Changes in deoxyadenosine production during human lymphocyte mitogenesis

Immunodeficient children who lack the purine metabolic enzyme adenosine deaminase have markedly elevated plasma concentrations of 2'-deoxyadenosine and adenosine. However, little information exists concerning the magnitude of endogenous 2'-deoxyadenosine and adenosine synthesis by normal human hematopoietic cells. In the present experiments, we have used the sensitive technique of high performance liquid chromatography to quantitate changes in 2'-deoxyadenosine and adenosine production during human lymphocyte mitogenesis. In the presence of the adenosine deaminase inhibitor deoxycoformycin, human T lymphocytes stimulated with phytohemagglutinin (PHA), and non-T lymphocytes stimulated with formalinized Staphylococcus aureus Cowan strain I, excreted 2'-deoxyadenosine into the cell medium. The nucleoside was detectable as early as 20 h after addition of mitogen. The time course of 2'-deoxyadenosine excretion correlated with the uptake of [methyl-3H]thymidine into nucleic acid. Mitogen-stimulated human lymphocytes produced only minimal amounts of adenosine. The results suggest that increased 2'-deoxyadenosine synthesis and release may normally accompany human lymphocyte mitogenesis.

Human placental deoxyadenosine and deoxyguanosine phosphorylating activity

We studied deoxyadenosine and deoxyguanosine phosphorylating activities in human placental cytosol. The specific activities of nucleoside kinase enzymes in nanomoles per h per mg +/- SD were as follows: adenosine kinase, 30 +/- 14; deoxyadenosine kinase, 12 +/- 2; deoxycytidine kinase, 0.30 +/- 0.04; and deoxyguanosine kinase, 27 +/- 16. Three major activities were resolved by ion exchange and affinity chromatography: deoxyguanosine-deoxycytidine kinase, deoxycytidine-deoxyadenosine kinase, and adenosine-deoxyadenosine kinase. Two other activities contained significant quantities of deoxyadenosine kinase. Deoxyguanosine-phosphorylating activity eluted as a single peak in association with deoxycytidine kinase. This deoxyguanosine-deoxycytidine kinase had an apparent molecular weight of 54,000, a Stokes radius of 31 A, and apparent Km values of 10, 130, and 14 microM for deoxyguanosine, deoxycytidine, and ATP, respectively. Four peaks of deoxyadenosine phosphorylating activity were resolved by affinity chromatography with AMP-Sepharose 4B. Adenosine-deoxyadenosine kinase had an apparent molecular weight of 38,000, a Stokes radius of 27.4 A, and apparent Km values of 0.4, 510, and 75 microM for adenosine, deoxyadenosine, and ATP, respectively. Attempts to distinguish whether adenosine-deoxyadenosine kinase was one enzyme with these two activities or two separate enzymes suggested that the former was the case. Deoxycytidine-deoxyadenosine kinase had apparent Km values of 0.7, 670, and 12 microM for deoxycytidine, deoxyadenosine, and ATP, respectively. Its apparent molecular weight was estimated to be 49,000 and its Stokes radius 30 A. Two other minor peaks of deoxyadenosine-phosphorylating activity had characteristics different from either deoxycytidine kinase or adenosine kinase-associated deoxyadenosine kinase. Our studies indicate that human placental cytosol contains a complex mixture of nucleoside kinase enzymes.

Increased susceptibility of fibroblasts from horses with severe combined immunodeficiency to growth inhibition by 2'-deoxyadenosine

The effect of adenosine, deoxyadenosine, guanosine, and deoxyguanosine on the growth rate of fibroblasts derived from normal horses, horses heterozygous for the severe combined immunodeficiency (SCID) trait (heterozygotes), and horses with SCID was studied. All four purines were found to inhibit growth in a dose-dependent manner, but only adenosine and deoxyadenosine were inhibitory at concentrations of less than 100 microM. No statistical difference in sensitivity to adenosine was detected between normal and SCID fibroblasts. Fibroblasts from SCID horses were, however, more sensitive to the growth inhibitory effects of deoxyadenosine than were fibroblasts from normal horses. Furthermore, following 24 hr of incubation with radiolabeled deoxyadenosine, radiolabeled deoxyATP concentrations were found to be twofold higher in SCID fibroblasts compared to those concentrations measured in normal fibroblasts cultured under identical conditions. Adenosine deaminase and S-adenosylhomocysteine hydrolase activities were normal in SCID fibroblasts. These findings suggest that SCID horses may have a defect in either transport or phosphorylation of deoxyadenosine, or in the utilization of deoxyATP.

Adenosine phosphorylase activity in mycoplasma-free growth media for mammalian cells

Mammalian cells have enzymes that deaminate adenosine to inosine, which can readily be phosphorolysed to hypoxanthine. They do not, however, possess enzymes to form adenine by the cleavage of adenosine. For this reason, the release of adenine from adenosine by mammalian cell cultures has usually been interpreted as indicating the presence of mycoplasma, a frequent microbial contaminant that contains high levels of adenosine phosphorylase. We found that some human lymphoblast cultures free of mycoplasma showed high levels of adenosine cleavage and that this activity resulted from adenosine phosphorylase in the bovine serum used as the culture growth supplement. A survey of 13 serum supplements disclosed that fetal bovine serum (six lots) contains the highest adenosine phosphorylase activity, ranging from 9 to 648 nmol adenine produced per hour per ml serum; newborn calf serum (four lots) has much less activity, ranging from 0 to 5 nmol adenine produced per hour per ml serum; and donor horse serum (three lots) contains no detectable activity. These results suggest that mycoplasma tests dependent on the presence of adenosine phosphorylase or other enzyme activities may give false-positives with cultures containing fetal bovine serum supplements.

