Carbohydrate and amino acid analyses of Giardia muris cysts

On the molecular and cellular effects of omeprazole to further support its effectiveness as an antigiardial drug

Research on Giardia lamblia has accumulated large information about its molecular cell biology and infection biology. However, giardiasis is still one of the commonest parasitic diarrheal diseases affecting humans. Additionally, an alarming increase in cases refractory to conventional treatment has been reported in low prevalence settings. Consequently, efforts directed toward supporting the efficient use of alternative drugs, and the study of their molecular targets appears promising. Repurposing of proton pump inhibitors is effective in vitro against the parasite and the toxic activity is associated with the inhibition of the G. lamblia triosephosphate isomerase (GlTIM) via the formation of covalent adducts with cysteine residue at position 222. Herein, we evaluate the effectiveness of omeprazole in vitro and in situ on GlTIM mutants lacking the most superficial cysteines. We studied the influence on the glycolysis of Giardia trophozoites treated with omeprazole and characterized, for the first time, the morphological effect caused by this drug on the parasite. Our results support the effectiveness of omeprazole against GlTIM despite of the possibility to mutate the druggable amino acid targets as an adaptive response. Also, we further characterized the effect of omeprazole on trophozoites and discuss the possible mechanism involved in its antigiardial effect.

Analysis of Giardia lamblia interactions with polymer surfaces using a microarray approach

The interaction of the waterborne protozoan parasite, Giardia lamblia, with polymeric materials was investigated by microarray screening of 652 polymers. Polymers were identified which either bound G. lamblia cysts or prevented their binding. Correlation of material properties such as wettability and surface roughness with cyst attachment revealed no influence of these factors upon Giardia adhesion. However, the study of polymer composition allowed the correlation of binding and generation of polymer structure function relationships; glycol and aromatic functionalities appeared to prevent adhesion, whereas secondary amine groups promoted adhesion, in agreement with previous literature. A significant reduction in attachment was observed following both cyst treatments with proteinase K and performing experiments at extremes of pH (2 and 12). It is suggested that proteinase K removes the proteins needed for specific surface interactions, whereas extremes of pH influence either protonation of the polymer or the surface charge of the cysts. The mechanism by which the protozoa attach to polymeric surfaces is proposed to be through ion-pair interactions. Improved understanding of G. lamblia surface interactions could assist in predicting transport and fate behavior in the environment and contribute to better design of water treatment processes, while the polymers identified in this work could find use in sensor applications and membrane filtration.

Cyst and encystment in protozoan parasites: optimal targets for new life-cycle interrupting strategies?

Certain protozoan parasites use survival strategies to reside outside the host such as the formation of cysts. This dormant and resistant stage results from the complex process of encystment that involves diverse molecular and cellular modifications. The stimuli and changes associated with cyst biogenesis are a matter of ongoing studies in human and animal protozoan parasites such as amoeba and Giardia species because blocking every step in the encystment pathway should, in theory, interrupt their life cycles. The present review thoroughly examines this essential process in those protozoan parasites and discusses the possibility of using that information to develop new kinds of anti-parasite specific and life cycle-interrupting drugs, aimed at holding back the dissemination of these infections.

Selective condensation drives partitioning and sequential secretion of cyst wall proteins in differentiating Giardia lamblia

Controlled secretion of a protective extracellular matrix is required for transmission of the infective stage of a large number of protozoan and metazoan parasites. Differentiating trophozoites of the highly minimized protozoan parasite Giardia lamblia secrete the proteinaceous portion of the cyst wall material (CWM) consisting of three paralogous cyst wall proteins (CWP1–3) via organelles termed encystation-specific vesicles (ESVs). Phylogenetic and molecular data indicate that Diplomonads have lost a classical Golgi during reductive evolution. However, neogenesis of ESVs in encysting Giardia trophozoites transiently provides basic Golgi functions by accumulating presorted CWM exported from the ER for maturation. Based on this “minimal Golgi” hypothesis we predicted maturation of ESVs to a trans Golgi-like stage, which would manifest as a sorting event before regulated secretion of the CWM. Here we show that proteolytic processing of pro-CWP2 in maturing ESVs coincides with partitioning of CWM into two fractions, which are sorted and secreted sequentially with different kinetics. This novel sorting function leads to rapid assembly of a structurally defined outer cyst wall, followed by slow secretion of the remaining components. Using live cell microscopy we find direct evidence for condensed core formation in maturing ESVs. Core formation suggests that a mechanism controlled by phase transitions of the CWM from fluid to condensed and back likely drives CWM partitioning and makes sorting and sequential secretion possible. Blocking of CWP2 processing by a protease inhibitor leads to mis-sorting of a CWP2 reporter. Nevertheless, partitioning and sequential secretion of two portions of the CWM are unaffected in these cells. Although these cysts have a normal appearance they are not water resistant and therefore not infective. Our findings suggest that sequential assembly is a basic architectural principle of protective wall formation and requires minimal Golgi sorting functions.

The protozoan Giardia lamblia is the leading cause for parasite-induced diarrhea with significant morbidity in humans and animals world-wide, and is transmitted by water-resistant cysts. Giardia has undergone substantial reductive evolution to a simpler organization than the last common eukaryotic ancestor, which makes it an interesting model to investigate basic cellular mechanisms. Its secretory system lacks a Golgi, but trophozoites induced to differentiate to cysts generate organelles termed encystation-specific vesicles (ESVs). Previous work shows that ESVs are most likely minimal pulsed Golgi-like compartments for exporting pre-sorted cyst wall material. We tested whether the sorting function associated with classical trans Golgi networks was also conserved in these organelles. By tracking immature and processed forms of the three cyst wall proteins during differentiation we discovered a novel sorting function which results in partitioning of ESV cargo and sequential secretion of the cyst wall material. Using live cell imaging we identified reversible formation of condensed cores as a mechanism for cargo partitioning. These observations suggest that the requirement for sequential secretion of extracellular matrix components protecting Giardia during transmission has prevented the complete secondary loss of the machinery to generate Golgi cisterna-like maturation compartments; indeed, the preserved functions have been placed under stage-specific control.

