Parasite-specific inhibition of the glycosylphosphatidylinositol biosynthetic pathway by stereoisomeric substrate analogues

The natural substrate for the first alpha-D-mannosyltransferase of glycosyl phosphatidylinositol biosynthesis in the protozoan parasite Trypanosoma bru cei is D-GlcN alpha 1-6-L-myo-inositol-1-P-sn-1,2-diacylglycerol, Here we s how that a diastereoisomer, D-GlcN alpha 1-6-L-myo-inositol-1-P-sn-1,2-diac ylglycerol, is an inhibitor of this enzyme in a trypanosomal cell-free syst em. Tests with other L-myo-inositol-containing compounds revealed that L-my o-inositol-l-phosphate is the principal inhibitory component and that methy lation of the 2-OH group of the L-myo-inositol residue abolishes any inhibi tion. Comparisons between the natural substrate and the inhibitors suggeste d that the inhibitors bind to the first alpha-D-mannosyltransferase by mean s of charge interactions with the l-phosphate group and/or hydrogen bonds i nvolving the 3-, 4-, and 5-OH groups of the L-myo-inositol residue, which a re predicted to occupy orientations identical to those of the I-phosphate a nd 5-, 4-, and 3-OH groups, respectively, of the D-myoinositol residue of t he natural substrate. However, additional experiments indicated that the 4- OH group of the D-myo-inositol residue is unlikely to be involved in substr ate recognition. None of the L-myo-inositol-containing compounds that inhib ited glycosylphosphatidylinositol (GPI) biosynthesis in a parasite cell-fre e system had any effect on GPI biosynthesis in a comparable human (HeLa) ce ll-free system, suggesting that other related parasite-specific inhibitors of this essential pathway might be developed.