Kinetic Studies of Adenosine Kinase from Ehrlich Ascites Tumor Cells

Concentration-dependent effects of tungstate on germination, growth, lignification-related enzymes, antioxidants, and reactive oxygen species in broccoli (Brassica oleracea var. italica L.)

The phyto-impact of tungstate is not frequently studied like other heavy metals especially in the sight of continuous accumulation of tungstate in the agriculture soils and water. Thus, the present study was aimed to investigate the supplementation of various tungstate concentrations (0, 1, 5, 10, 50, and 100) to germination water (mg L^-1) or clay soil (mg kg^-1) on germination and metabolism of broccoli. Lower concentrations (1-10 mg L^-1) accelerated germination process and reciprocally were recorded at the highest one (100 mg L^-1). The promoter effect of lower concentrations on seedlings growing on tungstate contaminated soil was underpinned from enhancement of pigments, metabolites, enzymatic and non-enzymatic antioxidants, and nitrate reductase. However, the highest concentration-noxious impacts perceived from oxidative damage and membrane integrity deregulation accompanied with no gain from increment of proline, superoxide dismutase, and glutathione-S-transferase. The depletion of phytochelatins and nitric oxide jointed with the enhancement of peroxidases, polyphenol oxidase, and phenylalanine ammonia-lyase at higher concentration reinforced lignin production which restricted plant growth. The results supported the hormetic effects of tungstate (beneficial at low concentrations and noxious at high concentration) on morphological and physiological parameters of broccoli seedlings. The stimulatory effect of tungstate on metabolic activities could serve as important components of antioxidative defense mechanism against tungstate toxicity.

Adenosine kinase: exploitation for therapeutic gain

Adenosine kinase (ADK; EC 2.7.1.20) is an evolutionarily conserved phosphotransferase that converts the purine ribonucleoside adenosine into 5'-adenosine-monophosphate. This enzymatic reaction plays a fundamental role in determining the tone of adenosine, which fulfills essential functions as a homeostatic and metabolic regulator in all living systems. Adenosine not only activates specific signaling pathways by activation of four types of adenosine receptors but it is also a primordial metabolite and regulator of biochemical enzyme reactions that couple to bioenergetic and epigenetic functions. By regulating adenosine, ADK can thus be identified as an upstream regulator of complex homeostatic and metabolic networks. Not surprisingly, ADK dysfunction is involved in several pathologies, including diabetes, epilepsy, and cancer. Consequently, ADK emerges as a rational therapeutic target, and adenosine-regulating drugs have been tested extensively. In recent attempts to improve specificity of treatment, localized therapies have been developed to augment adenosine signaling at sites of injury or pathology; those approaches include transplantation of stem cells with deletions of ADK or the use of gene therapy vectors to downregulate ADK expression. More recently, the first human mutations in ADK have been described, and novel findings suggest an unexpected role of ADK in a wider range of pathologies. ADK-regulating strategies thus represent innovative therapeutic opportunities to reconstruct network homeostasis in a multitude of conditions. This review will provide a comprehensive overview of the genetics, biochemistry, and pharmacology of ADK and will then focus on pathologies and therapeutic interventions. Challenges to translate ADK-based therapies into clinical use will be discussed critically.

Analysis of adenosine by RP-HPLC method and its application to the study of adenosine kinase kinetics

An RP-HPLC method for the analysis of adenosine (ADO) has been developed and validated. In the present study, we report an RP-HPLC-based method with modifications of mobile phase and shorter retention time that substantially improved the efficiency of ADO analysis. The HPLC separation of the ADO was achieved on a C18 column, using a mobile phase consisting of water, containing 7% v/v ACN, at a flow rate of 0.8 mL/min. The column effluent was monitored by UV detection at 260 nm. A linear response was achieved over the concentration range of 0.25-100.00 micromol/L. The analytical method inter- and intra-run accuracy and precision were better than +/- 15%. The LOQ was 0.25 micromol/L, with ADO detection in the range of 6.25 pmol per sample. The method has been applied to the study of adenosine kinase (AK) kinetics.

