Purification and characterization of encystment-induced glucosamine 6-phosphate isomerase in Giardia.

Giardia intestinalis encystment results in the incorporation of galactosamine (GalN) (a cystwall specific sugar) into outer cyst wall filaments [1,2]. GalN is synthesized during encystment from endogenous glucose by an inducible enzyme pathway [3] and the first reaction unique to GalN synthesis is the conversion of fructose-6phosphate (F6P) to glucosamine 6-phosphate (G1cN6P) [4,5]. In G. intestinalis, this reaction is catalyzed by G1cN6P isomerase (2-amino-2-deoxy-D-glucosamine 6-phosphate ketol isomerase, EC 5.3.1.10) which deaminates G1cN6P to F6P and NH3 and aminates F6P with NH3 to produce GlcN6P [3,6,7]. These activities as well as those of the other GalN synthetic enzymes of G. intestinalis [3] are induced when trophozoites encyst in the presence of bile. In bacterial and yeast systems, the isomerase is regarded as a catabolic enzyme involved in the degradation of amino sugars [7,8]. In contrast, an in vivo anabolic role has been suggested for Giardia G1cN6P isomerase [3], some other eukaryotes [9-11], and an Escherichia coli K-12 mutant lacking G1cN6P synthase activity [12]. This paper describes the purification of G1cN6P isomerase from encysting G. intestinalis and represents the first characterization of a purified enzyme specifically induced during the encystment of a protozoan.