Ultrastructure of cyst differentiation in parasitic protozoa

Cysts represent a phase in the life cycle of biphasic parasitic protozoa that allow them to survive under adverse environmental conditions. Two events are required for the morphological differentiation from trophozoite to cyst and from cyst to trophozoite: the encystation and excystation processes. In this paper, we present a review of the ultrastructure of the encystation and excystation processes in Entamoeba invadens, Acanthamoeba castellanii, and Giardia lamblia. The comparative electron microscopical observations of these events here reported provide a morphological background to better understand recent advances in the biochemistry and molecular biology of the differentiation phenomena in these microorganisms.

Secretory organelles of pathogenic protozoa

Secretory processes play an important role on the biology and life cycles of parasitic protozoa. This review focus on basic aspects, from a cell biology perspective, of the secretion of (a) micronemes, rhoptries and dense granules in members of the Apicomplexa group, where these organelles are involved in the process of protozoan penetration into the host cell, survival within the parasitophorous vacuole and subsequent egress from the host cell, (b) the Maurer's cleft in Plasmodium, a structure involved in the secretion of proteins synthesized by the intravacuolar parasite and transported through vesicles to the erythrocyte surface, (c) the secretion of macromolecules into the flagellar pocket of trypanosomatids, and (d) the secretion of proteins which make the cyst wall of Giardia and Entamoeba, with the formation of encystation vesicles.

Kinetic and physical characterization of the inducible UDP-N-acetylglucosamine pyrophosphorylase from Giardia intestinalis

The UDP-N-acetylglucosamine pyrophosphorylase in Giardia intestinalis (GiUAP) is one of the five inducible enzymes to synthesize UDP-GalNAc, which is an important precursor for cyst wall synthesis. The recombinant UDP-N-acetylglucosamine pyrophosphorylase (rGiUAP) and its mutants G108A and G210A were expressed and identified by SDS-PAGE, size-exclusion chromatography, Western hybridization, and MALDI mass spectrometry. Sequence comparison with other eukaryotic UAPs has identified three specific motifs. Within these motifs alanine substitution for Gly(108) or Gly(210) dramatically reduced the pyrophosphate synthesis, suggesting these amino acids are catalytic residues. Besides, the rGiUAP was found to have relaxed binding to other uridine-based nucleotides, suggesting the substrate binding pocket is specific to uridine rather than phosphate group(s). Moreover, thermal denaturation analysis showed a significant increase in T(m) for the rGiUAP and G108A upon binding of the substrate Mg-UTP. In contrast, G210A showed a decreased T(m) upon binding of Mg-UTP. These results showed that binding of Mg-UTP increases protein stability of the rGiUAP, and the catalytic residue Gly(210) plays a significant role in stabilizing the protein structure. Such stabilization effect induced by substrate binding might be physiologically important as it favors the production of UDP-GlcNAc and hence the downstream GalNAc, which is crucial to survival of Giardia. These results help to define the essential amino acids for catalysis in the GiUAP and reveal the role of Mg-UTP binding in regulation of protein stability.

Giardia intestinalis: Molecular characterization of UDP-N-acetylglucosamine pyrophosphorylase

The flagellated protozoan Giardia intestinalis is one of the most prevalent human-infective parasites with a worldwide distribution. This parasite must encyst to complete the life cycle and N-acetylgalactosamine is produced from endogenous glucose for cyst wall synthesis during the transformation. UDP-N-acetylglucosamine pyrophosphorylase in G. intestinalis (GiUAP, EC 2.7.7.23) is the fourth enzyme in the inducible pathway of N-acetylgalactosamine biosynthesis, catalysing the conversion of N-acetylglucosamine-1-P to UDP-N-acetylglucosamine. In this study the gene GiUAP was cloned and sequenced from the Portland 1 strain using PCR techniques. It has an ORF of approximately 1.3 kb and contains no introns. BLAST and ClustalW analysis of the deduced amino acid sequence revealed significant similarities to other eukaryotic UAPs with putative active sites identified. Southern hybridization showed that GiUAP exists as a single-copy gene and it was shown to have two transcripts by RT-PCR and Northern hybridization. RLM-RACE identified both 5' and 3' untranslated regions and suggested the transcripts exist as a 5'-capped mRNA, with the use of two tandem polyadenylation sites to generate two unusually long giardial 3' untranslated regions of approximately 522 bp and approximately 3 kb. Moreover, a recombinant protein (rGiUAP) was expressed in E. coli and subjected to physical characterizations. Surprisingly the results obtained in this study were significantly different from those reported for the GiUAP in MR4 strain, suggesting this gene is under different transcription control in different strains of G. intestinalis. This report describes the molecular characterization of GiUAP and provides an opportunity to explore the control of gene expression during encystation of the parasite.

Contribution of microscopy to a better knowledge of the biology of Giardia lamblia

Giardia lamblia is a flagellated protozoan of great medical and biological importance. It is the causative agent of giardiasis, one of the most prevalent diarrheal disease both in developed and third-world countries. Morphological studies have shown that G. lamblia does not present structures such as peroxisomes, mitochondria, and a well-elaborated Golgi complex. In this review, special emphasis is given to the contribution made by various microscopic techniques to a better knowledge of the biology of the protozoan. The application of video microscopy, immunofluorescence confocal laser scanning microscopy, and several techniques associated with transmission electron microscopy (thin section, enzyme cytochemistry, freeze-fracture, deep-etching, fracture-flip) to the study of the cell surface, peripheral vesicles, endoplasmic reticulum-Golgi complex system, and of the encystation vesicles found in trophozoites and during the process of trophozoite-cyst transformation are discussed.

