Different effect of dGTP on 2'-deoxyadenosine metabolism in mitochondria and cytosol

Two enzymes participating in 2'-deoxyadenosine (dAdo) metabolism: dAdo kinase (dAdoK EC 2.7.1.76) and adenosine deaminase (ADA, EC 3.5.4.4) were partially purified from rat liver mitochondria and cytosol and influence of nucleosides and nucleotides on the activity of these enzymes were investigated. Mitochondrial and cytosol dAdoK are separate proteins, while ADA from both subcellular fractions possesses similar physical properties. dGTP, a competitive inhibitor of mitochondrial dAdoK, inhibits cytosol ADA in a mixed way but activates mitochondrial ADA and cytosol dAdoK. A possible effect of dGTP on dAdo metabolism in mitochondria and cytosol is discussed.

Cytostasis induced in L1210 murine leukaemia cells by the S-adenosyl-L-methionine decarboxylase inhibitor 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine may be due to hypusine depletion

The effects of inhibition of the capacity to form spermidine and spermine on cell growth were investigated using murine leukaemia L1210 cells and 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine (MDL 73811, AbeAdo), an enzyme-activated irreversible inhibitor of S-adenosyl-L-methionine decarboxylase. Putrescine levels were increased 80-fold, and spermidine and spermine levels were greatly reduced after a 3-day exposure to a maximally inhibitory dose of 10 microM-AbeAdo. Addition of AbeAdo to the culture medium inhibited the growth of L1210 cells measured 3 days later in a dose-dependent manner, but, even at a dose of 10 microM, which was maximally effective, exposure to AbeAdo was not immediately cytostatic. However, the growth rate of L1210 cells chronically exposed to 10 microM-AbeAdo declined steadily until day 12, when the cells stopped growing. L1210 cells exposed to AbeAdo for 12 days could not be rescued from cytostasis by removal of the drug from the culture, but could be rescued by exposure to exogenous spermidine or spermine, indicating that the growth-inhibitory effects of AbeAdo were a result of spermidine and/or spermine depletion. It is suggested that elevated intracellular putrescine in AbeAdo-treated cells sustained limited growth in the absence of physiological levels of spermidine and spermine until certain critical and specific physiological role(s) fulfilled by spermidine (and/or spermine) became deficient resulting in cytostasis. N-(3-Aminopropyl)-1,4-diamino-cis-but-2-ene, a spermidine analogue that is a substrate for deoxyhypusine synthase, was able to mimic the effects of spermidine in reversing AbeAdo-induced cytostasis. Spermidine analogues such as 5,5-dimethylspermidine, which are not substrates for deoxyhypusine synthase, were not active in this way. These results provide evidence that the formation of hypusine in the protein-synthesis initiation factor eIF-5A may be a critical role of spermidine essential for cell growth.

Effect of thiamin on cordycepin sensitivity in Saccharomyces cerevisiae

The sensitivity of Saccharomyces cerevisiae to the antibiotic cordycepin (3'-deoxyadenosine) was found to be decreased by the addition of thiamin to the growth medium. A thiamin transport mutant of S. cerevisiae was also found to be resistant to the growth inhibitory effect of cordycepin. Not only the thiamin uptake but also adenosine uptake by this mutant cell was markedly reduced compared to those by the parent yeast cells. This strongly suggested that cordycepin, an analog of adenosine, is virtually taken up by the thiamin transport system of growing yeast cells; thus the drug sensitivity is decreased by the presence of thiamin in the growth medium.

Selective reactivities of isocyanates towards DNA bases and genotoxicity of methylcarbamoylation of DNA

The reactivities of methyl isocyanate (MIC) and phenyl isocyanate (PIC) with DNA, and the genotoxicity of MIC were investigated. MIC and PIC reacted with the exocyclic amino group of deoxycytidine, deoxyadenosine and deoxyguanosine to produce carbamoylated products. The reactions of both isocyanates with deoxycytidine were 2 and 4 orders of magnitude higher than with deoxyadenosine and deoxyguanosine, respectively. To explore the genotoxicity of MIC, M13mp9 RF DNA was modified with MIC and then introduced into E. coli. The plaque-forming efficiencies of DNA decreased with increasing dose levels, and the decreases were more pronounced in Uvr endonuclease-deficient strains (uvrA, uvrB and uvrC) than in the Uvr endonuclease-proficient strain, JM103. The differences in survival in JM103 and uvr- strains suggest that the methylcarbonyl adducts can be removed by the uvr excision-repair system. Modification of M13mp9 RF DNA with MIC induced MIC-dose-related, SOS-dependent mutations in the beta-galactosidase locus. These results demonstrate the genotoxic response of MIC-modified DNA in E. coli.