Mutational analysis of the active-site residues crucial for catalytic activity of adenosine kinase from Leishmania donovani

Leishmania donovani adenosine kinase (LdAdK) plays a pivotal role in scavenging of purines from the host. Exploiting interspecies homology and structural co-ordinates of the enzyme from other sources, we generated a model of LdAdK that led us to target several amino acid residues (namely Gly-62, Arg-69, Arg-131 and Asp-299). Replacement of Gly-62 with aspartate caused a drastic reduction in catalytic activity, with decreased affinity for either substrate. Asp-299 was found to be catalytically indispensable. Mutation of either Arg-131 or Arg-69 caused a significant reduction in kcat. R69A (Arg-69-->Ala) and R131A mutants exhibited unaltered K(m) for either substrate, whereas ATP K(m) for R69K increased 6-fold. Importance of both of the arginine residues was reaffirmed by the R69K/R131A double mutant, which exhibited approx. 0.5% residual activity with a large increase in ATP K(m). Phenylglyoxal, which inhibits the wild-type enzyme, also inactivated the arginine mutants to different extents. Adenosine protected both of the Arg-69 mutants, but not the R131A variant, from inactivation. Binding experiments revealed that the AMP-binding property of R69K or R69A and D299A mutants remained largely unaltered, but R131A and R69K/R131A mutants lost their AMP binding ability significantly. The G62D mutant did not bind AMP at all. Free energy calculations indicated that Arg-69 and Arg-131 are functionally independent. Thus, apart from the mandatory requirement of flexibility around the diglycyl (Gly-61-Gly-62) motif, our results identified Asp-299 and Arg-131 as key catalytic residues, with the former functioning as the proton abstractor from the 5'-OH of adenosine, while the latter acts as a bidentate electrophile to stabilize the negative charge on the leaving group during the phosphate transfer. Moreover, the positive charge distribution of Arg-69 probably helps in maintaining the flexibility of the alpha-3 helix needed for proper domain movement. These findings provide the first comprehensive biochemical evidence implicating the mechanistic roles of the functionally important residues of this chemotherapeutically exploitable enzyme.

Expression inEscherichia coliof a recombinant adenosine kinase fromSaccharomyces cerevisiae: purification, kinetics and substrate analyses

The Saccharomyces cerevisiae ADO1 gene is known to encode a homologue of eukaryotic adenosine kinases. This gene was expressed in Escherichia coli as a recombinant protein fused to a polyhistidine tag by using the rhamnose-inducible bacterial promoter rhaB. The recombinant protein was purified to apparent homogeneity and its ability to phosphorylate different substrates was evaluated. Adenosine (Km 3 microM) is its primary substrate. In addition, it also phosphorylates, albeit less efficiently, 3'-deoxyadenosine (cordycepin; Km 1.84 mM) and 3'-amino-3'-deoxyadenosine (Km 0.26 mM). Other kinetic properties of the recombinant enzyme have also been determined.

Pentavalent ions dependency is a conserved property of adenosine kinase from diverse sources: identification of a novel motif implicated in phosphate and magnesium ion binding and substrate inhibition

The catalytic activity of adenosine kinase (AK) from mammalian sources has previously been shown to exhibit a marked dependency upon the presence of pentavalent ions (PVI), such as phosphate (PO4), arsenate, or vanadate. We now show that the activity of AK from diverse sources, including plant, yeast, and protist species, is also markedly enhanced in the presence of PVI. In all cases, PO4 or other PVI exerted their effects primarily by decreasing the Km for adenosine and alleviating the inhibition caused by high concentrations of substrates. These results provide evidence that PVI dependency is a conserved property of AK and perhaps of the PfkB family of carbohydrate kinases which includes AK. On the basis of sequence alignments, we have identified a conserved motif NXXE within the PfkB family. The N and E of this motif make close contacts with Mg2+ and PO4 ions in the crystal structures of AK and bacterial ribokinase (another PfkB member which shows PVI dependency), implicating these residues in their binding. Site-directed mutagenesis of these residues in Chinese hamster AK have resulted in active proteins with greatly altered phosphate stimulation and substrate inhibition characteristics. The N239Q mutation leads to the formation of an active protein whose activity was not stimulated by PO4 or inhibited by high concentrations of adenosine or ATP. The activity of the E242D mutant protein was also not significantly altered in the presence of phosphate. Although PO4 had no effect on the KmAdenosine for this mutant, the KmATP, K(i)Adenosine, and K(i)ATP were significantly decreased. In contrast to these mutations, N239L or E242L mutant proteins showed greatly decreased activity with an altered Mg2+ requirement. These observations support the view that N239 and E242 play an important role in the binding of PO4 and Mg2+ ions required for the catalytic activity of adenosine kinase.