Cyst wall synthase: N-acetylgalactosaminyltransferase activity is induced to form the novel N-acetylgalactosamine polysaccharide in the Giardia cyst wall

Uridine-5'-diphospho-N-acetylgalactosamine (UDP-GalNAc) is required in the formation of the outer filamentous wall of Giardia and is synthesized by inducible enzymes in the cytosol of encysting trophozoites. In this study, an inducible enzyme activity that is associated with a particle population isolated from encysting Giardia is reported, and this activity exclusively incorporates [1-(14)C]GalNAc (from UDP-[(14)C]GalNAc) into an ethanol precipitate with the same properties as the filamentous cyst wall of GIARDIA: This ethanol precipitate exhibits characteristics of Giardia cyst wall filaments in that both contain GalNAc as the only sugar moieties and are SDS-insoluble, proteinase- and alkali-resistant and acid-hydrolysable. However, since the precise chemical nature of the ethanol precipitate remains unknown, this enzyme activity is referred to tentatively as cyst wall synthase (CWS). CWS activity peaks in cells between 24 and 36 h of encystment and exhibits a high affinity and marked specificity for UDP-GalNAc as its substrate. UDP-N-acetylglucosamine, UDP-glucose, UDP-galactose, D-glucosamine and D-galactosamine were not incorporated into the ethanol precipitate. Partially purified CWS activity exhibits an apparent K(m) of 0.048 mM for UDP-GalNAc, a V(max) of 0.70 nmol x min(-1) (mg protein)(-1) and a requirement for divalent cations in the following order of preference: Ca(2+), Mg(2+)>Co(2+)>>>Mn(2+), Zn(2+). EDTA inhibits CWS activity.

The Ultrastructure of the Cyst Wall of Giardia lamblia

Giardiasis is the most common human protozoal infection. In their cystic phase, giardias are protected from the environment by a filamentous cyst wall made up of carbohydrates, proteins, and by two outer membranes separated from the plasma membrane of the parasite by a peripheral space. The present transmission electron microscope observations of G. lamblia cysts of human origin suggest that the extracellular peritrophic space originates from the growth, elongation, and fusion of large cytoplasmic vacuoles. As the large clear vacuoles grew in size, flattening against the inner face of the plasma membrane, they formed a single vacuole that surrounded the body of the parasite, eventually forming two outer membranes. In mature Giardia cysts, the original plasma membrane of the trophozoite becomes the outermost membrane of the cyst wall (CM1). The large vacuoles form a second membrane surrounding the cyst (CM2), and also form a third membrane (CM3), that becomes the new plasma membrane of the trophozoite. During excystation CM1 and CM2 attach to each other and fragment, leaving abundant membrane residues in the peritrophic space. Knowledge of the biochemical composition and functional properties of the complex outer membranous system of G. lamblia cysts here described will be of use to understand the survival of Giardia cysts in the environment, a major factor responsible for the high prevalence of giardiasis worldwide.

Fine structure of the biogenesis of Giardia lamblia encystation secretory vesicles

Synthesis, transport, and assembly of the extracellular cyst wall is the hallmark of Giardia lamblia encystation. Much is known of the biochemical pathways and their regulation. However, from a cell biology point of view, the biogenesis of the encystation specific vesicles (ESVs) that transport cyst wall proteins to the periphery of the cell is poorly understood. Therefore, we exploited a number of complementary ultrastructural approaches to test the hypothesis that the formation of ESVs utilizes a novel regulated secretory pathway. We analyzed parasites at different stages of encystation in vitro by electron microscopy of thin sections, freeze fracture replicas, and three-dimensional reconstruction from serial sections of cells fixed for cytochemical localization of the endoplasmic reticulum (ER) marker, glucose 6-phosphatase. We also used a stereological approach to determine the area occupied by the ER, clefts, ESVs, and cyst wall. Taken together, our kinetic data suggest that some ER cisternae first dilate to form clefts, which enlarge into the ESVs. Living non-encysting and early-encysting trophozoites were labeled around the periphery of both nuclei with C(6)-NBD-ceramide. At 18-21 h, outward migration of some ESVs frequently caused protrusions at the periphery of encysting trophozoites. The presence of lysosome-like peripheral vesicles between the ESV and plasma membrane of the cell was confirmed using acridine orange, an acidic compartment marker. Our data suggest that G. lamblia has a novel secretory pathway in which certain functions of the ER and Golgi co-localize spatially and temporally. These studies will increase understanding of the evolutionary appearance of regulated secretory pathways for assembly of a primitive extracellular matrix in an early diverging eukaryote.

Transcription regulation is demonstrated for five key enzymes in Giardia intestinalis cyst wall polysaccharide biosynthesis

The cyst wall of Giardia intestinalis contains proteins and a novel N-acetylgalactosamine (GalNAc) polysaccharide, which is its major constituent. GalNAc is not present in growing trophozoites, but is synthesized during encystment via an inducible pathway of enzymes that produce UDP-GalNAc from fructose 6-phosphate. This report focuses on the regulation of these enzymes and thus the genes for glucosamine 6-phosphate N-acetyltransferase (GNA), phosphoacetylglucosamine mutase (AGM), UDP-N-acetylglucosamine pyrophosphorylase (UAP), and UDP-N-acetylglucosamine 4-epimerase (UAE) were cloned and expressed in Escherichia coli. Each of these expressed enzymes had the predicted activity and was used to generate antibodies. Northern and Western blot analyses demonstrated that both the mRNA and protein levels for all of these enzymes increase during encystment. Nuclear run-on assays of these and the previously analyzed glucosamine 6-phosphate deaminase (GNP; glucosamine 6-P isomerase) showed that all of the genes responsible for UDP-GalNAc synthesis during encystment are induced at the transcription level.

The Giardia intestinalis filamentous cyst wall contains a novel beta(1-3)-N-acetyl-D-galactosamine polymer: a structural and conformational study

Assembly of a protective cyst wall by Giardia is essential for the survival of the parasite outside the host intestine and for transmission among susceptible hosts. The structure of the G. intestinalis filamentous cyst wall was studied by chemical methods, mass spectrometry, and (1)H nuclear magnetic resonance spectroscopy. Isolated cyst wall material contains carbohydrate and protein in a ratio of 3:2 (w/w), and the carbohydrate moiety is composed of a beta(1-3)-N-acetyl-D-galactopyranosamine homopolymer. Conformational analysis by molecular dynamics and persistence length calculations of GalNAc oligomers in solution demonstrated a flexible structure consisting of left- and right-handed helical elements. It is most likely that in the solid state, the polysaccharide forms ordered helices or possibly multiple helical structures having strong interchain interactions. The highly insoluble nature of the Giardia cyst wall must be due to these strong interchain interactions and, probably, a strong association between the carbohydrate and the protein moiety.

