpha1-6-D-myo-inositol-1-HPO4-sn-1,2-dipalmitoylglycerol, consisting of 22 variants of the D-GlcN or lipid components, were tested in trypanosomal and human (HeLa) cell-free systems. The assays measured the abilities of the analogues to act as substrates or inhibitors of the enzymes of glycosylphosphatidylinositol biosynthesis downstream of GlcNAc-phosphatidylinositol (GlcNAc-PI) de-N-acetylase. One compound, 4-deoxy-D-GlcNalpha1-6-D-myo-inositol-1-HP04-Sn-1,2-dipalmitoylglycerol, proved to be an inhibitor of both the trypanosomal and HeLa pathways, whereas 4-O-methyl-D-GlcNalpha1-6-D-myo-inositol-1-HPO(4)sn-1,2-dipalmitoylglycerol and the 4'-epimer, D-GalN-alpha1-6-D-myo-inositol-1-HPO4-sn-1,2-dipalmitoylglycerol, were neither substrates nor inhibitors. The results with other analogues showed that the 6-OH of the alpha-D-GlcN residue is not required for substrate recognition in the trypanosomal and human pathways, whereas the 3-OH group is essential for both. Parasite-specific recognition of the P-linked analogue D-GlcNbeta1-6-D-myo-inositol-1-HPO4-sn-1,2-dipalmitoylglycerol is striking. This suggests that, like the GlcNAc-PI de-N-acetylase, the trypanosomal glycosylphosphatidylinositol alpha-mannosyltransferases, inositol acyltransferse and ethanolamine phosphate transferase, do not recognize the 2-, 3-, 4-, and 5-OH groups of the D-myo-inositol residue, whereas the human inositol acyltransferase and/or first alpha-mannosyltransferase recognizes one or more of these groups. All of the various lipid analogues tested served as substrates in both the trypanosomal and HeLa cell-free systems, suggesting that a precise lipid structure and stereochemistry are not essential for substrate recognition. However, an analogue containing a single C18:0 alkyl chain in place of sn-1,2-dipalmitoylglycerol proved to be a better substrate in the trypanosomal than in the HeLa cell-free system. These findings should have a bearing on the design of future generations of specific inhibitors of the trypanosomal glycosylphosphatidylinositol biosynthetic pathway.