Crystal structures of Toxoplasma gondii adenosine kinase reveal a novel catalytic mechanism and prodrug binding

Adenosine kinase (AK) is a key purine metabolic enzyme from the opportunistic parasitic protozoan Toxoplasma gondii and belongs to the family of carbohydrate kinases that includes ribokinase. To understand the catalytic mechanism of AK, we determined the structures of the T. gondii apo AK, AK:adenosine complex and the AK:adenosine:AMP-PCP complex to 2.55 A, 2.50 A and 1.71 A resolution, respectively. These structures reveal a novel catalytic mechanism that involves an adenosine-induced domain rotation of 30 degrees and a newly described anion hole (DTXGAGD), requiring a helix-to-coil conformational change that is induced by ATP binding. Nucleotide binding also evokes a coil-to-helix transition that completes the formation of the ATP binding pocket. A conserved dipeptide, Gly68-Gly69, which is located at the bottom of the adenosine-binding site, functions as the switch for domain rotation. The synergistic structural changes that occur upon substrate binding sequester the adenosine and the ATP gamma phosphate from solvent and optimally position the substrates for catalysis. Finally, the 1.84 A resolution structure of an AK:7-iodotubercidin:AMP-PCP complex reveals the basis for the higher affinity binding of this prodrug over adenosine and thus provides a scaffold for the design of new inhibitors and subversive substrates that target the T. gondii AK.

Crystal structures of Toxoplasma gondii adenosine kinase reveal a novel catalytic mechanism and prodrug binding

Adenosine kinase (AK) is a key purine metabolic enzyme from the opportunistic parasitic protozoan Toxoplasma gondii and belongs to the family of carbohydrate kinases that includes ribokinase. To understand the catalytic mechanism of AK, we determined the structures of the T. gondii apo AK, AK:adenosine complex and the AK:adenosine:AMP-PCP complex to 2.55 A, 2.50 A and 1.71 A resolution, respectively. These structures reveal a novel catalytic mechanism that involves an adenosine-induced domain rotation of 30 degrees and a newly described anion hole (DTXGAGD), requiring a helix-to-coil conformational change that is induced by ATP binding. Nucleotide binding also evokes a coil-to-helix transition that completes the formation of the ATP binding pocket. A conserved dipeptide, Gly68-Gly69, which is located at the bottom of the adenosine-binding site, functions as the switch for domain rotation. The synergistic structural changes that occur upon substrate binding sequester the adenosine and the ATP gi phosphate from solvent and optimally position the substrates for catalysis. Finally, the 1.84 A resolution structure of an AK:7-iodotubercidin:AMP-PCP complex reveals the basis for the higher affinity binding of this prodrug over adenosine and thus provides a scaffold for the design of new inhibitors and subversive substrates that target the T. gondii AK.

The influence of inorganic phosphate on the activity of adenosine kinase

The enzyme adenosine kinase (AK; EC 2.7.1.20) shows a dependence upon inorganic phosphate (Pi) for activity. The degree of dependence varies among enzyme sources and the pH at which the activity is measured. At physiological pH, recombinant AK from Chinese hamster ovary (CHO) cells and AK from beef liver (BL) show higher affinities for the substrate adenosine (Ado), larger maximum velocities and lower sensitivities to substrate inhibition in the presence of Pi. At pH 6.2, both BL and CHO AK exhibit almost complete dependence on the presence of Pi for activity. The data show that both enzymes exhibit increasing relief from substrate inhibition upon increasing Pi and the inhibition of BL AK is almost completely alleviated by the addition of 50 mM Pi. The affinity of CHO AK for Ado increases asymptotically from K(m) 6.4 microM to a limit of 0.7 microM upon the addition of increasing Pi from 1 to 50 mM. The concentration of Ado necessary to invoke substrate inhibition also increases asymptotically from K(i) 32 microM to a limit of 69 microM at saturating concentrations of phosphate. In the presence of increasing amounts of Pi, the maximal velocity of activity increases hyperbolically. The effect that phosphate exerts on AK may be either to protect the enzyme from inactivation at high adenosine and H(+) concentrations or to stabilize substrate binding at the active site.

