Purification and characterization of the adenine phosphoribosyltransferase and hypoxanthine-guanine phosphoribosyltransferase activities from Leishmania donovani

The adenine phosphoribosyltransferase (APRTase) and hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) activities from promastigotes of Leishmania donovani have been purified to homogeneity using ammonium sulfate precipitation, DEAE-cellulose exclusion, and either AMP-agarose (APRTase) or GTP-agarose (HGPRTase) affinity chromatography. The specific activities of the affinity-purified APRTase and HGPRTase fractions were 326-fold and 1341-fold greater than those in the 40-80% ammonium sulfate precipitate, respectively. The purified APRTase migrated as a single band on sodium dodecyl sulfate (SDS) polyacrylamide gels with a size of 29 kDa, while HGPRTase was also determined to be homogeneous by SDS gel electrophoresis with a size of 24 kDa. In addition, a mutant cell line, APPB2, partially deficient in APRTase activity, still contained quantities of purifiable APRTase protein, while a clonal secondary derivative of the APPB2 cell line that is completely deficient in APRTase activity, APPB2-640A3, failed to express purifiable APRTase protein. The homogeneous enzymes possessed apparent Km values for their nucleobase substrates between 2.0 and 5.0 microM, and both enzymes were inhibited by their immediate or ultimate reaction endproducts, APRTase by AMP and PPi and HGPRTase by GMP, GTP, and PPi. The generation of homogeneous preparations of APRTase and HGPRTase protein will serve as a prerequisite for the generation of immunological and molecular biological probes to analyze the leishmanial phosphoribosyltransferases.

Purification of hypoxanthine-guanine phosphoribosyltransferase of Plasmodium lophurae

Hypoxanthine-guanine phosphoribosyltransferase (EC 2.4.2.8) was isolated from the malarial parasite, Plasmodium lophurae. The apparent pI, as determined by chromatofocusing, was 7.6. The native molecular weight was 79,000. The pH profile of HGPRT exhibited a broad pH optimum. With hypoxanthine as substrate maximal activity was achieved from pH 6.0-10.0, and with guanine as substrate maximal activity occurred from pH 7.5-9.5. The enzyme exhibited Michaelis-Menten kinetics with all substrates. The Km values were 3.8 microM (hypoxanthine), 2.4 microM (guanine), 6.2 microM (6-mercaptopurine), 7.6 microM (6-thioguanine), and 360 microM (8-azahypoxanthine). 6-Thioinosine, 9-beta-arabinofuranosylhypoxanthine, 6-chloropurine, xanthine and azaguanine were inhibitors of the P. lophurae enzyme. From the substrate and inhibitor data it appears that the sixth position on the purine ring plays a major role in enzyme activity.

Purification and characterization of hypoxanthine-guanine phosphoribosyltransferase from Schistosoma mansoni. A potential target for chemotherapy

Hypoxanthine phosphoribosyltransferase and guanine phosphoribosyltransferase activities are essential for the supply of guanine nucleotides in Schistosoma mansoni schistosomules. In crude extracts of adult S. mansoni, these two activities co-elute in size exclusion, ion exchange, and chromatofocusing chromatography and exhibit similar stabilities to heat treatment, suggesting that they are associated in one enzyme protein hypoxanthine-guanine phosphoribosyltransferase. This enzyme has been purified by a combination of heat treatment at 85 degrees C and chromatofocusing chromatography with elution at an apparent pI of 5.27 +/- 0.15. Pore gradient electrophoresis of the native enzyme followed by subsequent activity staining demonstrate an enzyme molecular weight of 105,000. The activity staining pattern remains the same whether hypoxanthine or guanine is used as the substrate, further supporting the existence of a single protein, hypoxanthine-guanine phosphoribosyltransferase. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified protein results in a single protein band with a subunit molecular weight estimate of 64,000, suggesting that the native enzyme is a dimer. Preliminary kinetic studies showed that the purified hypoxanthine-guanine phosphoribosyltransferase reacted with guanine at a rate twice as fast as it did with hypoxanthine, but it did not act on xanthine at all. A full-length mouse neuroblastoma hypoxanthine-guanine phosphoribosyltransferase cDNA clone pHPT5 and a plasmid pSV2-gpt containing the xanthine-guanine phosphoribosyltransferase gene for Escherichia coli were utilized as probes on Southern blots of S. mansoni DNA digests, and no significant hybridization was found under relatively relaxed conditions. Polyclonal antibodies made against human erythrocyte hypoxanthine-guanine phosphoribosyltransferase and E. coli xanthine-guanine phosphoribosyltransferase were tested in enzyme-linked immunosorbent assays of S. mansoni protein extracts, and no detectable cross-reacting protein was found. S. mansoni hypoxanthine-guanine phosphoribosyltransferase thus may bear rather limited homology to mammalian hypoxanthine-guanine phosphoribosyltransferase or bacterial xanthine-guanine phosphoribosyltransferase and could be an attractive target for antischistosomal chemotherapeutic drug design.