Specific toxicity of 2-chlorodeoxyadenosine toward resting and proliferating human lymphocytes

2-Chlorodeoxyadenosine (CdA), an adenosine-deaminase-resistant purine deoxynucleoside, is markedly toxic toward human T-lymphoblastoid cell lines in vitro and is an effective agent against L1210 leukemia in vivo. The present studies have examined the toxicity, and in some cases, metabolism, of CdA in (1) multiple established human cell lines of varying phenotype, (2) leukemia and lymphoma cells taken directly from patients, (3) normal bone marrow cells, and (4) normal peripheral blood lymphocytes. Nanomolar concentrations of CdA blocked the proliferation of lymphoblastoid cell lines with a high ratio of deoxycytidine kinase to deoxynucleotidase. The drug had virtually no effect on the growth of cell lines derived from solid tissues. The CdA inhibited the spontaneous uptake of tritiated thymidine by many T and non-T, non-B acute lymphoblastic leukemia cell specimens at concentrations less than or equal to 5 nM. The same concentrations did not impair either thymidine uptake or granulocyte-monocyte colony formation by normal bone marrow cells. In common with deoxyadenosine, but unlike several other agents affecting purine and purine metabolism, CdA was lethal to resting normal T lymphocytes and to slowly dividing malignant T cells. In both resting and proliferating lymphocytes, the CdA was phosphorylated by deoxycytidine kinase and entered a rapidly turning over nucleotide pool. Dividing lymphocytes also incorporated abundant CdA into DNA. The selective toxicity of CdA toward both dividing and resting lymphocytes may render the drug useful as an immunosuppressive or antileukemic agent.

Effect of 2'-deoxycoformycin infusion on S-adenosylhomocysteine hydrolase and the amount of S-adenosylhomocysteine and related compounds in tissues of mice

Mice were given constant infusions of the adenosine deaminase inhibitor, 2'-deoxycoformycin, by i.p. implantation of microosmotic pumps, delivering the compound at a rate of 0.16 mg hr-1 kg-1. In accordance with published data, we observed that adenosine deaminase in most tissues was nearly completely inhibited. In addition, the S-adenosylhomocysteine hydrolase activity decreased slowly and showed a half-life in liver of about 4 hr. The rate and extent of the inactivation were highest in spleen. The amounts of adenosine, 2'-deoxyadenosine, S-adenosylhomocysteine, and S-adenosylmethionine were determined in treated animals and control animals. The tissue levels of adenosine and, to a lesser degree, S-adenosylhomocysteine and S-adenosylmethionine were critically dependent on the procedure used for processing the tissues. Lowest concentrations were observed when the organs were frozen in situ by liquid nitrogen. Treatment with 2'-deoxycoformycin induced no or a moderate increase in tissue content of adenosine and S-adenosylhomocysteine, whereas the amount of 2'-deoxyadenosine increased markedly, especially in spleen and thymus. 2'-Deoxycoformycin treatment caused an increase in adenosine and 2'-deoxyadenosine, but not S-adenosylhomocysteine, in serum of mice.

Cordycepin analog of (A2'p)2A: evidence that it functions as a prodrug of cordycepin

The effect of the cordycepin trimer analog of (A2'p)2A on cell growth, cell viability and nucleic acid synthesis was assessed in human colon carcinoma cell line HT-29 in vitro. The cordycepin analog, (3'dA2'p)2(3)'dA reduced 24 hr cell growth by 50% at 10(-4)M and decreased cell viability by 98% under these conditions. The cytotoxicity and inhibitory effects of (3'dA2'p)2(3)'dA on DNA and RNA synthesis were potentiated 5-10-fold by the presence of the adenosine deaminase inhibitor, 2'-deoxycoformycin, and closely resembled those of the parent drug, cordycepin. Chromatographic analyses of the stability of (3'dA2'p)2(3)'dA in the tissue culture medium indicated that it was hydrolyzed to the dimer and monomer forms with a half life of approximately 2 hr. No intact (3'dA2'p)2(3)'dA was detectable intracellularly, but large concentrations of cordycepin nucleotide metabolites were formed, particularly in the presence of 2'-deoxycoformycin.

Renal secretion of purine nucleosides and their analogs in mice

Previous results have indicated that 2'-deoxyadenosine (dAdo) and 2'-deoxytubercidin (dTub) are secreted by the mouse kidney. Secretion of dTub appeared to occur via the organic cation carrier [J. F. Kuttesch, Jr. et al., Biochem. Pharmac. 31, 3387 (1982)]. In the current study, the structural specificity of the secretory system for d Tub was probed by evaluating the renal clearance of several sugar-modified dTub analogs. The following sugar-modified derivatives also underwent apparent secretion: 3'-deoxy, arabinosyl, and xylosyl. These results suggest a lack of structural specificity of the secretory system for dTub. Tubercidin was apparently reabsorbed, analogous to the observation in mice that adenosine clearance is less than that of inulin. In related experiments, a transport maximum for dAdo could not be demonstrated due to the marked pharmacologic activity of dAdo. Cimetidine was found to selectively inhibit the organic cation secretory system since it blocked the renal secretion of tetraethylammonium but not that of p-amminohippurate in mice. Correspondingly, cimetidine prevented the renal secretion dTub; however, cimetidine did not inhibit the renal secretion of dAdo nor the renal reabsorption of Ado. These results suggest that renal secretion of dTub occurs via the organic cation carrier. The mechanisms for the renal secretion of dAdo and for the renal reabsorption of Ado may be unique and independent of the organic cation system.