Uptake and metabolism of adenosine mediate a meiosis-arresting action on mouse oocytes

This study was carried out to evaluate the possible role of adenosine uptake and metabolism in mediating the inhibitory actions of this nucleoside on spontaneous mouse oocyte maturation. Uridine blocked 3H-adenosine uptake by oocyte-cumulus cell complexes (OCCs) and cumulus cell-enclosed oocytes (CEOs) by 82-85%, whereas uptake by denuded oocytes (DOs) was suppressed by 97%. Uridine had no effect on germinal vesicle breakdown (GVB) in CEOs when meiotic arrest was maintained with hypoxanthine or hypoxanthine plus adenosine but reversed the combined inhibitory action of these purines in DOs. Five of six adenosine analogs that bind to purinoceptors demonstrated meiosis-arresting activity but not in relation to their relative affinities for inhibitory or stimulatory adenosine receptors and only at high concentrations. Moreover, in DOs, uridine reversed the inhibitory effect of 2-chloroadenosine and 5'-N-ethylcarboxamidoadenosine, two receptor agonists that are poor substrates for adenosine-metabolizing enzymes. Results of experiments with adenosine kinase inhibitors showed that methylmercaptopurine riboside (MMPR) and tubercidin, but not 5'-amino-5'-deoxyadenosine, reversed meiotic arrest maintained by hypoxanthine +/- adenosine, but this required an additional inhibitory action on de novo purine synthesis. Inhibition of de novo purine synthesis alone was not sufficient because azaserine failed to reverse meiotic arrest. MMPR was a very potent meiosis-inducing agent, completely reversing meiotic arrest in CEOs and DOs in the presence of a variety of meiotic inhibitors. The adenosine deaminase inhibitor deoxycoformycin had opposite effects on oocyte maturation depending on the presence or absence of adenosine: the inhibitory action of hypoxanthine alone was bolstered, but the meiosis-arresting action of adenosine was reversed. These data therefore indicate that at low adenosine concentrations phosphorylation predominates, but at higher adenosine concentrations deaminated products contribute to the meiotic inhibition. This idea was borne out by the ability of inosine to mimic the synergistic interaction of adenosine with hypoxanthine. The action of adenosine is not due to deamination to inosine and conversion to nucleotides through the hypoxanthine salvage pathway because adenosine-mediated inhibition was not compromised in oocytes from mutant mice unable to salvage hypoxanthine.

Structure of human adenosine kinase at 1.5 A resolution

Adenosine kinase (AK) is a key enzyme in the regulation of extracellular adenosine and intracellular adenylate levels. Inhibitors of adenosine kinase elevate adenosine to levels that activate nearby adenosine receptors and produce a wide variety of therapeutically beneficial activities. Accordingly, AK is a promising target for new analgesic, neuroprotective, and cardioprotective agents. We determined the structure of human adenosine kinase by X-ray crystallography using MAD phasing techniques and refined the structure to 1.5 A resolution. The enzyme structure consisted of one large alpha/beta domain with nine beta-strands, eight alpha-helices, and one small alpha/beta-domain with five beta-strands and two alpha-helices. The active site is formed along the edge of the beta-sheet in the large domain while the small domain acts as a lid to cover the upper face of the active site. The overall structure is similar to the recently reported structure of ribokinase from Escherichia coli [Sigrell et al. (1998) Structure 6, 183-193]. The structure of ribokinase was determined at 1.8 A resolution and represents the first structure of a new family of carbohydrate kinases. Two molecules of adenosine were present in the AK crystal structure with one adenosine molecule located in a site that matches the ribose site in ribokinase and probably represents the substrate-binding site. The second adenosine site overlaps the ADP site in ribokinase and probably represents the ATP site. A Mg2+ ion binding site is observed in a trough between the two adenosine sites. The structure of the active site is consistent with the observed substrate specificity. The active-site model suggests that Asp300 is an important catalytic residue involved in the deprotonation of the 5'-hydroxyl during the phosphate transfer.

