Identification of the active sites of human and schistosomal hypoxanthine-guanine phosphoribosyltransferases by GMP-2',3'-dialdehyde affinity labeling

Pyrimidine metabolism in schistosomes: A comparison with other parasites and the search for potential chemotherapeutic targets

Schistosomes are responsible for the parasitic disease schistosomiasis, an acute and chronic parasitic ailment that affects >240 million people in 70 countries worldwide. It is the second most devastating parasitic disease after malaria. At least 200,000 deaths per year are associated with the disease. In the absence of the availability of vaccines, chemotherapy is the main stay for combating schistosomiasis. The antischistosomal arsenal is currently limited to a single drug, Praziquantel, which is quite effective with a single-day treatment and virtually no host-toxicity. Recently, however, the question of reduced activity of Praziquantel has been raised. Therefore, the search for alternative antischistosomal drugs merits the study of new approaches of chemotherapy. The rational design of a drug is usually based on biochemical and physiological differences between pathogens and host. Pyrimidine metabolism is an excellent target for such studies. Schistosomes, unlike most of the host tissues, require a very active pyrimidine metabolism for the synthesis of DNA and RNA. This is essential for the production of the enormous numbers of eggs deposited daily by the parasite to which the granulomas response precipitates the pathogenesis of schistosomiasis. Furthermore, there are sufficient differences between corresponding enzymes of pyrimidine metabolism from the host and the parasite that can be exploited to design specific inhibitors or "subversive substrates" for the parasitic enzymes. Specificities of pyrimidine transport also diverge significantly between parasites and their mammalian host. This review deals with studies on pyrimidine metabolism in schistosomes and highlights the unique characteristic of this metabolism that could constitute excellent potential targets for the design of safe and effective antischistosomal drugs. In addition, pyrimidine metabolism in schistosomes is compared with that in other parasites where studies on pyrimidine metabolism have been more elaborate, in the hope of providing leads on how to identify likely chemotherapeutic targets which have not been looked at in schistosomes.

Inhibition of erythrocyte phosphoribosyltransferases (APRT and HPRT) by Pb2+: a potential mechanism of lead toxicity

Many reports show that red blood cells of people exposed to lead have a decreased ATP concentration, decreased adenylate energy charge value and many metabolic and morphological abnormalities. Since the synthesis of nucleotides in erythrocytes occurs only through salvage pathways, we hypothesized that a decrease in nucleotide concentrations may be caused by lead-induced inhibition of erythrocyte phosphoribosyltransferases: adenine APRT (EC 2.4.2.7) and hypoxanthine-guanine HPRT (EC 2.4.2.8). These enzymes enable the reutilization of purine bases (adenine, guanine, hypoxanthine) converting them to mononucleotides (AMP, GMP, IMP), substrates for the synthesis of high-energy nucleotides. To confirm the hypothesis two experiments were performed: (i) in vitro, using a lysate of human erythrocytes incubated (5, 10, 30min) with lead ions (100microM, 10microM, 1microM, 500nM, 100nM lead acetate) and 100microM sodium acetate for the control, (ii) in vivo, using a lysate of rat erythrocytes taken from rats chronically exposed to lead (0.1% lead acetate in drinking water for 9 months, resulting in whole blood lead concentration 7microg/dL). The activities of APRT and HPRT were determined using HPLC method, which allowed concurrent determination of the activity of both enzymes in erythrocyte lysates. We have shown that, lead ions: (i) moderately inhibit both phosphoribosyltransferases in erythrocytes, this influence being detectable even at very low concentrations (ii) participate in hemolysis, the intensity of which negatively correlates with the activity of phosphoribosyltransferases. Our results indicate the necessity of further research on the role of lead-induced APRT and HPRT inhibition as one of the mechanisms of lead toxicity.

Potential chemotherapeutic targets in the purine metabolism of parasites

Parasites are responsible for a wide variety of infectious diseases in human as well as in domestic and wild animals, causing an enormous health and economical blight. Current containment strategies are not entirely successful and parasitic infections are on the rise. In the absence of availability of antiparasitic vaccines, chemotherapy remains the mainstay for the treatment of most parasitic diseases. However, there is an urgent need for new drugs to prevent or combat some major parasitic infections because of lack of a single effective approach for controlling the parasites (e.g., trypanosomiasis) or because some serious parasitic infections developed resistance to presently available drugs (e.g., malaria). The rational design of a drug is usually based on biochemical and physiological differences between pathogens and host. Some of the most striking differences between parasites and their mammalian host are found in purine metabolism. Purine nucleotides can be synthesized by the de novo and/or the so-called "salvage" pathways. Unlike their mammalian host, most parasites studied lack the pathways for de novo purine biosynthesis and rely on the salvage pathways to meet their purine demands. Moreover, because of the great phylogenic separation between the host and the parasite, there are in some cases sufficient distinctions between corresponding enzymes of the purine salvage from the host and the parasite that can be exploited to design specific inhibitors or "subversive substrates" for the parasitic enzymes. Furthermore, the specificities of purine transport, the first step in purine salvage, diverge significantly between parasites and their mammalian host. This review highlights the unique transporters and enzymes responsible for the salvage of purines in parasites that could constitute excellent potential targets for the design of safe and effective antiparasitic drugs.