Membrane-associated dipeptidyl peptidase IV is involved in encystation-specific gene expression during Giardia differentiation

Giardia is a flagellated protozoan that resides in the upper small intestine of its vertebrate host and is the most common cause of defined waterborne diarrhoea worldwide. Giardia trophozoites undergo significant biological changes to survive outside the host by differentiating into infective cysts. Encystation is thus essential for transmission of the parasite among susceptible hosts. In the present study, we report that bestatin, a competitive inhibitor of aminopeptidases, blocks cyst formation in vitro by abolishing the expression of encystation-specific genes, such as those coding for cyst wall proteins. Bestatin does not affect proliferating trophozoites, indicating that its effect is encystation-specific. Using biochemical and molecular biological approaches, we identified the enzyme inhibited by bestatin and cloned its corresponding gene. Sequence similarity indicated that this enzyme belongs to a family of dipeptidyl peptidases. Our results suggest that a specific proteolytic event caused by a constitutively expressed membrane-associated dipeptidyl peptidase IV is necessary for encystation of Giardia.

Properties of group I allergens from grass pollen and their relation to cathepsin B, a member of the C1 family of cysteine proteinases

Expansins are a family of proteins that catalyze pH-dependent long-term extension of isolated plant cell walls. They are divided into two groups, alpha and beta, the latter consisting of the grass group I pollen allergens and their vegetative homologs. Expansins are suggested to mediate plant cell growth by interfering with either structural proteins or the polysaccharide network in the cell wall. Our group reported papain-like properties of beta-expansin of Timothy grass (Phleum pratense) pollen, Phl p 1, and suggested that cleavage of cell wall structural proteins may be the underlying mechanism of expansin-mediated wall extension. Here, we report additional data showing that beta-expansins resemble ancient and modern cathepsin B, which is a member of the papain (C1) family of cysteine proteinases. Using the Pichia pastoris expression system, we show that cleavage of inhibitory prosequences from the recombinant allergen is facilitated by its N-glycosylation and that the truncated, activated allergen shows proteolytic activity, resulting in very low stability of the protein. We also show that deglycosylated, full-length allergen is not activated efficiently and therefore is relatively stable. Motif and homology search tools detected significant similarity between beta-expansins and cathepsins of modern animals as well as the archezoa Giardia lamblia, confirming the presence of inhibitory prosequences, active site and other functional amino-acid residues, as well as a conserved location of these features within these molecules. Lastly, we demonstrate by site-directed mutagenesis that the conserved His104 residue is involved in the catalytic activity of beta-expansins. These results indicate a common origin of cathepsin B and beta-expansins, especially if taken together with their previously known biochemical properties.

The activity of a developmentally regulated cysteine proteinase is required for cyst wall formation in the primitive eukaryote Giardia lamblia

Giardia is an intestinal parasite that belongs to the earliest diverging branch of the eukaryotic lineage of descent. Giardia undergoes adaptation for survival outside the host's intestine by differentiating into infective cysts. Encystation involves the synthesis and transport of cyst wall constituents to the plasma membrane for release and extracellular organization. Nevertheless, little is known about the molecular events related to cyst wall biogenesis in Giardia. Among the components of the cyst wall there are two proteins that we have previously identified and characterized: CWP1 (26 kDa) and CWP2 (39 kDa). Expression of these proteins is coordinately induced, and both concentrated within encystation-specific secretory vesicles before their extracellular polymerization. Although highly similar to each other at the amino terminus, CWP2 includes a COOH-terminal 121-amino acid extension. Here, we show that this extension, rich in basic residues, is cleaved from CWP2 before cyst wall formation by an intracellular cysteine proteinase activity, which is induced during encystation like CWPs. Specific inhibitors prevent release of cyst wall materials, abolishing cyst wall formation. We also report the purification, cloning, and characterization of the encystation-specific cysteine proteinase responsible for the proteolytic processing of CWP2, which is homologue to lysosomal cathepsin C. Encystation-specific cysteine proteinase ESCP possesses unique characteristics compared with cathepsins from higher eukaryotes, such as a transmembrane domain and a short cytoplasmic tail. These features make this enzyme the most divergent cathepsin C identified to date and provide new insights regarding cyst wall formation in Giardia.

Giardia intestinalis glucosamine 6-phosphate isomerase: the key enzyme to encystment appears to be controlled by ubiquitin attachment

The cyst wall of the parasitic protozoan, Giardia intestinalis, is composed of a polymer of N-acetylgalactosamine, the precursor of which is synthesized by an inducible enzyme pathway. The first enzyme in this pathway, glucosamine 6-phosphate isomerase, is transcriptionally regulated. During encystment and in mature cysts this isomerase appears to be modified by ubiquitin attachment. Thus, it might be targeted for destruction by an ubiquitin-mediated pathway, suggesting that glucosamine 6-phosphate isomerase expression is tightly regulated.

Reversible interruption of Giardia lamblia cyst wall protein transport in a novel regulated secretory pathway

To survive in the environment and infect a new host, Giardia lamblia secretes an extracellular cyst wall using a poorly understood pathway. The two cyst wall proteins (CWPs) form disulphide-bonded heterodimers and are exported via novel encystation-specific secretory vesicles (ESVs). Exposure of eukaryotic cells to dithiothreitol (DTT) blocks the formation of disulphide bonds in nascent proteins that accumulate in the endoplasmic reticulum (ER) and induces an unfolded protein response (UPR). Proteins that have exited the ER are not susceptible. Exposure to DTT inhibits ESV formation by > 85%. Addition of DTT to encysting cells causes rapid (t1/2 < 10 min), reversible disappearance of ESVs, correlated with reduction of CWPs to monomers and reformation of CWP oligomers upon removal of DTT. Neither CWPs nor ESVs are affected by mercaptoethanesulphonic acid, a strong reducing agent that does not penetrate cells. DTT does not inhibit the overall protein secretory pathway, and recovery does not require new protein synthesis. We found evidence of protein disulphide isomerases in the ESV and the surface of encysting cells, in which they may catalyse initial CWP folding and recovery from DTT. This is the first suggestion of non-CWP proteins in ESVs and of enzymes on the giardial surface. DTT treatment did not stimulate a UPR, suggesting that Giardia may have diverged before the advent of this conserved form of ER quality control.