Human brain hypoxanthine guanine phosphoribosyltransferase: structural and functional comparison with erythrocyte hypoxanthine guanine phosphoribosyltransferase

A rapid and simple method, based on GMP Sepharose affinity chromatography, was used for the purification of human brain hypoxanthine guanine phosphoribosyltransferase. A single protein band was detected by polyacrylamide gel electrophoresis of the native purified enzyme. A subunit molecular weight of 25,000 was estimated by SDS gel electrophoresis. The Km values for hypoxanthine and phosphoribosyl pyrophosphate were 50 and 111 microM, respectively. The Ki values for GMP and IMP with phosphoribosyl pyrophosphate were 21 and 37 microM, respectively. The purified enzyme from human brain did not differ significantly from the human erythrocyte one in amino acid composition. The brain and erythrocyte hypoxanthine guanine phosphoribosyltransferases showed complete immunochemical identity on Ouchterlony double diffusion.

Purification and characterisation of chicken brain hypoxanthine-guanine phosphoribosyltransferase

Hypoxanthine-guanine phosphoribosyltransferase enzyme (EC 2.4.2.8) from chicken brain has been purified 10 000-fold to homogeneity. The molecular mass of the native enzyme is 85 kDa, with four subunits, each of 26 kDa, and exerts its maximum activity at pH 10.0. The Km values for hypoxanthine and guanine are 5.2 and 1.8 microM, respectively. The half-life of the enzyme is 30 min at 85 degrees C. Monoclonal antibodies were raised against the native purified enzyme and were used for purification of enzyme to homogeneity. The monoclonal antibody did not bind to the active centre of the enzyme.

Hypoxanthine phosphoribosyltransferase from human brain: purification and partial characterization

A facile and rapid purification procedure, based upon the heat denaturation of extraneous proteins and GMP-Sepharose affinity chromatography, has been used to purify hypoxanthine phosphoribosyltransferase from human brain. A homogeneous enzyme preparation, as judged by sodium dodecyl sulfate and gradient polyacrylamide gel electrophoresis, was obtained. The subunit molecular weight of the enzyme was estimated as 24,000 by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The native molecular weight, determined by gradient gel electrophoresis, was approximately 100,000. These results suggest human brain hypoxanthine phosphoribosyltransferase is a tetramer, consistent with recent results reported for the human erythrocyte enzyme. At least three charge variant forms of the human brain enzyme were distinguished by nondenaturing polyacrylamide gel electrophoresis, electrofocusing, and chromatofocusing. Acidic pI values of approximately 5.7, 5.5, and 5.0 were estimated for the three major species.

Studies of the kinetic mechanism of hypoxanthine-guanine phosphoribosyltransferase from yeast

An assay procedure, utilizing high pressure liquid chromatography, has been designed which allows both reactions catalyzed by hypoxanthine-guanine phosphoribosyltransferase to be monitored simultaneously. Using this procedure and the theories described by Huang (Huang, C. V. (1979) Methods. Enzymol. 63, 486-500) for alternate substrate kinetic analysis, we have determined that purified hypoxanthine-guanine phosphoribosyltransferase from yeast catalyzes the formations of both IMP and GMP through the use of an Ordered Bi Bi kinetic mechanism, and that guanine is highly preferred over hypoxanthine as substrate in the forward reaction. This proposed kinetic mechanism has been confirmed using flow dialysis experiments in which a binary enzyme-5-phosphoribosyl-alpha-1-pyrophosphate complex was characterized but where enzymic complexes, with either guanine or hypoxanthine, were not detected. Also consistent with this kinetic mechanism was our observation that an exchange of label between [14C]guanine or [14C]hypoxanthine and their respective nucleotides (GMP and IMP) was not catalyzed by hypoxanthine-guanine phosphoribosyltransferase. However, a significant exchange of label between [32P]pyrophosphate and 5-phosphoribosyl-alpha-1-pyrophosphate is observed upon incubation with this enzyme, suggesting that hypoxanthine-guanine phosphoribosyltransferase may exist, in part, as a phosphoribosyl-enzyme complex in the presence of 5-phosphoribosyl-alpha-1-pyrophosphate.

