Regulation of carbohydrate metabolism during Giardia encystment

Giardia intestinalis trophozoites encyst when they are exposed to bile. Dur ing encystment, events related to the inducible synthesis of a novel N-acet yl-D-galactosamine (GalNAc) homopolymer, occur. Within the first 6 h of enc ystment, mRNA for glucosamine 6-P isomerase (GPI), the first inducible enzy me unique to this pathway appears, oxygen uptake rates double from non-ency sting levels, and metronidazole (MTZ) inhibits oxygen uptake. Within 12 h, GPI and its activity are detectable and OU decreases 50% from non-encysting levels: glucose's stimulation and MTZ's inhibition of oxygen uptake cease. In contrast, aspartate uptake remained constant throughout the 40 h monito red. Two genes, gpi 1 and 2 encode for GPI, but only gpil is expressed duri ng encystment. Glucosamine 6-P (GlcN6P). the synthetic product of GPI, acti vates UDP-N-acetylglucosamine (UDP-GlcNAc) pyrophosphorylase, a downstream enzyme. 3 to 5-fold in the direction of UDP-GlcNAc synthesis. UDP-GlcNAc is epimerized to UDP-GalNac and UDP-GalNAc is polymerized by "cyst wall synth ase" (beta 1 --> 3 GalNAc transferase) into a highly insoluble beta 1,3-lin ked homopolymer. This GalNAc polysaccharide, the major component of cyst wa ll filaments forms, in conjunction with polypeptides, the outer cyst wall o f Giardia.