Glycoconjugate structures of parasitic protozoa

Glycoconjugates are abundant and ubiquitious on the surface of many protozoan parasites. Their tremendous diversity has implicated their critical importance in the life cycle of these organisms. This review highlights our current knowledge of the major glycoconjugates, with particular emphasis on their structures, of representative protozoan parasites, including Leishmania, Trypanosoma, Giardia, Plasmodia, and others.

Regulation of carbohydrate metabolism during Giardia encystment

Giardia intestinalis trophozoites encyst when they are exposed to bile. During encystment, events related to the inducible synthesis of a novel N-acetyl-D-galactosamine (GalNAc) homopolymer, occur. Within the first 6 h of encystment, mRNA for glucosamine 6-P isomerase (GPI), the first inducible enzyme unique to this pathway appears, oxygen uptake rates double from non-encysting 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 monitored. Two genes, gpi 1 and 2 encode for GPI, but only gpi1 is expressed during encystment. Glucosamine 6-P (GlcN6P), the synthetic product of GPI, activates 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 synthase" (beta 1 --> 3 GalNAc transferase) into a highly insoluble beta 1,3-linked homopolymer. This GalNAc polysaccharide, the major component of cyst wall filaments, forms, in conjunction with polypeptides, the outer cyst wall of Giardia.

The most abundant glycoprotein of amebic cyst walls (Jacob) is a lectin with five Cys-rich, chitin-binding domains

The infectious stage of amebae is the chitin-walled cyst, which is resistant to stomach acids. In this study an extraordinarily abundant, encystation-specific glycoprotein (Jacob) was identified on two-dimensional protein gels of cyst walls purified from Entamoeba invadens. Jacob, which was acidic and had an apparent molecular mass of approximately 100 kDa, contained sugars that bound to concanavalin A and ricin. The jacob gene encoded a 45-kDa protein with a ladder-like series of five Cys-rich domains. These Cys-rich domains were reminiscent of but not homologous to the Cys-rich chitin-binding domains of insect chitinases and peritrophic matrix proteins that surround the food bolus in the insect gut. Jacob bound purified chitin and chitin remaining in sodium dodecyl sulfate-treated cyst walls. Conversely, the E. histolytica plasma membrane Gal/GalNAc lectin bound sugars of intact cyst walls and purified Jacob. In the presence of galactose, E. invadens formed wall-less cysts, which were quadranucleate and contained Jacob and chitinase (another encystation-specific protein) in secretory vesicles. A galactose lectin was found to be present on the surface of wall-less cysts, which phagocytosed bacteria and mucin-coated beads. These results suggest that the E. invadens cyst wall forms when the plasma membrane galactose lectin binds sugars on Jacob, which in turn binds chitin via its five chitin-binding domains.

UDP-N-acetylglucosamine pyrophosphorylase, a key enzyme in encysting Giardia, is allosterically regulated

Giardia synthesizes UDP-GalNAc during cyst wall formation (encystment) via a pathway of inducible enzymes similar to that used to synthesize chitin or peptidoglycan and that includes the UTP-requiring UDP-N-acetylglucosamine pyrophosphorylase. Although it has never been reported as a regulatory enzyme in any system studied to date, kinetic data including Hill plots demonstrate clearly that UDP-N-acetylglucosamine pyrophosphorylase activity, purified from encysting Giardia, is allosterically activated anabolically by physiological levels of glucosamine 6-phosphate (3 microm). Capillary electrophoresis demonstrates that within 24 h after trophozoites are induced to encyst, the level of glucosamine 6-phosphate increases 3-fold over that of non-encysting cells and that by 48 h into encystment the level of glucosamine 6-phosphate has decreased to non-encysting levels or below. UDP-N-acetylglucosamine pyrophosphorylase protein is present constitutively in encysting as well as non-encysting cells. UDP-N-acetylglucosamine pyrophosphorylase immunoaffinity purified from encysting and non-encysting cells exhibited the same molecular weight, amino acid composition, and circular dichroism spectra. Moreover, regardless of whether the enzyme came from encysting or non-encysting cells, the change in its circular dichroism spectra and up to a 6-fold increase in its specific activity anabolically were due to its activation with glucosamine 6-phosphate. Thus, the data support the idea that UDP-N-acetylglucosamine pyrophosphorylase is a major regulatory point in amino sugar synthesis in encysting Giardia and that its allosteric anabolic activation may shift the equilibrium of this pathway toward UDP-GalNAc synthesis.

Stage-specific expression and targeting of cyst wall protein-green fluorescent protein chimeras in Giardia

In preparation for being shed into the environment as infectious cysts, trophozoites of Giardia spp. synthesize and deposit large amounts of extracellular matrix into a resistant extracellular cyst wall. Functional aspects of this developmentally regulated process were investigated by expressing a series of chimeric cyst wall protein 1 (CWP1)-green fluorescent protein (GFP) reporter proteins. It was demonstrated that a short 110 bp 5' flanking region of the CWP1 gene harbors all necessary cis-DNA elements for strictly encystation-specific expression of a reporter during in vitro encystation, whereas sequences in the 3' flanking region are involved in modulation of steady-state levels of its mRNA during encystation. Encysting Giardia expressing CWP1-GFP chimeras showed formation and maturation of labeled dense granule-like vesicles and subsequent incorporation of GFP-tagged protein into the cyst wall, dependent on which domains of CWP1 were included. The N-terminal domain of CWP1 was required for targeting GFP to regulated compartments of the secretory apparatus, whereas a central domain containing leucine-rich repeats mediated association of the chimera with the extracellular cyst wall. We show that analysis of protein transport using GFP-tagged molecules is feasible in an anaerobic organism and provides a useful tool for investigating the organization of primitive eukaryotic vesicular transport.