Neurotoxicity of deoxycoformycin: effect of constant infusion on adenosine deaminase, adenosine, 2'-deoxyadenosine and monoamines in the mouse brain

The tight-binding adenosine deaminase inhibitor, 2'-deoxycoformycin (dCF), was continuously infused into mice by intraperitoneal implantation of microosmotic pumps delivering the compound at a rate of 0.16 mg hr-1 kg-1 for up to 6 days. The activity of cerebral adenosine deaminase was nearly totally inhibited. The amount of adenosine and 2'-deoxyadenosine was determined in the brain frozen in liquid nitrogen through the intact skull bone. The concentration of adenosine was about 1 nmol/g, and was essentially not altered following treatment with deoxycoformycin. Deoxycoformycin induced a progressive increase in cerebral content of 2'-deoxyadenosine, which after 1 day of treatment equalled the amount of adenosine. The concentrations of serotonin, dopamine and noradrenaline in the brain were not altered.

Terminal incorporation of 2'-deoxyadenosine into polyadenylate segments of polyadenylated RNA in G1-phase-arrested human T-lymphoblasts

In the presence of the adenosine deaminase inhibitor erythro-9-[3(2-hydroxynonyl)]adenine microM concentrations of 2'-deoxyadenosine (dAdo) are toxic to nondividing human lymphoid cells and induce G1-phase arrest in T-leukemic lymphoblasts, effects which appear to be independent of ribonucleotide reductase inhibition by accumulated 2'-deoxyadenosine 5'-triphosphate. We sought to determine if 2'-deoxyadenosine 5'-triphosphate had effects similar to those of other cytotoxic adenosine analogues which are incorporated into polyadenylated RNA [poly(A)+ RNA]. In the presence of erythro-9-[3-(2-hydroxynonyl)]adenine, 8-14C]dAdo, at minimal cytostatic concentrations, was incorporated into the polyadenylate segments of cytoplasmic poly(A)+ RNA in the human T-leukemic lymphoblast line CCRF-CEM, and 70% of incorporated dAdo was in the 3'-terminal position. No DAdo was found in enzyme hydrolysates of nonpolyadenylated regions of poly(A)+ RNA or of poly(A)-RNA. Enzymic hydrolysis of polyadenylated segments from labeled poly(A)+ RNA yielded adenosine:dAdo ratios of approximately 55:1.

[Chromatographic method for quantitative determination of cordycepin in rat brain tissue after intraperitoneal injection of the antibiotic]

A procedure is developed for quantitative estimation of intraperitoneally administered unlabelled cordycepin in acid soluble pool isolated from rat brain tissue. The fraction containing 2'- and 3'-deoxyriboadenosine (cordycepin) in acid soluble pool of rat brain was isolated after consecutive chromatography on columns of Dowex 1 X 8, Dihydroxyboryl-SP 500 and Sephasorb-HP. At the same time, 2'-deoxyriboadenosine was isolated from the acid soluble pool fraction of control animals. Content of the antibiotic in brain tissue was estimated by its difference in these two fractions (control/experiment).

5'-Deoxyadenosine metabolism in various mammalian cell lines

5'-Deoxyadenosine (5'-dAdo) was rapidly cleaved to adenine by cell-free, dialyzed extracts of Chinese hamster ovary (CHO), Novikoff rat hepatoma and HeLa cells in a phosphate-dependent reaction, but not by extracts from L929, L1210 and P388 cells. Radioactivity from [5'-3H]5'-dAdo was incorporated into the acid-soluble pool (uptake) by whole CHO, Novikoff and HeLa cells almost as rapidly as from labeled adenosine or adenine (all at 5 microM extracellular concentration). Radioactivity in the acid-soluble pool was mainly associated with a component identified as 5-deoxyribose-1-phosphate. Compared to ribose-1-phosphate, 5-deoxyribose-1-phosphate was metabolically highly stable. A second labeled component, however, was formed slowly and accumulated mainly in the medium. Its formation was greatly stimulated by hypoxanthine and, under conditions where their deamination was not blocked, by adenosine and 2'- and 3'-deoxyadenosine. The second product was 5'-deoxyinosine synthesized from hypoxanthine and 5-deoxyribose-1-phosphate by purine nucleoside phosphorylase. Cleavage of 5'-dAdo by whole cells was dependent on the continuous removal of the product adenine, since uptake was greatly reduced in cells deficient in adenine phosphoribosyl transferase and 50 microM adenine strongly inhibited 5'-dAdo cleavage. The results are consistent with the view that 5'-dAdo is a substrate for 5'-methylthioadenosine phosphorylase and that its use as a non-metabolizable substrate for the nucleoside transport measurements is limited to cells lacking this enzyme.

Inhibition of ribosomal translocation by peptidyl transfer ribonucleic acid analogues

The activity of peptidyl-tRNALys-CpCp2'dA was measured in an in vitro poly(A)-dependent polypeptide synthesizing system derived from Escherichia coli. It has already been shown that Lys-tRNALys-CpCp2'dA is active as an acceptor and Ac2-Lys-tRNALys-Cp2'dA can donate its peptidyl residue but that the overall poly(A)-dependent synthesis of polylysine does not take place with Lys-tRNALys-CpCp2'dA [Wagner, T., Cramer, F., & Sprinzl, M. (1982) Biochemistry 21, 1521-1529]. This is due to the efficient inhibition of the EF-G-dependent translocation of the peptidyl-tRNA CpCp2'dA from the ribosomal A to the ribosomal P site. In addition, the EF-G-dependent release of the deacylated tRNALys-CpCp2'dA from the ribosomes is also inhibited. The action of the elongation factor G or some other ribosomal component participating in the translocation process requires the presence of the 2'-hydroxyl group on the terminal adenosine of tRNA. If this hydroxyl group is not present on the tRNA, the ribosomes remain locked in their pretranslocational state.