Photoaffinity labeling on magnetic microspheres (PALMm) methodology for topographic mapping: preparation of PALMm reagents and demonstration of biochemical relevance

Photoaffinity labeling (PAL) is a technique widely used for identifying the binding-site within proteins. Although the classic method is both versatile and powerful, it suffers significant disadvantages, such as the need to radiolabel the PAL ligand, and the need to conduct highly complicated separations of both the labeled protein and the labeled peptides derived from it. Here, we propose a novel and universal methodology--Photo-Affinity Labeling on Magnetic microspheres (PALMm) designed to simplify and shorten the PAL protocol. In this context, we describe the preparation of PALMm reagents and the evaluation of their biochemical relevance regarding two ATP-binding enzymes: hexokinase and apyrase.

Unusual substrate specificity of a chimeric hypoxanthine-guanine phosphoribosyltransferase containing segments from the Plasmodium falciparum and human enzymes

Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) catalyzes the phosphoribosylation of hypoxanthine and guanine by transferring the phosphoribosyl moiety from phosphoribosylpyrophosphate (PRPP) on to N9 in the purine base, resulting in the formation of inosine monophosphate (IMP) and guanosine monophosphate (GMP). Xanthine is an additional substrate for the Plasmodium falciparum HGXPRT. Our aim has been to elucidate structural features in HGPRT that govern substrate specificity. We have addressed this problem by engineering chimeric HGPRTs, which contain segments from both the parasite and human enzymes. Four chimeric enzymes were engineered (DS1-DS4), of which the chimeric enzyme, DS1, in which the first 49 residues of human HGPRT were replaced with the corresponding residues from the P. falciparum enzyme, exhibited additional specificity for xanthine. None of the switched residues forms a part of the purine or PRPP binding region in the available crystal structures of HG(X)PRTs. Our data on the chimeric enzyme DS1 provide the first evidence that the N-terminal approximately 50 amino acids, although not proximal to the active site in the crystal structure, can in fact modulate substrate specificity. DS1 exhibits a reduced k(cat) for hypoxanthine and guanine, while its K(m) for these oxopurine bases remains largely unchanged. Its specific activity for xanthine is comparable with hypoxanthine but five times more than that for guanine.

Structural and functional studies of UDP-glucuronosyltransferases

UDP-Glucuronosyltransferases (UGTs) are glycoproteins localized in the endoplasmic reticulum (ER) which catalyze the conjugation of a broad variety of lipophilic aglycon substrates with glucuronic acid using UDP-glucuronic acid (UDP-GIcUA) as the sugar donor. Glucuronidation is a major factor in the elimination of lipophilic compounds from the body. In this review, current information on the substrate specificities of UGT1A and 2B family isoforms is discussed. Recent findings with regard to UGT structure and topology are presented, including a dynamic topological model of UGTs in the ER. Evidence from experiments on UGT interactions with inhibitors directed at specific amino acids, photoaffinity labeling, and analysis of amino acid alignments suggest that UDP-GIcUA interacts with residues in both the N- and C-terminal domains, whereas aglycon binding sites are localized in the N-terminal domain. The amino acids identified so far as crucial for substrate binding and catalysis are arginine, lysine, histidine, proline, and residues containing carboxylic acid. Site-directed mutagenesis experiments are critical for unambiguous identification of the active-site architecture.

The conserved serine-tyrosine dipeptide in Leishmania donovani hypoxanthine-guanine phosphoribosyltransferase is essential for catalytic activity

Crystal structures of hypoxanthine-guanine phosphoribosyltransferase (HGPRT) proteins have implied that the translocation of a flexible loop containing a highly conserved Ser-Tyr dipeptide is necessary for the protection of the proposed oxocarbonium ion transition state of the enzyme (Eads, J. C., Scapin, G. T., Xu, Y., Grubmeyer. C., and Sacchettini, J. C. (1994) Cell 78, 325-334; Schumacher, M. A., Carter, D., Roos, D. S., Ullman, B., and Brennan, R. G. (1996) Nature Struct. Biol. 3, 881-887). An essential role for this Ser-Tyr dyad in HGPRT catalysis has now been verified biochemically and genetically for the Leishmania donovani HGPRT employing a combination of protein modifying reagents and site-directed mutagenesis. Incubation of HGPRT with either tetranitromethane or diethyl pyrocarbonate inactivated the enzyme completely, and peptide sequence analysis revealed that tetranitromethane treatment modified the Tyr residue within the Ser95-Tyr96 dipeptide. Analysis of site-directed mutants confirmed that both amino acids were vital for phosphoribosylation activity. Mutant HGPRTs, S95A, S95E, Y96F, and Y96V, exhibited dramatic reductions in their catalytic capabilities of 2-3 orders of magnitude, whereas HGPRTs containing conservative substitutions, S95C and S95T, displayed only a 2-3-fold decrease in kcat. Km values for the substrates of the forward and reverse reactions were largely unchanged for all HGPRT constructs, except for a 4-5-fold decrease in the Km value of the Y96F and Y96V mutants for phosphoribosylpyrophosphate. Expression of L. donovani hgprt constructs in Escherichia coli indicated that wild type and S95T HGPRTs complemented bacterial phosphoribosyltransferase deficiencies, whereas the S95A and S95C mutants complemented weakly, and the S95E, Y96F, and Y96V HGPRT did not support bacterial growth. These data authenticate that the Ser-Tyr dipeptide that is conserved among all members of the HGPRT family is essential for phosphoribosylation of purine nucleobases by HGPRT.