Developmental gene regulation in Giardia lamblia: first evidence for an encystation-specific promoter and differential 5' mRNA processing

Giardia lamblia must encyst to survive in the environment and subsequently infect new hosts. We investigated the expression of glucosamine-6-phosphate isomerase (Gln6PI), the first enzyme required for biosynthesis of N-acetylgalactosamine, for the major cyst wall polysaccharide. We isolated two Gln6PI genes that encode proteins with large areas of identity, but distinctive central and terminal regions. Both recombinant enzymes have comparable kinetics. Interestingly, these genes have distinct patterns of expression. Gln6PI-A has a conventional, short 5' untranslated region (UTR), and is expressed at a low level during vegetative growth and encystation. The Gln6PI-B gene has two transcripts - one is expressed constitutively and the second species is highly upregulated during encystation. The non-regulated Gln6PI-B transcript has the longest 5'-UTR known for Giardia and is 5' capped or blocked. In contrast, the Gln6PI-B upregulated transcript has a short, non-capped 5'-UTR. A small promoter region (< 56 bp upstream from the start codon) is sufficient for the regulated expression of Gln6PI-B. Gln6PI-B also has an antisense overlapping transcript that is expressed constitutively. A shorter antisense transcript is detected during encystation. This is the first report of a developmentally regulated promoter in Giardia, as well as evidence for a potential role of 5' RNA processing and antisense RNA in differential gene regulation.

TechLab and alexon Giardia enzyme-linked immunosorbent assay kits detect cyst wall protein 1

A Giardia lamblia antigen detected by the TechLab Giardia Test (TechLab, Inc., Blacksburg, Va.) and the Alexon ProSpecT Giardia microplate assay (Alexon, Inc., Sunnyvale, Calif.) was purified by immunoaffinity chromatography from supernatant fluids of encystment cultures. Two major proteins (Mr 22,000 and 26,000) were observed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Coomassie staining that did not resemble the GSA65 antigen reportedly detected by the Alexon test. These proteins reacted intensely with the monoclonal antibodies used in both commercial enzyme-linked immunosorbent assays (ELISAs). Both proteins had identical N-terminal amino acid sequences and were identified as cyst wall protein 1 (CWP1). The 26-kDa form appeared early during encystment followed by the appearance of the 22-kDa form. Recombinant CWP1 (Mr 26,000) was strongly positive in both commercial tests. CWP1 was stable in human stool specimens, resistant to degradation by proteases and N- and O-glycanases, and unaffected by oxidation with sodium periodate. Two minor proteins with Mrs of 32,000 and 39,000 were detected in CWP1 preparations by using a sensitive fluorescent protein stain. Both were identified as CWP2, and neither reacted with the monoclonal antibodies from the commercial tests. We analyzed 535 stool specimens for CWP1 by using both commercial ELISAs and resolved discrepant results by using routine ova and parasite examination (O&P) and on immunofluorescence antibody assay. The presence of CWP1 correlated well between both ELISAs (98.7% correlation). Our results demonstrate that both commercial ELISAs detect CWP1, which is a useful diagnostic marker because it is highly stable, is secreted in large amounts by encysting trophozoites, and correlates well with O&P.

The Molecular Mechanisms of Giardia Encystation

The protozoan parasite Giardia lamblia is transmitted as an environmentally resistant cyst. The encystation process is attracting attention not only from the viewpoint of disease transmission, but also as a model for differentiation. Here, Hugo Luján, Michael Mowatt and Theodore Nash discuss molecular events underlying this process, including the induction of expression and transport of cyst wall proteins and the induction of Golgi-like activity. They also propose that the signal for encystation derives from cholesterol deprivation in the lower small intestine.

Cloning of two putative Giardia lamblia glucosamine 6-phosphate isomerase genes only one of which is transcriptionally activated during encystment

The biosynthesis of the carbohydrate component of the cyst wall of the protozoan parasite Giardia lamblia, a polymer of N-acetylgalactosamine (GalNac), is by a pathway that is initiated with the conversion of fructose 6-phosphate to glucosamine 6-phosphate by an aminating isomerase, glucose 6-phosphate isomerase. This enzyme appears only after Giardia trophozoites are induced to start the production of cyst wall components after bile is added. To investigate whether induction of glucosamine 6-phosphate isomerase is by protein modification or by transcription activation, its gene was cloned and sequenced. Two genes, gpi1 and gpi2, encoding putative glucosamine 6-phosphate isomerases were identified but one, gpi1 was expressed. The transcript for gpi1 appeared not earlier than 6 h after cells were induced with bile salts. These results show that the first enzyme in the pathway leading to GalNac synthesis in encysting Giardia cyst wall biosynthesis is under transcriptional control.

Trichomonas vaginalis and Tritrichomonas foetus: expression of chitin at the cell surface

The expression of chitin as a structural component of Trichomonas vaginalis and Tritrichomonas foetus was demonstrated by using enzymatic hydrolysis by recombinant (rec-) chitinase, chemical analysis, lectin, fluorescent Calcofluor and antibody binding, glycosidases of known specificity, high-performance liquid chromatography (HPLC), and flow cytometry. Chitinous structures were characterized by their insolubility in hot alkali and by releasing glucosamine on hydrolysis with 6 N HCl. N,N'-Diacetylchitobiose and N,N,'N''-triacetylchitotriose were identified by HPLC as enzymatic hydrolysis products of the alkali-resistant polysaccharide. The location of chitin on the surface of T. vaginalis and T. foetus was inferred from the decreased reactivity with whole parasites of ligands such as Lycopersicon esculentum (TOL) and Solanum tuberosum lectins, fluorescent Calcofluor, and anti-chitin antibody, after cell treatment with rec-chitinase. Binding of [125I]TOL showed that, in T. vaginalis and T. foetus, the numbers of lectin receptors per cell were 4.2 x 10(5) and 3.0 x 10(5), respectively. Binding of the lectin to the trichomonad surface was markedly decreased by treatment with rec-chitinase. TOL interaction with the parasites was not affected by N-acetyl-beta-D-glucosaminidase treatment, showing that the lectin receptors consisted of beta-linked GlcNAc polymers and not of terminal beta-linked GlcNAc residues.