Human hypoxanthine-guanine phosphoribosyltransferase. Purification and characterization of mutant forms of the enzyme

Erythrocyte hypoxanthine-guanine phosphoribosyltransferase has been highly purified from five unrelated patients with a deficiency of this enzyme. Affinity chromatography using either GMP-Sepharose or an immunoadsorbent was the most productive step in the purifications. The specific activity of the purified enzyme was unchanged for patients L. P. and G. S., and slightly decreased for patient R. H., as compared to control subjects. Enzyme from patient I. V. and from patient E. S. exhibited markedly reduced specific activities when purified to near homogeneity. The level of immunoreactive protein in patient I. V. appeared to be significantly higher than normal. The apparent subunit molecular weight of the enzyme from patient G. S. was decreased by approximately 1000 while it was increased by approximately 400 from patient I. V. The isoelectric points of the subunit isozymes were shifted to higher pH values from patients I. V. and E. S., and to lower pH values from patient L. P.; the subunit isozymes from patient G. S. were identical with normal. These studies provide direct evidence for the existence of at least four different mutations in the structural gene for hypoxanthine-guanine phosphoribosyltransferase. The four different mutant forms of human hypoxanthine-guanine phosphoribosyltransferase that have been identified are named as follows: patient L. P., HPRTToronto; patient G. S., HPRTLondon; patient E. S., HPRTKinston; and patient I. V., HPRTMunich.

Purification and characterization of hypoxanthine-guanine phosphoribosyltransferase from Saccharomyces cerevisiae

Hypoxanthine-guanine phosphoribosyltransferase (HPRT) was purified 12 000-fold to homogeneity from yeast by a three-step procedure including acid precipitation, anion-exchange chromatography, and guanosine 5' -monophosphate affinity chromatography. The enzyme is a dimer consisting of two, probably identical, subunits of Mr 29 500. The enzyme recognized hypoxanthine and guanine, but not adenine or xanthine, as substrates. An antiserum against both native and denatured enzyme has been raised and shown to be specific for the enzyme. The antiserum has no affinity for Chinese hamster or human HPRT but does recognize subunits of yeast HPRT as well as some cyanogen bromide fragments of the enzyme.

Purification and characterization of hypoxanthine/guanine phosphoribosyltransferase in bovine snout epidermis

Hypoxanthine/guanine phosphoribosyltransferase was purified from bovine snout epidermis, about 600-fold by a combination method of centrifugation, ammonium sulfate fraction, Sephadex G-200 and DEAE cellulose chromatography. Enzymatic properties of the purified enzyme were determined as follows: pH optimum 7.2, temperature optimum 56 degrees C, and 82,000 in molecular weight. In the presence of phosphoribosyl pyrophosphate, the enzyme was extremely heat-stable. The enzyme displayed Michaelis-Menten kinetics with apparent Michaelis constants for hypoxanthine, guanine and phosphoribosyl pyrophosphate of 1.59, 20.4 and 72.6 microM respectively.

Studies of an unusually basic hypoxanthine-guanine phosphoribosyltransferase

Hypoxanthine-guanine phosphoribosyltransferase (EC 2.4.2.8) from beef brain has been purified 3100-fold to apparent homogeneity using a purification procedure based on GMP-Sepharose affinity chromatography. The native enzyme has a molecular weight of 84,000 as determined by gel filtration studies. A subunit molecular weight of 26,000 was obtained by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, suggesting that the enzyme is a trimer. Two forms of the enzyme have been separated by nondenaturing polyacrylamide gel electrophoresis and isoelectric focusing. Basic pI values of 7.85 and 8.10 were obtained for the two forms. These values are much higher than have been observed with any other purified phosphoribosyltransferase. The amino acid composition of the enzyme is 18 Lys, 6 His, 9 Arg, 1 Trp, 6 Cys, 28 Asx, 12 Thr, 16 Ser, 19 Glx, 10 Pro, 23 Gly, 16 Ala, 17 Val, 5 Met, 11 Ile, 19 Leu, 9 Tyr, and 8 Phe. An unusual basic amino acid, yet to be identified, was also present. The enzyme exhibits Km values of 0.42 microM for guanine, 0.99 microM for hypoxanthine, 18.6 microM for P-Rib-PP in the presence of guanine, and 2.9 microM for P-Rib-PP in the presence of hypoxanthine.

Characterization of the subunit composition of HGPRTase from human erythrocytes and cultured fibroblasts