Renal transport of 2'-deoxytubercidin in mice

Previous results [J. F. Kuttesch, Jr. and J. A. Nelson, Cancer Chemother, Pharmac. 8, 221 (1982)] from this laboratory indicate that mechanisms exist for renal secretion of 2'-deoxyadenosine and possibly for reabsorption of adenosine in humans and in mice. Since significant metabolism of these purine nucleosides occurs even in the presence of adenosine deaminase inhibitors, the renal handling of a compound which is not significantly metabolized by the deaminase or by kinases was studied. Unlike 2'-deoxyadenosine itself, the 2'-deoxyadenosine analog, [4-amino-7-(2'-deoxy-beta-D-erythro-pentofuranosyl)-pyrrolo-(2,3-d)pyrimidine; 2'-deoxytubercidin], is not significantly metabolized by mammalian tissues. In mice, the renal plasma clearance of 2'-deoxytubercidin exceeded that of inulin by about 3-fold. Also, mouse kidney slices concentratively accumulated 2'-deoxytubercidin by a saturable and metabolically dependent process. The uptake by mouse kidney slices was inhibited by classical substrates for the organic cation secretory system (tetraethylammonium, choline and N1-methylnicotinamide) but was not markedly inhibited by classical substrates for the organic anion secretory system (p-aminohippurate, phenol red and probenecid). Since 2'-deoxytubercidin inhibited the active, concentrative uptake of [14C]tetraethylammonium, but failed to inhibit the uptake of p-[14C]aminohippurate by mouse kidney slices, it is concluded that 2'-deoxytubercidin may be secreted by the organic cation system. Additional studies are required, however, to unequivocally establish the relationships between 2'-deoxytubercidin, 2'-deoxyadenosine and tetraethylammonium renal secretory mechanisms.

Effect of red cell transfusions, thymic hormone and deoxycytidine in severe combined immunodeficiency due to adenosine deaminase deficiency

A child with severe combined immunodeficiency due to adenosine deaminase deficiency was treated with thymic hormone (TP1) alone, thymic hormone with red cell transfusions and finally with a combination of these and parenteral deoxycytidine. TP1 alone produced no benefit while the addition of red cell transfusions resulted in only partial immunological restoration despite improvement in clinical and biochemical findings. Deoxycytidine given subcutaneously produced no additional benefit in any of the parameters measured.

S-Adenosylhomocysteinase: mechanism of inactivation by 2'-deoxyadenosine and interaction with other nucleosides

S-Adenosylhomocysteinase (SAHase), a tetrameric enzyme, is inactivated by 2'-deoxyadenosine (2'dAdo) in a time-dependent process [Hirshfield, M. S. (1979) J. Biol. Chem. 254, 22-25]. It has been proposed that inactivation involves oxidation of 2'dAdo at C-3' by enzyme-bound nicotinamide adenine dinucleotide (NAD), subsequent proton abstraction at C-2', and elimination of adenine. This results in irreversible formation of enzyme-bound NADH and of adenine (Ade) and inactivation [Abeles, R. H., TAshjian, A. H., Jr., & Fish, S (1980) Biochem. Biophys. Res. Commun. 95, 612-617]. It has now been established that upon inactivation of SAHase with deoxy[2'(R)-3H]adenosine, 3H2O is formed. This is consistent with the proposed mechanism and of 3H2O release shows that maximally two of the four subunits participate in the reaction that results in 3H2O release. Reaction of SAHase with 2'dAdo results in reduction of two of the enzyme-bound NAD molecules. However, all four NAD molecules can be reduced by NaBH4, but only two are reduced to C-4 NADH. When the enzyme is inactivated with adenine-labeled 2'dAdo, radioactivity corresponding to 0.5-1.0 mumol of 2'dAdo binds tightly per micromole of subunit. This radioactive material is not removed from the enzyme by extensive dialysis but can be displaced by unlabeled 2'dAdo or Ade. After denaturation of the complex, radioactive material is released. Of this material 80-90% is adenine and less than 1% 2'dAdo. 2'dAdo also binds tightly to the enzyme reduced with NaBH4. Upon denaturation mostly adenine (80-90%) is released. Reaction of [2'-3H]2'dAdo with enzyme reduced with NaBH4 does not result in 3H2O formation. We conclude that the enzyme catalyzes the release of adenine from 2'dAdo by two mechanisms: One involves formation of 3'keto-2'dAdo and subsequent elimination of adenine. The other does not involve oxidation of 2'dAdo and probably is a hydrolytic process. It is proposed that the ability of the enzyme to carry out the hydrolytic process is a direct consequence of the manner in which 2'dAdo as well as the normal substrate binds to the enzyme, i.e., hydrogen-bond interaction of the protein with the adenine moiety and distortion of the ribose ring. When adenine-labeled adenosine is added to the enzyme, radioactivity corresponding to 0.5 mumol/mumol of subunit is associated with the protein after gel filtration. Of the radioactive material bound to the protein, 20% is adenine, 15% is adenosine, and the remaining radioactivity is present in unidentified compounds. The adenine bound to the enzyme does not participate in the catalytic process, and we conclude that it is bound to two of the subunits that do not participate in catalysis. Possible, these two subunits have a regulatory function. SAHase probably consists of two nonequivalent pairs of subunits. Only one pair participates in catalysis, but all four subunits probably bind Ado and 2'dAdo. We have confirmed the fact that the carbocyclic analogue of adenosine inactivates SAHase [Guranowski, A., Montgomery, J. A., 110-115]...