Study of the substrate-binding properties of bovine liver adenosine kinase and inhibition by fluorescent nucleoside analogues

Adenosine kinase (AK) catalyzes the phosphorylation of adenosine to AMP with ATP as phosphate donor. Intrinsic fluorescence of bovine liver AK was shown previously to be a sensitive probe to quantify the binding of substrates to the enzyme [Elaloui, A., Divita, G., Maury, G., Imbach, J.-L. & Goody, R. S. (1994) Eur. J Biochem. 221, 839-846]. AK contains two catalytic, sites: a high-affinity site, which binds adenosine and AMP selectively; and a site for ATP and ADP. In the present work, these two sites were characterized by combining the quenching of protein fluorescence induced by the binding of the ligands and the fluorescence enhancement observed upon binding of the N-methylanthraniloyl-derivated nucleotides or adenosine. A new fluorescent analog of adenosine, 5'-N-methylanthraniloyl-adenosine, was synthesized and shown to bind selectively to the high-affinity adenosine-binding site with an affinity similar to that of adenosine (Kd 1 microM). In contrast, 2'(3')-N-methylanthraniloyl derivatives of ATP, adenosine (5')tetraphospho(5')adenosine (Ap4A), and adenosine (5')pentaphospho(5')adenosine (Ap5A), bind to the enzyme at the ATP site. Methylantraniloyl derivatives of ATP and adenosine were used as tools for selective characterization of a series of adenosine analogues. The bisubstrate inhibitors Ap4A and Ap5A bind to the ATP site with high affinity and apparently not to the adenosine site, thus acting more as ATP analogues than true bisubstrate ligands. The binding properties of a series of adenosine analogues were strongly dependent on the structural modifications on adenosine. The analogues modified at positions 2' or 3' show similar affinities for AK as that of adenosine, whereas adenosine analogues modified at the base present a relatively low affinity for the enzyme.

Cloning and characterization of cDNA for adenosine kinase from mammalian (Chinese hamster, mouse, human and rat) species. High frequency mutants of Chinese hamster ovary cells involve structural alterations in the gene

The enzyme adenosine kinase constitutes the major purine nucleoside phosphorylating activity in mammalian cells. In view of its central role in adenosine metabolism, which is an important physiological regulator, an understanding of the primary structure of adenosine kinase is of much interest. Using microsequence information from peptides derived from purified Syrian hamster liver enzyme, we have succeeded in isolating full length cDNA clones encoding adenosine kinase from Chinese hamster ovary cells and mouse 3T3 cells. The open reading frames in these clones consist of 334 and 335 amino acids and encode proteins of molecular masses 37364 Da and 37489 Da, respectively. In addition, the coding and upstream sequences for adenosine kinase from human (HeLa cells) and rat liver have also been cloned and sequenced. Transfection of an adenosine-kinase-deficient mutant (selected for resistance to the adenosine analog toyocamycin) of Chinese hamster ovary cells with a plasmid containing the cloned adenosine kinase cDNA, leads to regaining of adenosine kinase activity in the transformed cell. The adenosine kinase transformants also simultaneously lost their toyocamycin resistance and became similarly sensitive to the analog as the parental wild-type Chinese hamster ovary cells. The cloned adenosine kinase cDNA was also used to examine structural changes in mutants affected in adenosine kinase. In Chinese hamster ovary cells, one type of mutant that lacks adenosine kinase activity and displays high degree of resistance to various adenosine analogs, is obtained at an unusually high spontaneous frequency (10(-4)-10(-3)). Results of Southern and northern-blot analysis provide evidence that this group of mutants involves gross structural alterations affecting the adenosine kinase gene. Such structural alterations are not observed in another type of mutant which exhibits increased resistance only to C-adenosine analogs. Sequence similarity searches indicate that several of the bacterial and yeast sugar kinases (ribokinase, fructokinase and inosine-guanosine kinase) exhibit limited but significant similarity to the mammalian adenosine kinase. The sequence similarity data support the possibility that adenosine kinase shares a common evolutionary ancestor with these protein sequences.