Hypoxanthine-guanine phosphoribosyltransferase is a ubiquitous human enzyme, the inherited deficiency of which leads to a specific metabolic-neurological syndrome. Native acrylamide isoelectric focusing revealed that the human enzyme consists of different numbers of isoenzymes depending on the tissue of origin. The erythrocytic enzyme has the most isoenzymes while the enzyme from cultured fibroblasts has only a single isoenzyme. The isoenzyme pattern of the erythrocytic enzyme changes on storage of the crude hemolysate at 4 C. Treatment of the stored crude hemolysate with 4.5 M urea and 0.35 mM beta-mercaptoethanol results in an isoenzyme pattern similar to that of the fresh crude extract. Thus the additional isoenzymes are generated on storage not by covalent modification of the enzyme but probably by binding of small molecules to the enzyme or to association of the enzyme molecules. Hypoxanthine-guanine phosphoribosyltransferase has been purified to 80% homogeneity in three steps, DEAE Sephadex chromatography, heat treatment at 85 C for 5 min, and hydroxylapatite chromatography. Denaturing two-dimensional gel electrophoresis of the erythrocytic enzyme revealed that the erythrocytic enzyme is composed of three major types of subunits (1-3) with the same molecular weight but different isoelectric points. In contrast, the fibroblast enzyme is composed of only a single type of subunit, which comigrates with subunit 1 of the erythrocytic enzyme. Since there is a single genetic locus in humans for HGPRTase (the enzyme is X linked) (Nyhan et al., 1967), the observed subunit modification of the erythrocyte enzyme appears to be the result of posttranslational modification. These findings provide a simple explanation for the observed electrophoretic properties of human HGPRTase. A patient with 0.5% of HGPRTase activity in his erythrocytes was found to have small amounts (greater than 0.5% but less than 5% of normal) of the erythrocytic HGPRTase subunits.

Purification and characterization of the hypoxanthine-guanine phosphoribosyltransferase from Saccharomyces cerevisiae

1. Hypoxanthine-guanine phosphoribosyltransferase (EC 2.4.2.8) from Saccharomyces cerevisiae was purified 9400-fold by affinity chromatography giving rise to an electrophoretically homogeneous preparation. 2. The molecular weight of the enzyme was determined by gel filtration with Sephadex G-100 and by sodium dodecylsulfate gel electrophoresis. Both methods reveal a molecular weight of 51,000. 3. The enzyme requires Mg2+ and has its pH optimum at 8.5. 4. Isoelectric focussing as well as gel electrophoresis of the purified extract reveals a single band which exhibits enzyme activity. The isoelectric point of the enzyme is 5.1. 5. The enzyme displays Michaelis-Menten kinetics with apparent Michaelis constants for hypoxanthine, guanine and phosphoribosylpyrophosphate of 23 microns, 18 microns, and 50 microns respectively.

Hypoxanthine guanine phosphoribosyltransferase (HGPRT) in Gilles de la Tourette syndrome

Hypoxanthine guanine phosphoribosyltransferase (HGPRT) and adenosine phosphoribosyltransferase (APRT) were examined from 11 individuals with Gilles de la Tourette syndrome, 10 of their first- or second-degree relatives, and 3 normal controls. It has been suggested that in some self-mutilating Tourette patients, HGPRT shows a time-related loss of activity at 4 degrees C, and an unusual isoelectrofocusing pattern. Although 3 patients experienced self-mutilation, no consistent abnormalities were found in the temperature-stability of their HGPRT at 4 degrees C and 70 degrees C, or in isoelectrofocusing of HGPRT purified by immunoprecipitation. An alteration of the purine metabolic pathway in Tourette syndrome has not been established.

Evidence against the existence of real isozymes of hypoxanthine phosphoribosyltransferase

A method for reducing the degree of heterogeneity in the electrophoretic enzyme activity pattern of hypoxanthine phosphoribosyltransferase preparations by incubation with a (magnesium) phosphoribosyl diphosphate substrate is described. Hypoxanthine phosphoribosyltransferase was isolated from human erythrocytes and Chinese hamster livers. A subunit molecular weight of 26000--27000 as reported by other authors was obtained for both enzymes by gel electrophoresis in the presence of dodecylsulfate. Gradient gel electrophoresis revealed that the native enzymes mainly have a molecular weight of 105000--110000 and are thus apparently tetrameric, when held in the active state by the presence of phosphoribosyl diphosphate. The dimeric enzyme with a molecular weight of 52000--55000, was also found under other conditions. The trimer occurred only in the absence of phosphoribosyl diphosphate, for instance by glycerol gradient centrifugation. The enzyme from human erythrocytes was partly degraded during purification in the absence of a protease inhibitor. The purified enzyme has a very low protease contamination level. Proteolysis is an additional cause of heterogeneity and might therefore explain earlier conflicting results. Since the heterogeneous nature of hypoxanthine phosphoribosyltransferase is caused only by the secondary processes of dissociation/association and, in the case of the human erythrocyte enzyme, degradation, we suggest that the use of the term 'isozyme' to describe the different forms should be avoided.

Human hypoxanthine-guanine phosphoribosyltransferase. Evidence for tetrameric structure

Hypoxanthine-guanine phosphoribosyltransferase (EC 2.4.2.8) has been purified 23,000-fold from normal human erythrocytes. The purification includes affinity chromatography on a GMP column. The subunit molecular weight of the enzyme obtained from this purification is 24,000. The finding of four protein species after cross-linkage of the highly purified enzyme with dimethylsuberimidate, dimethyladipimidate, and glutaraldehyde suggests that the enzyme may exist in the native state as a tetramer.