Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UMLS:C0017536 (giardiasis)
 1,714 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Giardia lamblia, a flagellated parasitic protozoan and the causative agent of giardiasis, lacks de novo purine biosynthesis and exists on salvage of adenine and guanine by adenine phosphoribosyltransferase and guanine phosphoribosyltransferase. Guanine phosphoribosyltransferase from G. lamblia crude extracts has been purified to apparent homogeneity by Sephacryl S-200 gel filtration followed by C-8-GMP-agarose and 2',3'-GMP-agarose affinity chromatography, resulting in an overall recovery of 77% and a purification of 83,000-fold. The molecular weight of the native enzyme as estimated by gel filtration and isokinetic sucrose gradients was found to be 58,000-63,000, with a subunit molecular weight of approximately 29,000, as shown by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Mono P chromatofocusing chromatography gives rise to a major activity peak eluting from the column at a pH of 6.75 and two minor activity peaks at pH of 5.3 and 5.2. Hypoxanthine and xanthine can be recognized by the enzyme as substrates but at Km values 20 times higher than that observed with guanine. G. lamblia guanine phosphoribosyltransferase is immunologically distinct from human hypoxanthine-guanine phosphoribosyltransferase and Escherichia coli xanthine-guanine phosphoribosyltransferase, and G. lamblia DNA fragments are incapable of hybridizing with mouse neuroblastoma hypoxanthine-guanine phosphoribosyltransferase DNA or E. coli xanthine phosphoribosyltransferase DNA under relatively relaxed conditions. All evidence presented suggests that G. lamblia guanine phosphoribosyltransferase may be qualified as a potential target for antigiardiasis chemotherapy.
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PMID:Purification and characterization of guanine phosphoribosyltransferase from Giardia lamblia. 308 75

Purine metabolism in Giardia lamblia was investigated by monitoring incorporation of radiolabeled precursors into purine nucleotides in the log-phase trophozoites cultivated in vitro in axenic media and incubated in buffered saline glucose. The lack of incorporation of formate, glycine, hypoxanthine, inosine, and xanthine into the nucleotide pool suggests the absence of de novo purine nucleotide synthesis and the inability to form IMP as the precursor of AMP and GMP in G. lamblia. Only adenine, adenosine, guanine, and guanosine were incorporated. Further analysis of the labeled nucleotides by HPLC indicated that adenine and adenosine are converted only to adenine nucleotides whereas guanine and guanosine are only incorporated into guanine nucleotides. There is no competition of incorporation between adenine/adenosine and guanine/guanosine, and there is no interconversion between adenine and guanine nucleotides. Results from analyzing [5'-3H]guanosine incorporation indicate that the ribose moiety is not incorporated with the guanine base. Assays of purine salvage enzymic activities in the crude extracts of G. lamblia revealed the presence of only four major enzymes; adenosine and guanosine hydrolases and adenine and guanine phosphoribosyl transferases. Apparently, G. lamblia has an exceedingly simple purine salvage system; it converts adenosine and guanosine to corresponding purine bases and then forms AMP and GMP by the actions of corresponding purine phosphoribosyl transferases. The guanine phosphoribosyl transferase in G. lamblia is interesting because it does not recognize either hypoxanthine or xanthine as substrate. It thus must have a unique substrate specificity and may be regarded as a potential target to attack as a rational approach to chemotherapeutic control of giardiasis.
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PMID:Purine salvage networks in Giardia lamblia. 660 8

Guanine phosphoribosyltransferase from Giardia lamblia, a key enzyme in the purine salvage pathway, is a potential target for anti-giardiasis chemotherapy. Recent structural determination of GPRTase (Shi, W., Munagala, N. R., Wang, C. C., Li, C. M., Tyler, P. C., Furneaux, R. H., Grubmeyer, C., Schramm, V. L., and Almo, S. C. (2000) Biochemistry 39, 6781-6790) showed distinctive features, which could be responsible for its singular guanine specificity. Through characterizing specifically designed site-specific mutants of GPRTase, we identified essential moieties in the active site for substrate binding. Mutating the unusual Tyr-127 of GPRTase to the highly conserved Ile results in 6-fold lower K(m) for guanine. A L186F mutation in GPRTase increased the affinity toward guanine by 3. 3-fold, whereas the corresponding human HGPRTase mutant L192F showed a 33-fold increase in K(m) for guanine. A double mutant (Y127I/K152R) of GPRTase retained the improved binding of guanine and also enabled the enzyme to utilize hypoxanthine as a substrate with a K(m) of 54 +/- 15.5 microm. A triple mutant (Y127I/K152R/L186F) resulted in further increased binding affinity with both guanine and hypoxanthine with the latter showing a lowered K(m) of 29.8 +/- 4.1 microm. Dissociation constants measured by fluorescence quenching showed 6-fold tighter binding of GMP with the triple mutant compared with wild type. Thus, by increasing the binding affinity of 6-oxopurine, we were able to convert the GPRTase to a HGPRTase.
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PMID:Converting the guanine phosphoribosyltransferase from Giardia lamblia to a hypoxanthine-guanine phosphoribosyltransferase. 1097 10