Self-activation of recombinant human lysosomal procathepsin D at a newly engineered cleavage junction, "short" pseudocathepsin D

To obtain a recombinant model of human cathepsin D with kinetic properties that are identical with native human liver enzyme, we have addressed the significant differences in structure and catalytic function between naturally occurring enzyme and bacterially derived pseudocathepsin D. Human procathepsin D was expressed in a baculovirus system to obtain correctly folded, glycosylated enzyme that upon acidification completely converts to the active intermediate, pseudocathepsin D. The oligosaccharide moieties of this recombinant enzyme contributed to about 5% of the apparent molecular mass of the enzyme, and the carbohydrate composition was quite similar to the native material. However, specificity constants (kcat/Km) of this glycosylated pseudoform for several synthetic chromogenic substrates were considerably less (33%-50%) than those for the native enzyme and were virtually identical with those observed with nonglycosylated pseudocathepsin D. A cleavable junction suitable for self-processing at the normal maturation point of human cathepsin D was engineered into procathepsin D according to known specificity requirements of this enzyme, and the construct was expressed using baculovirus. Following experiments that demonstrated that the new proenzyme failed to process to the expected point, the new cleavage junction was moved 6 residues toward the amino terminus of procathepsin D and expressed in Escherichia coli. After refolding, the protein containing the newly engineered junction self-processed, generating a shortened mutant form of pseudocathepsin D that is 6 residues longer at the amino terminus than the native material. The kinetic properties of this newly engineered pseudoform proved to be identical with those of the native enzyme, thus establishing an improved recombinant model for this important aspartic proteinase.

Production of adenosine and nucleoside analogs by the exchange reaction catalyzed by rat liver adenosine kinase

We have previously shown [8] that rat liver adenosine kinase can produce [14C]AMP from [14C]adenosine (Ado) and unlabelled adenosine monophosphate (AMP), in the absence of ATP, by an exchange reaction. In this study, we investigated whether Ado or AMP could be replaced in this exchange reaction by other nucleosides or nucleoside monophosphates (NMP), respectively. In the presence of 1 mM of the unlabelled NMP analogs 7-deazaadenosine (tubercidin) 5'-monophosphate, 6-chloropurine riboside 5'-monophosphate, or N6-methyl-AMP, [14C]AMP was formed from 20 microM [14C]Ado at up to 50% of the rate recorded with 1 mM unlabelled AMP. In the presence of 0.2 mM of the unlabelled analog nucleosides tubercidin, N6-methyladenosine, or 6-methylmercaptopurine riboside, [14C]Ado was generated from 1 mM [14C]AMP at up to 60% of the rate recorded with 0.2 mM unlabeled Ado. Small amounts of [14C]Ado were also formed from the natural nucleosides 5-amino-4-imidazolecarboxamide (AICA) riboside or 2'-deoxyadenosine. Administration of therapeutic anticancer and antiviral nucleosides that can serve as substrates for the exchange reaction catalyzed by adenosine kinase might, thus, result in a net production of Ado, a potent autacoid with physiological effects in numerous tissues.

Reversal of methylmercaptopurine ribonucleoside cytotoxicity by purine ribonucleosides and adenine