Inactivation of Tritrichomonas foetus and Schistosoma mansoni purine phosphoribosyltransferases by arginine-specific reagents

The arginine-specific reagents phenylglyoxal and butane-2,3-dione irreversibly inactivate the Tritrichomonas foetus hypoxanthine-guanine-xanthine phosphoribosyltransferase (HGXPRT) and Schistosoma mansoni hypoxanthine-guanine phosphoribosyltransferase (HGPRT). The inactivation of the tritrichomonal enzyme by phenylglyoxal follows time-dependent and concentration-dependent pseudo-first-order kinetics. Complete protection against inactivation is afforded by the addition of 25 microM GMP, whereas 5-phosphoribosyl-1-diphosphate (PRibPP) at 50-250 microM can only slow down the inactivation, without being protective. Digestion of [7-(14)C]phenylglyoxal-modified enzyme with trypsin and separation of the peptides by reverse-phase HPLC shows that only one radioactive peak is greatly diminished by incubation with 25 microM GMP or 1 mM PRibPP. Mass-spectral analysis identifies Arg155 as the target site of two molecules of phenylglyoxal that is protected by the substrates. This amino acid residue is positioned next to Tyr156, which is a highly conserved aromatic residue among all the purine phosphoribosyltransferases (PRT) and is always found stacked on top of the purine substrate. This may explain why phenylglyoxal labeling of Arg155 inactivates the enzyme and why GMP can protect Arg155 more effectively than PRibPP. Among the purine PRT in our possession, only schistosomal HGPRT, the only other enzyme that contains an arginine residue at the corresponding location (Arg187), was susceptible to phenylglyoxal and butane-2,3-dione. The presence of Lys185-Phe186 and Ser179-Trp180 at the corresponding locations in human HGPRT and Giardia lamblia GPRT, respectively, may explain their resistance to phenylglyoxal. Thus, Arg155 in T. foetus HGXPRT and Arg187 in S. mansoni HGPRT will be attractive targets for future studies.

Probing the active site of Tritrichomonas foetus hypoxanthine-guanine-xanthine phosphoribosyltransferase using covalent modification of cysteine residues

The hypoxanthine-guanine-xanthine phosphoribosyltransferase (HGXPRTase) of Tritrichomonas foetus was inactivated by the thiol reagents iodoacetate and 5,5'-dithiobis(2-nitrobenzoic acid) (Nbs2). Iodoacetate inactivates the enzyme in a time-dependent and concentration-dependent manner that follows pseudo-first-order kinetics. However, the observation that total inactivation with iodoacetate was not achieved suggests that none of the reactive cysteine residues is directly involved in the catalytic activity of the enzyme. Nbs2 caused 50% inactivation rapidly, which was followed by gradual modifications of an additional three cysteine residues leading to complete enzyme inactivation. Analysis of the inactivation using the method developed by Tsou (1962) revealed that modification of two cysteine residues by Nbs2 is sufficient to impair the HGXPRTase activity. Tryptic digestion of HGXPRTase labeled with iodo[2-14C]acetic acid, followed by fractionation of the digest by HPLC and sequence analysis of the labeled peptides allowed the identification of Cys71, Cys129, Cys132, and Cys148 as the reactive cysteine residues. GMP and 5-phosphoribosyl-1-diphosphate provided complete protection against HGXPRTase inactivation by iodoacetate and against carboxymethylation of Cys129, Cys132, and Cys148, Cys71 was not protected by either substrate against iodoacetate, but its carboxymethylation caused no loss in enzyme activity either. There was also no substrate protection of Cys71 against Nbs2, which, however, caused 50% inactivation of the enzyme. Replacing the thionitrobenzoate (Nbs) moiety from Cys71 with cyanide resulted in a gradual recovery of the enzyme activity, which indicates that a steric hindrance at the active site was introduced by Nbs but removed by cyanide. Thus, our results demonstrate that although the reactive cysteine residues in HGXPRTase are not directly involved in the catalytic activity, modification of cysteine residues 129, 132, and 148 by iodoacetate or Nbs2 hinders substrate binding which can, in turn, protect the cysteine residues from modifications. The substrate protection of Cys129 and Cys148 is probably also indicative of a conformational change in the protein structure brought about by substrate binding.