Binding of 2',5'-dideoxyadenosine to brain membranes. Comparison to P-site inhibition of adenylate cyclase

Membranes from rat cerebral cortex and striatum contain a relatively large number of high-affinity binding sites for [3H]2',5'-dideoxyadenosine, [3H]adenine arabinoside, and [3H]adenosine. The binding of [3H]2',5'-dideoxyadenosine and [3H]adenine arabinoside was virtually unaffected by relatively specific agonists and antagonists for adenosine receptors, such as 2-chloroadenosine, N6-phenylisopropyladenosine or theophylline. Binding of [3H]adenosine was partially blocked by such receptor ligands. The specific binding of all three ligands was antagonized by a variety of adenosine analogs which inhibit adenylate cyclase by interaction with the so-called P-site associated with this enzyme. However, potencies of adenosine analogs as P-site inhibitors of adenylate cyclase and as antagonists of binding do not correlate well. 5'-Methylthioadenosine had high potency and efficacy versus binding of [3H]2',5'-dideoxyadenosine but had virtually no effect on activity of adenylate cyclase. 2-Fluoroadenosine was less potent than adenosine as an antagonist of specific binding of [3H]2',5'-dideoxyadenosine, while 2-fluoroderivatives of adenosine, adenine arabinoside and adenine xylofuranoside were more potent than the parent compounds as P-site inhibitors. The significance of the binding sites for [3H]2',5'-dideoxyadenosine remains unclear, but their presence complicates the use of [3H]adenosine and certain analogs as ligands for adenosine membrane sites associated with adenylate cyclase.

Metabolism of adenosine and 2'-deoxy-adenosine by fetal mouse calvaria in culture

The metabolism of endogenous adenosine and 2-deoxy-adenosine was studied in cultures of fetal mouse calvaria. Adenosine deamination was the most important pathway of metabolism. This was blocked by erythro-2-(2-hydroxy-3-nonyl)adenine (1 microM). Albumin in the medium could not account for the deaminase activity. The disappearance of adenosine from the medium was not influenced by two inhibitors of adenosine transport, dipyridamole and dilazep, but was competitively inhibited by 2-deoxy-adenosine. During culture there was a net increase in adenosine and inosine, possibly originating from damaged cells.

Quantitative estimate of unlabelled cordycepin in acid-soluble pool isolated from rat brain tissue after intraperitoneal injection of the inhibitor

A technique for the quantitative estimation of intraperitoneally injected unlabelled cordycepin in an acid-soluble pool (ASP) isolated from rat brain tissue is suggested. It consists in consecutive chromatography of ASP on Dowex 1 X 8, Dihydroxyboryl = SP500 and Sephasorb-HP. The fraction containing 2'-deoxyriboadenosine and 3'-deoxyriboadenosine (cordycepin) has been isolated from brain tissue ASP of experimental animals after a cordycepin injection. 2'-Deoxyriboadenosine fraction has been isolated from tissue ASP of the control animals not subjected to an inhibitor injection. Brain tissue antibiotic content has been estimated by the difference in nucleoside quantity values (microM) in these two fractions (control/experiment).

Studies on the mechanism of Escherichia coli DNA polymerase I large fragment. Chain termination and modulation by polynucleotides

Homopolymer replication systems and measurement of precise product chain length have been used to elucidate two new points about the mechanism of Escherichia coli DNA polymerase I large fragment: chain termination as a function of product chain length is multiphasic, and polynucleotides exert a secondary effect in the mechanism of this enzyme. During replication of (dT)800 or (dA)800 with short oligonucleotides as primer, DNA polymerase I large fragment was processive, catalyzing hundreds of dNMP incorporations during each cycle of binding to the template-primer, incorporation, and termination. Our observations indicated, however, that polynucleotides could terminate chain elongation and that this effect probably occurred through interaction at a secondary binding site on the enzyme. Thus, in the presence of higher levels of template-primer, early termination occurred and the relatively short product molecules could be resolved by gel electrophoresis. Incubation conditions were adjusted so that the number of product molecules at each chain length was equal to the actual number of termination events, and, therefore, the statistical chance for termination as a function of product chain length could be calculated. These termination probability values depended upon specific incubation conditions, such as dNTP level and whether the primer was a ribo- or deoxyribonucleotide, and interestingly, the values changed as the chain length of the product increased. For the first 5 to 10 dMP residues added to the primer, termination probability declined with each dNMP addition, but then remained constant for the addition of the next 20 to 40 dNMP residues. These results are discussed in the context of a kinetic model representing two stages of synthesis during the formation of each product molecule.

Metabolic defects in immunodeficiency diseases

There are two ways to find the cause of primary immunodeficiency diseases. One approach is to start with the immune defect and work backwards, using in vitro techniques to define where the primary abnormality lies in the immune response. Immunologists favour this approach for obvious reasons; and it is not without virtue since, for example, it has shown that the defect in most cases of primary hypogammaglobulinaemia is due to a failure of B lymphocyte maturation. The alternative approach is to screen empirically for defects in biochemical pathways in the hope of finding a clue which will eventually lead to the underlying disorder. This is a sensible approach in diseases which are clearly inherited (e.g. X-linked hypogammaglobulinaemia) but is less attractive in a disease such as late onset hypogammaglobulinaemia which is not obviously inherited. In practice, such procedures involve screening the urine for abnormalities in the quality or quantity of excreted compounds. Another way is to screen for abnormalities in organelle integrity by measuring the activity of various enzymes in subcellular fractions. In reality, the clue to the metabolic defect is usually discovered by accident, the prime example in our field being the discovery of adenosine deaminase (ADA) deficiency.