Mechanisms of Giardia lamblia differentiation into cysts

Microbiologists have long been intrigued by the ability of parasitic organisms to adapt to changes in the environment. Since most parasites occupy several niches during their journey between vectors and hosts, they have developed adaptive responses which allow them to survive under adverse conditions. Therefore, the life cycles of protozoan and helminthic parasites are excellent models with which to study numerous mechanisms involved in cell differentiation, such as the regulation of gene expression, signal transduction pathways, and organelle biogenesis. Unfortunately, many of these studies are very difficult because the conditions needed to elicit developmental changes in parasites remain undetermined in most cases. Recently, several interesting findings were reported on the process of differentiation of Giardia lamblia trophozoites into cysts. G. lamblia is a flagellated protozoan that inhabits the upper small intestine of its vertebrate host and is a major cause of enteric disease worldwide. It belongs to the earliest identified lineage among eukaryotes and therefore offers a unique insight into the progression from primitive to more complex eukaryotic cells. The discovery of a specific stimulus that induces trophozoites to differentiate into cysts, the identification and characterization of encystation-specific molecules, the elucidation of novel biochemical pathways, and the development of useful reagents and techniques have made this parasite an excellent model with which to study differentiation in eukaryotic cells. In this review, we summarize the most recent fundings on several aspects of Giardia differentiation and discuss the significance of these findings within the context of current knowledge in the field.

A primitive enzyme for a primitive cell: the protease required for excystation of Giardia

Protozoan parasites of the genus Giardia are one of the earliest lineages of eukaryotic cells. To initiate infection, trophozoites emerge from a cyst in the host. Excystation is blocked by specific cysteine protease inhibitors. Using a biotinylated inhibitor, the target protease was identified and its corresponding gene cloned. The protease was localized to vesicles that release their contents just prior to excystation. The Giardia protease is the earliest known branch of the cathepsin B family. Its phylogeny confirms that the cathepsin B lineage evolved in primitive eukaryotic cells, prior to the divergence of plant and animal kingdoms, and underscores the diversity of cellular functions that this enzyme family facilitates.

Changes in lipid composition during in vitro encystation and fatty acid desaturase activity of Giardia lamblia

Lipids of axenically-cultured Giardia lamblia trophozoites were compared with those of cells undergoing in vitro encystation. Although the lipid composition of the organisms grossly resembled those of low-bile or high-bile culture media, differences were clearly detected. Encysting trophozoites incubated in a high-bile medium for 24 h had a higher concentration of unsaturated fatty acids in the total cellular lipids than did nonencysting trophozoites. The organism, but not the medium, contained linoleate and linolenate, suggesting that G. lamblia desaturates oleate. The presence of a fatty acid desaturase activity in the organism was demonstrated by the conversion of a radiolabeled monounsaturated fatty acid (oleate) to radiolabeled polyunsaturated fatty acids. Triglycerides, a common form of storage lipids, were unusually low in G. lamblia, but steryl esters (which can also serve as reserves) were abundant. Steryl esters increased during encystation of G. lamblia. The changes observed in G. lamblia lipids (increased fatty acid unsaturation and the accumulation of storage lipids) are consistent with parasite differentiation into a cyst stage that is able to survive outside the host at reduced temperatures and reduced available nutrient resources. This study also demonstrated that G. lamblia not only has the capacity to de novo synthesize isoprenoid lipids (ubiquinone, prenylated proteins), but it can also metabolize fatty acids by the addition of double bonds.

Formation of the Giardia cyst wall: studies on extracellular assembly using immunogold labeling and high resolution field emission SEM

Encystment of the intestinal protozoan, Giardia, is a key step in the life cycle that enables this parasite to be transmitted from host to host via either fecal oral, waterborne, or foodborne transmission. The process of encystment was studied by localizing cyst wall specific antigens with immunofluorescence for light microscopy and immunogold staining for field emission scanning electron microscopy. Chronological sampling of Giardia cultures stimulated with endogenous bile permitted identification of an intracellular and extracellular phase in cyst wall formation, a process which required a total of 14-16 h. The intracellular phase lasted for 8-10 h, while the extracellular phase, involved the appearance of cyst wall antigen on the trophozoite membrane, and the assembly of the filamentous layer, a process requiring an additional 4-6 h for completion of mature cysts. The extracellular phase was initiated with the appearance of cyst wall antigen on small protrusions of the trophozoite membrane (approximately 15 nm), which became enlarged with time to caplike structures ranging up to 100 nm in diameter. Caplike structures involved with filament growth were detected over the entire surface of the trophozoite including the adhesive disc and flagella. Encysting cells rounded up, lost attachment to the substratum, and became enclosed in a layer of filaments. Late stages in encystment included a "tailed" cyst in which flagella were not fully retracted into the cyst. Clusters of cysts were seen in which filaments at the surface of one cyst were connected with the surface of adjacent cysts or the "tailed" processes of adjacent cysts, suggesting that the growth of cyst wall filaments may be at the terminal end. In conclusion, the process of encystment has been shown to consist of two morphologically different stages (intracellular and extracellular) which requires 16 h for completion. Further investigation of the extracellular stage with regard to assembly of the filamentous layer of the cyst wall may lead to innovative methods for interfering with production of an intact functional cyst wall, and thereby, regulation of viable Giardia cyst release from the host.

Inhibition of Giardia lamblia excystation by antibodies against cyst walls and by wheat germ agglutinin

Although excystation is crucial to the initiation of infection by Giardia lamblia, little is known about the regulation of this important process. We have been able to reliably induce excystation in vitro by mimicking cyst passage through the stomach and upper small intestine by the exposure of in vitro-derived cysts to an acidic, reducing environment (stage I) followed by protease treatment at a slightly alkaline pH (stage II). Preexposure of cysts to polyclonal rabbit antiserum against purified cyst walls (PCWs) or to wheat germ agglutinin (WGA) inhibited excystation by > 90%. Adsorption of either ligand with PCWs eliminated inhibition, demonstrating specificity for cyst wall epitopes. Inhibition by WGA was reversed by either chitotriose or sialic acid, while inhibition by polyclonal antibodies against PCWs (anti-PCW) was reversed only by sialic acid, which also inhibited binding of both ligands to intact cysts and to cyst wall antigens in immunoblots. Binding of anti-PCW did not affect acidification of cyst cytoplasm during stage I. Exposure of cysts to anti-PCW and WGA prior to, but not after, stage II was sufficient to inhibit excystation, and inhibition could be partially reversed by increasing the protease concentration during stage II. A 7- to 10-fold higher proportion of WGA- and anti-PCW-treated cysts than control cysts remained intact after stage II. Our results suggest that these ligands, which bind cyst wall epitopes, inhibit excystation, most likely by interfering with proteolysis of cyst wall glycoproteins during stage II.

