Macechko P T, Steimle P A, Lindmark D G, Erlandsen S L, Jarroll E L
Department of Biology, Cleveland State University, OH.
Mol Biochem Parasitol. 1992 Dec;56(2):301-9. doi: 10.1016/0166-6851(92)90179-n.
Galactosamine, a Giardia filamentous cyst wall specific-sugar, is below the limits of detection in non-encysting trophozoites. Radiolabeling studies suggest that Giardia synthesize galactosamine primarily from endogenous glucose rather than salvage it from the environment. Enzymes responsible for galactosamine synthesis from glucose are induced during encystment and have been characterized in crude homogenates and in supernatant (soluble) fractions. These enzymes (specific activity; time after encystment is induced for maximal activity; x-fold increase) include glucosamine 6-phosphate isomerase (in the deaminating direction, 167 mU mg protein-1; 20 h; x 182-fold; in the aminating direction, 258 mU mg protein-1; 20 h; x 13-fold), glucosamine 6-phosphate N-acetylase (11 mU mg protein-1; 20 h; x 20-fold), phosphoacetylglucosamine mutase (160 mU mg protein-1; 20 h; x 12-fold), UDP-N-acetylglucosamine pyrophosphorylase (22 mU mg protein-1; 48 h; x 8-fold), and UDP-N-acetylglucosamine 4'-epimerase (13 mU mg protein-1; 48 h; x 4000-fold). This represents the first report of these enzymes and of an inducible carbohydrate-synthesizing pathway in any protozoan.
半乳糖胺是贾第虫丝状包囊壁特有的糖类，在非包囊滋养体中的含量低于检测限。放射性标记研究表明，贾第虫主要从内源性葡萄糖合成半乳糖胺，而非从环境中获取。负责从葡萄糖合成半乳糖胺的酶在包囊形成过程中被诱导产生，并且已在粗匀浆和上清液（可溶性）组分中得到鉴定。这些酶（比活性；诱导包囊形成后达到最大活性的时间；增加倍数）包括6-磷酸葡糖胺异构酶（脱氨基方向，167 mU mg蛋白-1；20小时；增加182倍；氨基化方向，258 mU mg蛋白-1；20小时；增加13倍）、6-磷酸葡糖胺N-乙酰基转移酶（11 mU mg蛋白-1；20小时；增加20倍）、磷酸乙酰葡糖胺变位酶（160 mU mg蛋白-1；20小时；增加12倍）、UDP-N-乙酰葡糖胺焦磷酸化酶（22 mU mg蛋白-1；48小时；增加8倍）和UDP-N-乙酰葡糖胺4'-表异构酶（13 mU mg蛋白-1；48小时；增加4000倍）。这是关于这些酶以及任何原生动物中可诱导碳水化合物合成途径的首次报道。