6-Methylmercaptopurine ribonucleoside-5'-phosphate (MeSPuRMP), the sole metabolite of 6-methylmercaptopurine ribonucleoside (MeSPuRib), is a strong inhibitor of purine de novo synthesis, inducing depletion of intracellular purine nucleotides and subsequent cell death in several tumor cell lines. In this study prevention of MeSPuRib cytotoxicity by compounds of the purine salvage pathway was studied in Molt F4 human malignant T-lymphoblasts. Adenosine, adenine and inosine were able to prevent depletion of the adenine nucleotide pool when used in combination with 0.5 microM MeSPuRib, but had virtually no effect on depletion of guanine nucleotides. Nevertheless, these three purine compounds were able to reduce the cytotoxic effects induced by MeSPuRib. Addition of guanosine to cells treated with 0.5 microM MeSPuRib normalized the guanine nucleotide pool, but adenine nucleotides remained depleted. Under these conditions, inhibition of cell growth was significantly decreased. With the combination of guanosine and 10 microM MeSPuRib, cytotoxicity was increased compared to 10 microM MeSPuRib alone, associated with a depletion of adenine nucleotides to 9% of untreated cells. Since cell growth and cell viability of Molt F4 cells are less inhibited by MeSPuRib under conditions where adenine nucleotide depletion is prevented by purine compounds (and where the other nucleotides are depleted) we conclude that depletion of adenine nucleotides is an important factor in MeSPuRib cytotoxicity.

Purine control of mouse oocyte maturation: evidence that nonmetabolized hypoxanthine maintains meiotic arrest

Hypoxanthine is present in preparations of follicular fluid and has been shown to suppress the spontaneous meiotic maturation of mammalian oocytes in vitro. The present experiments examined the possible role of hypoxanthine metabolism in mediating this meiotic arrest. Four putative inhibitors of the enzyme, hypoxanthine phosphoribosyltransferase (HPRT), which metabolizes hypoxanthine to inosine monophosphate, were tested on lysates of oocyte-cumulus cell complexes. At a concentration of 1 mM, 6-mercapto-9-(tetrahydro-2-furyl)-purine (MPTF) and 6-mercaptopurine (6-MP) suppressed enzymatic activity by 86% and 98%, respectively, while 6-azauridine and 2,6-bis-(hydroxyamino)-9-beta-D-ribofuranosyl-purine had no effect. MPTF and 6-MP increased the inhibitory effect of hypoxanthine on germinal vesicle breakdown, but the other agents did not. The 2 active agents had similar effects on salvage activity and hypoxanthine-maintained meiotic arrest in denuded oocytes. Also, oocytes from XO mice were more sensitive to the meiosis-arresting action of hypoxanthine than oocytes from XX littermates, which have twice the HPRT activity. The actions of the HPRT inhibitors were not due to their conversion to nucleotides via HPRT and negative feedback on purine de novo synthesis, because azaserine and 6-methylmercaptopurine riboside, which are more potent inhibitors of de novo synthesis, had a stimulatory, rather than inhibitory, effect on hypoxanthine-arrested oocytes. Furthermore, several lines of evidence indicate that metabolism of hypoxanthine to xanthine and uric acid by xanthine oxidase does not mediate the inhibitory action of this purine base on meiotic maturation. The data therefore suggest that nonmetabolized hypoxanthine is responsible for the meiotic arrest observed, most likely through suppression of cAMP degradation.

Reaction kinetics and inhibition of adenosine kinase from Leishmania donovani

The reaction kinetics and the inhibitor specificity of adenosine kinase (ATP:adenosine 5'-phosphotransferase, EC 2.7.1.20) from Leishmania donovani, have been analysed using homogeneous preparation of the enzyme. The reaction proceeds with equimolar stoichiometry of each reactant. Double reciprocal plots of initial velocity studies in the absence of products yielded intersecting lines for both adenosine and Mg2+-ATP. AMP is a competitive inhibitor of the enzyme with respect to adenosine and noncompetitive inhibitor with respect to ATP. In contrast, ADP was a noncompetitive inhibitor with respect to both adenosine and ATP, with inhibition by ADP becoming uncompetitive at very high concentration of ATP. Parallel equilibrium dialysis experiments against [3H]adenosine and [gamma-32P]ATP resulted in binding of adenosine to fre enzyme. Tubercidin (7-deazaadenosine) and 6-methyl-mercaptopurine riboside acted as substrates for the enzyme and were found to inhibit adenosine phosphorylation competitively in vitro. 'Substrate efficiency (Vmax/Km)' and 'turnover numbers (Kcat)' of the enzyme with respect to specific analogs were determined. Taken together the results suggest that (a) the kinetic mechanism of adenosine kinase is sequential Bi-Bi, (b) AMP and ADP may regulate enzyme activity in vivo and (c) tubercidin and 6-methylmercaptopurine riboside are monophosphorylated by the parasite enzyme.