Adenosine- and deoxyadenosine-mediated altered pyrimidine metabolism in human adeosine deaminase-deficient lymphoblasts

To evaluate the importance of altered pyrimidine synthesis in the relationship between immune dysfunction and deficiency of ADA, this pathway has been evaluated in normal and ADA-deficient human lymphoblasts. Adenosine and deoxyadenosine effect comparable reductions of radiolabeled bicarbonate incorporation in uridine nucleosides of both normal and deficient cells, with this effect being more prolonged in deficient cells. In ADA-deficient lymphoblasts this altered pyrimidine metabolism is accompanied by reduced intracellular concentrations of PP-ribose-P with adenosine but not deoxyadenosine. The altered PP-ribose-P levels are accompanied by orotic acid accumulation, but this block does not fully account for the net reduction in uridine nucleotide synthesis. This suggests interference of the pyrimidine pathway at another locus, possibly by altered carbamyl phosphate synthetase activity. The mechanism for deoxyadenosine-mediated inhibition is obscure. Despite the demonstrable adenosine- and deoxyadenosine-mediated alterations of pyrimidine metabolism, these changes are not accompanied by marked interference of cell growth for both normal and ADA-deficient lymphoblasts. These data support the hypothesis that altered pyrimidine metabolism is not the basis for the immune disorder in patients with deficiency of ADA.

Kinetic considerations for the regulation of adenosine and deoxyadenosine metabolism in mouse and human tissues based on a thymocyte model

Metabolic regulation at a branch point may be determined primarily by relative enzyme activities and affinity for common substrate. Adenosine and deoxyadenosine are both phosphorylated and deaminated and their metabolism was studied in intact mouse thymocytes. From kinetic considerations of two activities competing for a common substrate, the deamination:phosphorylation ratio, vd/vk, at high nucleoside concentration, [S] congruent to infinity, is equal to Vd/Vk, or 34 and 1090 for adenosine and deoxyadenosine, respectively. At low substrate concentrations, [S] congruent to o, vd/vk is equal to VdKkm/VkKdm, or 0.7 and 285 for adenosine and deoxyadenosine, respectively. The analysis was extended to other mouse and human tissues by measurement of adenosine kinase, deoxyadenosine kinase and adenosine deaminase activities. All tissues were found to preferentially deaminate deoxyadenosine. Three tissue types were apparent with respect to adenosine metabolism: those which preferentially phosphorylate adenosine at all concentrations, those which switch from phosphorylation to deamination between low and high adenosine concentration and those for which deamination is quantatively important at all concentrations. Lymphoid tissues are representative of the latter category. The kinetic approach we describe offers a means of predicting nucleoside metabolism over a range of concentration which may be technically difficult to otherwise measure. The phosphorylation of adenosine and deoxyadenosine was also studied in intact thymocytes in the presence of adenosine deaminase inhibitors. The rate of deoxyadenosine phosphorylation was unaffected by coformycin or EHNA, whereas adenosine phosphorylation decreased with increasing substrate concentrations to 18% the rate in the absence of adenosine deaminase inhibitors.

Modulation of the interaction of benzo[a]pyrene with a hamster tracheal epithelial cell line

A hamster tracheal cell line rapidly absorbed and subsequently metabolized benzo[a]pyrene (B[a]P). At 10(-7) M B[a]P, less than 5% of the applied hydrocarbon was retained by the cell after 8 h incubation. However, at higher concentrations up to 20% was retained. During a subsequent 24 h incubation in the absence of polycyclic hydrocarbon, the residual B[a]P was metabolized and, for the most part, excreted. Analysis of B[a]P bound to DNA showed that alkylation increased as a consequence of metabolism of the B[a]P retained after replacement of the culture medium. Analysis of DNA repair is therefore markedly affected by the contribution from this continued alkylation. At 10(-7) M B[a]P, DNA alkylation was rapidly completed, and within 24 h a marked reduction in total adducts was observed. During this phase, a specific removal was observed of two adducts that have been tentatively identified as derived from deoxyadenosine. A slower second phase of repair was followed for up to 5 days at which time 25% of the adducts still remained in DNA. During this slow phase, the repair appeared to show preference for two of the four remaining adducts. In cells that were incubated for 8 h with 10(-6) M B[a]P, no rapid early phase of repair was seen during the following 24 h because of the continued alkylation. Thereafter, only a slow repair was observed. The deoxyadenosine adducts were still detectable 5 days after treatment suggesting that their repair was inhibited or saturated at this high concentration of B[a]P.

Identification of the mechanism of activation of 9-beta-D-arabinofuranosyladenine in human lymphoid cells using mutants deficient in nucleoside kinases

The biochemical basis of cellular resistance to 9-beta-D-arabinofuranosyladenine (ara-A) and its natural purine derivative, deoxyadenosine, was investigated with two mutants of cultured human T-lymphoblastoid CCRF-CEM cells. One mutant that lacked deoxycytidine kinase activity, designated CEM/ara-C, retained about 10% of wild-type deoxyadenosine kinase and deoxyguanosine kinase activity each but maintained normal adenosine kinase or thymidine kinase activity. This suggested that in these human T-lymphoblastoid cells, as in other previously studied mammalian cells, deoxycytidine and purine deoxyribonucleosides are phosphorylated by the same enzyme. Despite this extensive reduction of purine nucleoside kinase activities, the cytotoxicity of ara-A or deoxyadenosine was not appreciably affected, decreasing by only 2.5- and 6-fold, respectively. A second mutant, isolated by selecting CEM/ara-C mutants that were resistant to ara-A, showed a 100-fold increase in resistance to ara-A cytotoxicity. This ara-A-resistant subline was deficient in the activities of two enzymes, deoxycytidine kinase and adenosine kinase, and showed a high degree of resistance to deoxyadenosine, adenosine, and pyrazofurin but not to pyrimidine analogs, such as 5-fluorodeoxyuridine or 5-fluorouridine. Further studies of ara-A and deoxyadenosine phosphorylation in wild-type and resistant cell lines disclosed that, although deoxycytidine kinase is the principal enzyme for their phosphorylation in vitro, their intracellular conversion to cytotoxic nucleotides depends on the joint action of deoxycytidine kinase and adenosine kinase rather than purine-specific deoxynucleoside kinase, as previously thought.

