SUBSTRATE-SPECIFICITY OF THE N-ACETYLGLUCOSAMINYL-PHOSPHATIDYLINOSITOL DE-N-ACETYLASE OF GLYCOSYLPHOSPHATIDYLINOSITOL MEMBRANE ANCHOR BIOSYNTHESIS IN AFRICAN TRYPANOSOMES AND HUMAN-CELLS

De-N-acetylation of N-acetylglucosaminyl-phosphatidylinositol (GlcNAc- PI) is the second step of glycosylphosphatidylinositol (GPI) membrane anchor biosynthesis in eukaryotes. This step is a prerequisite for the subsequent mannosylation of glucosaminyl-phosphatidylinositol (GlcN-P I) which leads to mature GPI membrane anchor precursors, which are tra nsferred to certain proteins in the endoplasmic reticulum. The substra te specificities of the GlcNAc-PI de-N-acetylase activities of African trypanosomes and human (HeLa) cells were studied with respect to the N-acyl groups (R) that could be removed from a series of GlcNR-PI subs trates, where R = acetyl (Ac), propionyl (Pr), butyryl(Bu), isobutyryl (iBu), pentanoyl (Pen) or hexanoyl (Hex). The data show that the tryp anosomal and HeLa enzymes had similar specificities and that the turno ver of GlcNR-PIs by the trypanosomal enzyme was in the order GlcNAc-PI > GlcNPr-PI much greater than GlcNBu-PI approximate to GlcNiBu-PI app roximate to GlcNPen-PI much greater than GlcNHex-PI. The trypanosome a nd HeLa de-N-acetylases were unable to de-N-acetylate mannosylated Glc NAc-PI intermediates, which explains why de-N-acetylation must precede mannosylation in the GPI biosynthetic pathway.