Characterization and immunoprotective properties of a monoclonal antibody against the major oocyst wall protein of Eimeria tenella

The oocyst wall of Eimeria spp. consists of a 10-nm-thick outer lipid layer and a 90-mm-thick inner layer of glycoprotein which has been described previously to be composed of a single major protein. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions and (125)I labelling of a oocyst wall fragments and of delipidated intact oocysts revealed a molecule of approximately 12 kDa as the major protein component of the oocyst wall of Eimeria tenella. An immunoglobulin M monoclonal antibody (c11B9F3) was produced against this 12-kDa oocyst wall protein sliced from a preparative SDS-polyacrylamide gel. Its reactivity by immunofluorescence against oocyst wall fragments and sporozoites or by immunoperoxidase assays of infected tissue sections was stage restricted to gametocytes and oocysts but pan-specific against all face of the oocyst wall. In chicks passively immunized with C11B9F3, oocyst output was significantly (P<0.01) reduced by 42 to 54% after homologous E. tenella infection and by 35% after heterologous Eimeria maxima infection compared with that of control groups. The results demonstrate the presence of a highly conserved, low-molecular-weight antigen on the oocyst wall and the gametocytes of Eimeria spp. which is a candidate for inclusion in a pan-specific, transmission-blocking vaccine against avian coccidiosis.

Identification of a novel Giardia lamblia cyst wall protein with leucine-rich repeats. Implications for secretory granule formation and protein assembly into the cyst wall

Giardia lamblia trophozoites, like most intestinal parasitic protozoa, undergo fundamental biological changes to survive outside the intestine of their mammalian host by differentiating into infective cysts. This complex process entails the coordinated production, processing, and transport of cyst wall constituents for assembly into a protective cyst wall. Yet, little is known about this process and the identity of cyst wall constituents. We previously identified a 26-kDa cyst wall protein, CWP1. In the present work, using monoclonal antibodies to cyst wall antigens, we cloned the gene that encodes a novel 39-kDa cyst wall protein, CWP2. Expression of CWP1 and CWP2 was induced during encystation with identical kinetics. Soon after synthesis, these two proteins combine to form a stable complex, which is concentrated within the encystation-specific secretory granules before incorporation into the cyst wall. Both proteins contain five tandem copies of a 24-residue leucine-rich repeat, a motif implicated in protein-protein interactions. Unlike CWP1, CWP2 has an extremely basic 121-residue COOH-terminal extension that might be involved in the sorting of these proteins to the secretory granules.

A lipoprotein-cholesterol-albumin serum substitute stimulates Giardia lamblia encystation vesicle formation

We found previously that the A6 clone of Giardia lamblia strain WB that did not encyst in vitro was blocked at an early stage in differentiation, as it did not form encystation secretory vesicles (ESV) efficiently or express cyst antigens, in comparison with the related clone C6. We now report that A6 formed ESV normally in the suckling mouse model. Therefore, we asked whether our serum-containing encystation media might lack a stimulus or component or contain an inhibitor of ESV formation to which A6 was especially sensitive. We found that replacing bovine serum with a lipoprotein-cholesterol solution and bovine serum albumin (LPC) in pre-encystation and encystation media increased ESV formation by both A6 and C6. The % of A6 cells with ESV increased from 8% in BS medium to 48% in LPC medium, compared with 64% and 98% for C6. Similarly, the average number of ESV/positive cell increased from 1.5 in BS medium to 7.7 in LPC medium for A6, and from 13.3 to 19.7 for C6. Moreover, in LPC encystation media, A6 expressed the cyst wall epitope recognized by monoclonal GCSA-1. Although formation of water-resistant cysts by A6 was increased > 60 fold in LPC media, the numbers of cysts remained only approximately 3-15% that of C6. This suggests that LPC may primarily affect early events in encystation and that A6 may require additional factors later in encystation.

Developmentally regulated expression of a Giardia lamblia cyst wall protein gene

The protozoan Giardia lamblia is an obligate parasite of the mammalian small intestine. We studied the expression of a gene that encodes a protein component of the cyst wall, a complex structure assembled during the differentiation of trophozoites to cysts and which is critical to survival of the parasite outside its mammalian host. Transcripts from the cyst wall protein gene increase more than 100-fold during encystation, reaching a maximum between 5 and 24 hours after induction. Cyst wall protein expression also increases dramatically during encystation, and, prior to its incorporation into the nascent cyst wall, the protein is contained within the encystation-specific vesicles of encysting trophozoites. The sequence of the cloned gene predicts an acidic, leucine-rich polypeptide of M(r) 26,000 that contains 5.3 tandemly arranged copies of a degenerate 24-amino-acid repeat. A hydrophobic amino-terminal peptide probably serves as the initial signal that targets this protein to a secretory pathway involving vesicular localization during encystation and, ultimately, secretion to form the cyst wall.

Oxygen uptake in cysts and trophozoites of Giardia lamblia

Oxygen uptake in cysts and trophozoites of the parasitic protozoan Giardia lamblia was examined. Both showed oxygen uptake activity, but that of cysts was only 10% to 20% that of trophozoites. Oxygen dependence of oxygen uptake in cysts and trophozoites showed oxygen maxima above which oxygen uptake decreased. The oxygen concentration at which the oxygen uptake rate was greatest was higher for trophozoites than for cysts. The effect of various inhibitors on cyst and trophozoite oxygen uptake suggested that flavoproteins and quinones play some role in oxygen uptake. The substrate specificities and the effect of inhibitors on G. lamblia trophozoites were similar to those observed for G. muris. Metronidazole, the drug most commonly used in treatment of giardiasis, inhibited oxygen uptake and motility in trophozoites; however, it had no obvious effect on either oxygen uptake or excystation in cysts. Menadione, a redox cycling naphthaquinone, first stimulated, then completely inhibited, oxygen uptake in cysts and trophozoites; a complete loss of cyst viability and trophozoite motility was also observed. The effect of menadione on G. lamblia may indicate that redox cycling compounds have potential as chemotherapeutic agents for the treatment of giardiasis.

Galactosamine-synthesizing enzymes are induced when Giardia encyst

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.