DNA alkylation by vinyl chloride metabolites: etheno derivatives or 7-alkylation of guanine?

The state of the literature led to a re-investigation of the alkylation products caused by vinyl chloride metabolites in DNA. When rat liver microsomes, an NADPH-regenerating system, DNA and [14C]vinyl chloride were incubated and, when the DNA was subsequently re-isolated and (enzymatically) hydrolyzed, chromatograms (on Aminex A-6) showed the presence of 1,N6-ethenodeoxyadenosine, 3,N4-ethenodeoxycytidine and 7-N-(2-oxoethyl)guanine (the product of hydrolysis of 7-N-(2-oxoethyl)-deoxyguanosine). By contrast, when rats were exposed to [1,2-14C]vinyl chloride and when the liver DNA of these rats was subjected to similar procedures, no radioactive 'etheno' derivatives could be detected, but a radioactive peak was eluted with 7-N-(2-oxoethyl)guanine. This peak could be transformed into 7-N-(2-hydroxyethyl)guanine; the chromatographic behaviour of which was identical to the reference compound used by Ostermann-Golkar et al. (Biochem. biophys. Res. Commun., 76 (1977) 259). Thus, it is concluded that the compound described by these authors, 7-N-(2-oxoethyl)guanine is in fact the major product of base alkylation in DNA after exposure to vinyl chloride.

Deoxycoformycin: neurological toxicity

Deoxycoformycin (DCF) is a tight-binding inhibitor of adenosine deaminase (ADA) currently undergoing phase I--II evaluation. Neurological toxicity has been a frequent and occasionally severe complication of treatment with this drug. A T-cell leukemia patient with an Ommaya reservoir was treated with DCF, and the pharmacokinetics of the drug in the cerebrospinal fluid and plasma were studied. DCF penetrates the cerebrospinal fluid and achieves levels as high as 1/10 the concurrent plasma levels. The accumulation of adenosine and deoxyadenosine in plasma, cerebrospinal fluid, and urine was monitored; the neuropharmacological effect of these metabolites is discussed.

In vivo formation of benzo(alpha)pyrene diol epoxide-deoxyadenosine adducts in the skin of mice susceptible to benzo(alpha)pyrene-induced carcinogenesis

The hydrocarbon-deoxyribonucleoside adducts formed in mouse skin DNA have been determined following topical application of an initiating dose of benzo(a)pyrene to Swiss mice, a strain shown to be susceptible to benzo(a)pyrene-induced skin carcinogenesis. Several DNA-bound products were formed, of which the major one (60% of total adducts), in agreement with other workers' findings, was derived from reaction of (+/-) 7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo(alpha)pyrene (BDE) with the exocyclic aminogroup of deoxyguanosine. A further product (9-10% of total adducts), previously observed only after microsomal activation of benzo(a)pyrene, was observed and co-chromatographed with a further metabolite of 9-hydroxybenzo(alpha)pyrene bound to an uncharacterized base in the DNA. Two otherr products (2-3% of total adducts) were also found in the in vivo studies which co-chromatographed with BPDE-deoxyadenosine adducts and arose from cis and trans addition of the exocyclic amino group of deoxyadenosine to the 7R form, but not the 7S form, of BPDE. In contrast to this, the major in vitro deoxyadenosine-bound products, formed following reaction of BPDE with calf-thymus DNA, were derived from the 7S form of BPDE, suggesting either stereoselective formation or reaction of the 7R form of BPDE in mouse skin in vivo. Similar amounts of BPDE-deoxyguanosine and BPDE-deoxyadenosine adducts, as well as those derived from further metabolism of 9-hydroxybenzo(alpha)pyrene were formed in three strains of mice reported to have widely differing susceptibilities to polycyclic hydrocarbon-induced skin carcinogenesis. The relevance of these different hydrocarbon-DNA adducts to carcinogenesis requires further investigation.

Deoxyguanosine kinase of neonatal mouse skin tissue

Deoxyguanosine kinase (ATP: deoxyguanosine 5'-phosphotransferase) activity has been identified in neonatal mouse skin tissue. This activity which has a molecular weight of 44 000 was shown to be highly specific for deoxyguanosine as a substrate. Unique to deoxynucleoside kinases was the observation that this enzyme possessed a pH optimum of 5.2. In dilute solutions catalytic activity is lost, however the enzymatic activity can be stabilized by the addition of 20 micrometer MgATP or ATP. Kinetic analysis gave an apparent Km for deoxyguanosine of 7 micrometer. Double-reciprocal plots of activity vs. MgATP concentration produced a broken line with the break occurring at 0.5 mM MgATP. Below this concentration an apparent Km for MgATP of 23 micrometer was measured; above 0.5 mM MgATP an apparent Km of 265 micrometer was calculated. Mouse skin deoxyguanosine kinase was strongly inhibited by dGTP, dGDP and UDp. dGTP was a competitive inhibitor of deoxyguanisine with an apparent Ki of 1.9 micrometer. UPD (K1,app = 3 micrometer), dGDP = 0.7 micrometer) (Ki,app = 0.07 micrometer] were competitive inhibitors of MgATP and dGTP ki,app = 0.07 micrometer) were competitive inhibitors of MGATP when the concentration of MgATRP was greater than 0.5 mM.