Substrate specificity of the purine-2'-deoxyribonucleosidase of Crithidia luciliae

The purine-2'-deoxyribonucleosidase of Crithidia luciliae catalyses an efficient deoxyribosyl transfer between a variety of purine bases, benzimidazole and 5,6-dimethylbenzimidazole. Since the deoxyriboside of a deoxyribosyl acceptor is necessarily also a substrate, the trans-N-deoxyribosylase activity of the enzyme allows a study of its specificity to be extended to a large number of purines and purine analogues. Amongst 27 different deoxyribosyl acceptors, only hypoxanthine gave rise to isomeric products. The introduction of methyl groups at appropriate positions in either purine or benzimidazole lowered the Michaelis constant, KB, for deoxyribosyl acceptors: by about 10-fold for 6-methylpurine (KB 351 +/- 87 microM) compared with purine (KB 3.91 +/- 0.8 mM) and by about 10(3)-fold for 5,6-dimethylbenzimidazole (KB 7.0 +/- 0.79 microM) compared with benzimidazole (Km,app. 7.8 +/- 2.4 mM). The maximal rates of deoxyribosyl transfer to different acceptors, on the other hand, varied by only 4.5-fold, and can be ascribed to decreases in the rate of release of the newly formed purine deoxyriboside from the enzyme.

Synthesis of 2'-deoxy-7,8-dihydro-8-oxoguanosine and 2'-deoxy-7,8-dihydro-8-oxoadenosine and their incorporation into oligomeric DNA

Reliable methods have been developed for the synthesis of the 3'-O-[(diisopropylamino) (2-cyanoethoxy)phosphino]-5'-O-(4,4'- dimethoxytrityl) derivatives of 2'-deoxy-7,8-dihydro-8-oxoguanosine (8-oxo-dGuo, 1) and 2'-deoxy-7,8-dihydro-8-oxoadenosine (8-oxo-dAdo, 2), and for the efficient incorporation of the latter into oligomeric DNA. Both methods rely on the conversion of the 2'-deoxy-8-bromopurine nucleosides 3 and 10 to their corresponding 2'-deoxy-8-(benzyloxy) nucleosides 4 and 12 followed by catalytic hydrogenation to generate the 8-oxo function at the C-8 position. The preparation of the phosphoramidites 8 and 19 required for the synthesis of a series of DNA oligomers was carried out under strictly anhydrous conditions. Failure to keep the systems dry resulted in great difficulties during the purification procedures, and erratic results when DNA synthesis was attempted. In the preparation of the DNA itself, it was found to be extremely important during the ammonia deprotection step to add an antioxidant. Otherwise aerial oxidation resulted in almost complete loss of the oligomer. However, when these special conditions were followed, oligomeric DNA containing 8-oxo-dGuo and 8-oxo-dAdo residues could be prepared in excellent yield. Analysis of selected DNA oligomers by enzymatic degradation and mass spectroscopic analysis confirmed the designated sequences and compositions.

Nucleoside transporters in Leishmania major: diversity in adenosine transporter expression or function in different strains

Cytotoxic nucleoside derivatives may become useful in the treatment of parasitic infections. As part of our drug development studies, the effect of a number of nucleosides (100 microM) on the cellular transport of 3H-adenosine and 3H-inosine (each at 1 microM) in promastigotes from four Leishmania major strains was investigated. When 3H-inosine was used as permeant, all strains exhibited essentially the same inhibition profile, with unlabeled inosine, guanosine, formycin B, and 3'-deoxyinosine being strongly inhibitory, and adenosine-related compounds such as 2'-deoxyadenosine and tubercidin being inactive. However, when 3H-adenosine was used as permeant, considerable differences in the inhibition profiles were noted among strains. Thus, both inosine transporter-selective nucleosides such as inosine and guanosine and adenosine transporter-selective nucleosides such as 2'-deoxyadenosine and tubercidin showed variable activity as inhibitors of 3H-adenosine transport in different strains. These observations indicated that an adenosine transporter was variably expressed in different strains, and that inhibition profiles for adenosine transport indicated cellular entry via both the inosine and adenosine transporters. The existence of different types of adenosine transporters as an alternative explanation could not be ruled out. The apparent uniform expression of an inosine transporter among different species and strains of Leishmania suggests that inosine derivatives may be useful as anti-leishmanial drugs.