Regulation of deoxyadenosine and nucleoside analog phosphorylation by human placental adenosine kinase

The enzymes responsible for phosphorylation of adenosine and nucleoside analogs are important in the pathogenesis of adenosine deaminase deficiency and for the activation of specific anticancer and antiviral drugs. We examined the role of adenosine kinase in catalyzing these reactions using an enzyme purified 4000-fold (2.1 umol/min/mg) from human placenta. The Km values of adenosine and ATP are 135 uM and 4 uM, respectively. Adenosine kinase phosphorylates adenine arabinoside with an apparent Km value of 1 mM using adenosine kinase assay conditions. The Km values for 6-methylmercaptopurine riboside and 5-iodotubercidin, substrates for adenosine kinase, are estimated to be 4.5 uM and 2.6 nM, respectively. These data indicate that dadenosine phosphorylation by adenosine kinase is primarily regulated by its Km, and the concentrations of Mg2+, ADP and AMP. The high Km values for phosphorylation of dadenosine and adenine arabinoside suggest that adenosine kinase may be less likely to phosphorylate these nucleosides in vivo than other enzymes with lower Km values. Adenosine kinase appears to be important for adenosine analog phosphorylation where the Michaelis constant is in the low micromolar range.

Adenosine kinase from bovine adrenal medulla

Adenosine kinase from bovine adrenal medulla was purified 1600-fold by using ammonium sulfate precipitation, gel filtration and affinity chromatography. Gel filtration yielded a relative molecular mass around 42000 and Michaelis constants were 0.2 microM for adenosine and 20 microM for MgATP. The enzyme showed a broad specificity for purine nucleoside triphosphate as phosphate donors. Both free Mg2+ and ATP were inhibitors. AMP was a competitive inhibitor with regard to adenosine and a non-competitive inhibitor versus MgATP, while ADP was a uncompetitive inhibitor with regard to adenosine and a non-competitive inhibitor versus MgATP. Adenosine kinase was strongly inhibited by the bis(adenylyl) polyphosphates Ap4A and Ap5A. These compounds inhibited the enzyme competitively versus MgATP (Ki = 0.06 microM for Ap4A and 0.4 microM for Ap5A) and uncompetitively with regard to adenosine. The results of the kinetic analysis suggest an ordered bi-bi mechanism, adenosine being the first substrate. The phosphorylation of adenosine was unaffected in the presence of vanadate ions.

Purification and properties of adenosine kinase from rat brain

Adenosine kinase (ATP:adenosine 5'-phosphotransferase, EC 2.7.1.20) has been purified to apparent homogeneity from rat brain by (NH4)2SO4 fractionation, affinity chromatography on AMP-Sepharose 4B, gel filtration with Sephadex G-100, and DE-52 cellulose column chromatography. The yield was 56% of the initial activity with a final specific activity of 7.8 mumol/min per mg protein. The molecular weight was estimated as 38 000 by gel filtration with Sephadex G-100 and 41 000 by acrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS). The enzyme catalyzed the phosphorylation of adenosine, deoxyadenosine, arabinoadenosine, inosine and ribavirin. The activity of deoxyadenosine phosphorylation was 20% that of adenosine phosphorylation. The pH optimum profile was biphasic; a sharp pH optimum at pH 5.5 and a broad pH optimum at pH 7.5-8.5. The Km value for adenosine was 0.2 microM and the maximum activity was observed at 0.5 microM. At higher concentrations of adenosine, the activity was strongly inhibited. The Km value for ATP was 0.02 mM and that for Mg2+ was 0.1 mM. GTP, dGTP, dATP and UTP were also proved to be effective phosphate donors. Co2+ was as effective as Mg2+, and Ca2+, Mn2+ or Ni2+ showed about 50% of the activity for Mg2+. The kinase is quite unstable, but stable in the presence of a high concentration of salt; e.g., 0.15 M KCl.