Uridine metabolism in the goldfish retina during optic nerve regeneration: whole retina studies

Accumulation of radioactivity from [3H]uridine in incubations of whole goldfish retinas is increased in the ipsilateral retina during a period of regeneration that follows unilateral optic nerve crush. Brief incubations to investigate the nature of enhanced labeling of the acid-soluble fraction showed a peak uptake 4 days following crush, with a gradual decrease to control levels by 21 days following crush. That nucleoside uptake may not mediate the effect is supported by the observation that the rate of uptake of 5'-deoxyadenosine, a nonmetabolizable nucleoside analog, is the same in post-crush (PC) and normal (N) retinal incubations. Following brief incubations of PC and N retinas with [3H]uridine, there is enhanced labeling in PC retinas relative to N retinas of recovered UMP, UDP, UTP, and uridine nucleotide sugars, whereas recovery of labeled uridine itself is slightly decreased. The results suggest that the increased accumulation of radioactivity in PC retinas following incubation with uridine reflects an increase in the activities of retinal uridine kinase and uridine nucleotide kinases.

A comparison of the kinetic properties of the common and rare variants of adenosine deaminase

Adenosine deaminase activity has been measured in red cells from individuals of known ADA phenotype (ADA 1, ADA 2-1, ADA 3-1, ADA 3-2) using adenosine and 2'-deoxyadenosine as substrates. No significant differences were observed among the phenotypes in their relative deaminase activity with the two substrates. However, evidence suggests the occurrence of an uncommon allele designated ADA1w determining low levels of ADA activity. The deaminase activities of the phenotypes were in the order ADA 1 greater than ADA 2-1 greater than ADA 3-1 greater than ADA 3-2 with both substrates. The relative activities of the alleles were estimated to be: ADA1 100%, ADA2 89%, ADA3 28% and ADA1w 67% with adenosine, and ADA1 100%, ADA2 87%, ADA3 39% and ADA1w 66% with 2'-deoxyadenosine. The Michaelis constants for adenosine and 2'-deoxyadenosine were determined for the different phenotypes. There were no significant differences in these values among the phenotypes.

Preferential inhibition of ribonucleic acid synthesis by a new thiosemicarbazone possessing antibacterial and antiparasitic properties

We determined the influence of the azacycloheptane derivative (H) of a 2-acetylpyridine thiosemicarbazone on growth and macromolecular synthesis in Escherichia coli AT-9. Thiosemicarbazone H caused bacteriostasis and a primary inhibition of ribonucleic acid synthesis; secondary effects included inhibition of deoxyribonucleic acid and protein synthesis. Addition of cooper or other transition elements was not necessary for these inhibitions.

Deoxyadenosine/deoxycytidine kinase from Bacillus subtilis. Purification, characterization, and physiological function

Three different deoxyribonucleoside kinases with specificities toward thymidine, deoxyguanosine, and deoxyadenosine/deoxycytidine, respectively, are identified in Bacillus subtilis. The deoxyadenosin/deoxycytidine kinase is purified 950-fold employing blue Sepharose CL-6B column chromatography. The two deoxyribonucleoside kinase activities copurify and are present in the same band after polyacrylamide gel electrophoresis. The molecular weight is determined by gel filtration to be 47,000. Cytidine, adenosine, arabinosylcytosine, and arabinosyladenine are substrates for the enzyme. The activities toward these substrates are less than 20% of the activities obtained with deoxyadenosin and deoxycytidine. The deoxycytidine and deoxyadenosine saturation curves are hyperbolic with Km values for both nucleosides around 5 microM. The maximal velocities for the two deoxyribonucleosides are nearly identical with GTP as phosphate donor. GTP is the best donor showing hyperbolic saturation curves and Km values around 150 microM depending on the deoxyribonucleoside concentration. dATP and dCTP are inhibitors when GTP is the phosphate donor. They may both act as phosphate donors themselves. A divalent metal ion is required, Mg2+ giving the highest activity. A spontaneous mutant, selected as resistant to 5-fluorodeoxycytidine, lacks both deoxycytidine and deoxyadenosine kinase activity, while it retains normal activities toward deoxyguanosine, deoxyuridine, and thymidine.

2'-Deoxy-2'-azidoadenosine triphosphate and 2'-deoxy-2'-fluoroadenosine triphosphate as substrates and inhibitors for Escherichia coli DNA-dependent RNA polymerase

The effects of 2'-substitutions of ATP on the substrate and inhibitor properties for RNA synthesis were studied in the poly(dAT)-dependent reaction of Escherichia coli RNA polymerase. In the presence of UTP, 2'-deoxy-2'-azidoadenosine 5'-triphosphate (AZTP) was incorporated into an acid-insoluble fraction at one-tenth of the rate of ATP incorporation; it thus acts as a competitive inhibitor for poly(AU) synthesis. On the other hand, another ATP analog, 2'-deoxy-2'-fluoroadenosine 5'-triphosphate (AfTP), was co-polymerized with UTP into acid-insoluble materials at a rate less than 1% of that of ATP incorporation; in addition, it exerted a strong but mixed-type inhibition on poly(AU) synthesis. Different modes of action of the two ATP analogs are discussed in connection with the specificity of substrate recognition by RNA polymerase.