A new and potent 2-5A analogue which does not require a 5'-polyphosphate to activate mouse L-cell RNase L

In order to explore the possibility of supplanting the requirement of a 5'-triphosphate moiety for the activation of the 2-5A-dependent endonuclease (RNase L) of mouse L-cells, two new tetrameric analogues of 2-5A were synthesized. The first tetramer, obtained by both a modified prebiotic synthetic approach as well as a phosphite triester solid phase oligonucleotide synthesis method, was p5'A2'p5'A2'p5'(br8A)2'p5'(br8A). The second oligonucleotide was derived from the former by a sequence involving periodate oxidation, reaction with n-hexylamine, and cyanoborohydride reduction, resulting in conversion of the 2'-terminal adenosine residue to 9-(3'-aza-4'-hexyl-1',2',3',4'-tetradeoxyhexopyranos-1(1)-yl)-8-++ +bromoadenine. Both of these oligomers, bearing only 5'-monophosphate groups, were found to be as potent as 2-5A itself as activators of the RNase L of mouse L-cells.

A novel route of ATP synthesis

Incorporation of the adenine moiety of 2'-deoxyadenosine (dAdo) into ATP, consistently observed in human erythrocytes, is a phenomenon which cannot be explained by the operation of any known pathway. We reported previously that this effect was not observed in adenine phosphoribosyltransferase-deficient erythrocytes showing that adenine must be an obligatory intermediate. However, generation of adenine from dAdo was difficult to reconcile with the operation of any known process in human cells, and involvement of S-adenosylhomocysteine hydrolase (SAH-hydrolase) was postulated. The present studies with intact human erythrocytes demonstrate that nucleoside analogues which inhibit SAH-hydrolase caused substantial attenuation of adenine transfer from dAdo into ATP. It was confirmed that dAdo is not a substrate of 5'deoxy-5'methylthioadenosine (5'MT-adenosine) phosphorylase. Inhibition of the transfer of the adenine moiety of dAdo into ATP did not correlate with inhibition of 5'MT-adenosine phosphorylase by nucleoside analogues. This report provides further evidence that the pathway involving nucleoside (adenosine) analogue binding to SAH-hydrolase, release of base and subsequent phosphoribosylation can operate in intact cells. The metabolic significance of this process relates to the possible generation of free bases (adenine) in the human body, ATP synthesis and nucleoside drug interconversions.

Uptake of the antitrypanosomal drug 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine (MDL 73811) by the purine transport system of Trypanosoma brucei brucei

An irreversible inhibitor of S-adenosyl-L-methionine decarboxylase (AdoMetDC), 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine (MDL 73811), was found to cure Trypanosoma brucei brucei and multidrug-resistant T. b. rhodesiense infections in mice [Bitonti, Byers, Bush, Casara, Bacchi, Clarkson, McCann & Sjoerdsma (1990) Antimicrob. Agents Chemother. 34, 1485-1490]. Doses of this drug which resulted in a rapid clearance of parasites from T. b. brucei-infected rats resulted in plasma levels of 50-60 microM-MDL 73811 and an intratrypanosomal MDL 73811 concentration of 1.9 mM within 10 min of administration [Byers, Bush, McCann & Bitonti (1991) Biochem. J. 274, 527-533[. Based on this finding we speculated that MDL 73811, which is an adenosine analogue, is a substrate for the trypanosome active purine transport system. We now report evidence that supports this hypothesis. MDL 73811 uptake by T. b. brucei in vitro was time- and temperature-dependent and was saturable over a time course in which MDL 73811 metabolism was undetectable, suggesting that MDL 73811 uptake is a transport-mediated phenomenon. Inhibition of MDL 73811 uptake by purine nucleosides is consistent with the drug being a substrate for the trypanosome purine transport system. The accumulation of MDL 73811 by cultured L1210 mouse leukaemia cells was significantly less than by trypanosomes exposed to the same pharmacologically relevant concentrations of MDL 73811. Given that the half-life of MDL 73811 in the plasma of rats and mice is approx. 10 min, it seems likely that the existence of a highly active parasite transport system for MDL 73811 is crucial for the sensitivity of trypanosomes towards MDL 73811 in vivo, and that the absence of active transport of MDL 73811 by the host's cells may play a role in the selectivity of this drug.

The rate of catabolism of dATP to deoxyadenosine during the growth of different cell lines in vitro

A method has been developed for the determination of the rate of formation of deoxyadenosine from dATP in cultured cell lines. The lowest rate was found in the T-cell-derived Molt cell line while it was about 70-fold higher in Balb c/3T3 mouse fibroblasts. In the B-cell-derived Raji cells and in the murine sarcoma cell line SEWA it had intermediary values. It is concluded that in some cell types like the 3T3-cells the catabolism of dATP to deoxyadenosine may have a significant regulatory effect on the cellular content of dATP.

Partial purification of a human DNA glycosylase acting on the cyclic carcinogen adduct 1,N6-ethenodeoxyadenosine

We previously reported that a variety of human cells and tissues contained a Mr35,000 DNA-binding protein which selectively recognized a single 1,N6-ethenoadenine in a defined 25-base double-stranded oligonucleotide (B. Rydberg et al., Proc. Natl. Acad. Sci. USA, 88: 6839-6842, 1991). We now demonstrate that incubation of the same duplex with 50-fold partially purified binding protein from human placenta results in release of the free 1,N6-ethenoadenine base, indicative of DNA glycosylase action. This enzyme activity appears unique in that it excises a cyclic adduct resulting from a known human carcinogen.

[The lethal and mutagenic action of 3H incorporated into the 2' position of the deoxyribose in the DNA of the extracellular phage lambda]

The lethal and mutagenic effects of 3H decay in 2' position of deoxyribose residues in DNA of extracellular lambda phage were studied, [2'-3H]-deoxyadenosine (3H-dA) or [2'-3H]-thymidine (3H-dT) being used as labelled DNA precursors. As estimated by the efficiency of the lethal and mutagenic actions of 3H decay in position 2' was significantly lower than that of the decay in the incorporated 3H-pyrimidines. The genetic effects of 3H decay in 2' position may be attributed to the radiation effect of beta-particles on DNA. In UV-irradiated E. coli cells, with the induced SOS repair, the mutagenic effect of 3H-dA in phage lambda is significantly higher than that of 3H-dT. This is perhaps related to the formation in DNA of AP-sites, resulting from 3H-decay in 2' position, and to the predominant incorporation of adenosine residues opposite to AP-sites during SOS repair.

Dimethylchrysene diol epoxides: mutagenicity in Salmonella typhimurium, tumorigenicity in newborn mice, and reactivity with deoxyadenosine in DNA

In contrast to 5-methylchrysene and 5,9-dimethylchrysene, 5,6-dimethylchrysene and 5,7-dimethylchrysene are weak tumor initiators on mouse skin. In order to investigate the basis for this, we have evaluated the mutagenic activities toward Salmonella typhimurium TA 100 and reactivity with DNA of (+/-)-anti-1,2-dihydroxy-3,4-epoxy-1,2,3,4-tetrahydro-5,6-dimethyl-ch rys ene (anti-5,6-diMeC-1,2-diol 3,4-epoxide) and anti-5,7- and anti-5,9-diMeC-1,2-diol 3,4-epoxide. The tumorigenic activities of anti-5,6- and anti-5,7-diMeC-1,2-diol 3,4-epoxides in newborn mice were also investigated. anti-5,9-diMeC-1,2-diol 3,4-epoxide was the most mutagenic of the three diol epoxides. anti-5,6-diMeC-1,2-diol 3,4-epoxide was highly tumorigenic in newborn mouse lung, with activity significantly greater than that of either anti-5-MeC- or anti-5,7-diMeC-1,2-diol 3,4-epoxide. Although the amounts of total binding of the diol epoxides to calf thymus DNA were similar, anti-5,6-diMeC-1,2-diol 3,4-epoxide bound extensively to deoxyadenosine residues. High binding to deoxyadenosine is related to the presence of a sterically hindered bay or fjord region as present in 5,6-diMeC, 7,12-dimethylbenz[a]anthracene, benzo-[g]chrysene, and benzo[c]phenanthrene. The conformations of the anti- and syn-diol epoxides of 5,6-diMeC and benzo[c]phenanthrene were similar, with both having pseudodiequatorial hydroxyl groups, in contrast to less sterically crowded diol epoxides. The high tumorigenicity of anti-5,6-diMeC-1,2-diol 3,4-epoxide in newborn mice is of interest with respect to its high deoxyadenosine binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynamics of the dATP pool in cultured mammalian cells

Conditions for labeling the dATP pool of V79 and 3T3 cells from [3H]deoxyadenosine (salvage) or [3H]adenine (via ribonucleotide reduction) were established. With deoxyadenosine the specific radioactivity of dATP reached a constant value after 60 min. In resting 3T3 cells this value was 30 times higher than in S-phase cells. Turnover of dATP and absolute rates of DNA synthesis and excretion of breakdown products of dATP were determined from the accumulation of isotope in various compartments and the specific activity of dATP. In S-phase cells the dATP pool had a half-life of 4 min, identical to that of dTTP determined earlier. Deoxyadenosine was the major breakdown product of dATP in the presence of an inhibitor of adenosine deaminase. The rate of deoxyadenosine excretion of V79 cells amounted to 4% of the rate of dATP incorporation into DNA. Inhibition of DNA replication increased deoxyadenosine excretion 5- to 10-fold, demonstrating a continued de novo synthesis of dATP, albeit at a slightly reduced rate. Our results fit a model involving a substrate cycle between dAMP and deoxyadenosine regulating the dATP pool, similar to the model of substrate cycles involved in the regulation of pyrimidine deoxyribonucleotide pools developed earlier.

Substrate specificity of human deoxycytidine kinase toward antiviral 2',3'-dideoxynucleoside analogs

Deoxycytidine (dCyd) kinase has been purified to homogeneity from human leukemic spleen, and the capacity of the enzyme to phosphorylate 2',3'-dideoxynucleoside (ddN) analogs that are clinically effective inhibitors of human immunodeficiency virus (HIV) replication was evaluated. Cytosine-containing ddN analogs, such as 2',3'-dideoxycytidine, 2',3'-dideoxy-2',3'-dehydrocytidine, and cytallene, were efficiently phosphorylated by dCyd kinase, while no phosphorylation of purine-containing ddN analogs was detected. dCyd kinase was completely inactive toward 2',3'-dideoxyadenosine (ddAdo), 2',3'-dideoxyinosine, 2',3'-dideoxyguanosine, and adenallene, although it was capable of phosphorylating both 2'-deoxyadenosine (dAdo) and 2'-deoxyguanosine (dGuo). The abilities of wild type and mutant human T lymphoblastoid CEM cells to accumulate ddAdo in situ and in vitro were also ascertained. Comparison of the abilities of intact wild type CEM cells and derivatives deficient in nucleoside transport, dCyd kinase, and/or adenosine (Ado) kinase to accumulate [3H]ddAdo-derived radioactivity revealed no significant differences among the wild type and mutant strains. However, ddAdo phosphorylating activity was decreased in extracts from Ado kinase-deficient cells but not in lysates prepared from cells genetically deficient in dCyd kinase activity. In comparative growth rate experiments, wild type, nucleoside transport-deficient, and dCyd kinase-deficient CEM cells were equally sensitive to ddAdo toxicity, while, interestingly, a deficiency in Ado kinase correlated with a 5-fold decreased growth sensitivity to the purine ddN. Insertion of an adenine phosphoribosyltransferase deficiency into the CEM cell lines did not influence ddAdo toxicity or incorporation rate. These results imply that Ado kinase may be an important factor in ddAdo phosphorylation by CEM cells. Furthermore, these studies demonstrate that cytosine- and purine-containing ddNs are transported and activated by independent pathways and, therefore, have important implications for anti-HIV therapy in that pyrimidine and purine ddNs might be used in combination for the treatment of acquired immunodeficiency syndrome.

Factors influencing the inhibition of repair of irradiation-induced DNA damage by 2'-deoxycoformycin and deoxyadenosine

Permeabilized L5178Y cells were used to investigate the mechanism underlying inhibition of the repair of irradiation-induced DNA strand breaks by 2'-deoxycoformycin combined with deoxyadenosine. Permeabilized cells repaired DNA strand breaks as effectively as did intact cells, and at deoxyadenosine concentrations that produced similar levels of deoxyadenosine triphosphate (dATP), repair of DNA strand breaks was inhibited by 2'-deoxycoformycin plus deoxyadenosine to a comparable extent in both types of cells. Accompanying the increase in intracellular dATP produced by 2'-deoxycoformycin combined with deoxyadenosine was a fall in levels of deoxythymidine triphosphate (dTTP), deoxyguanosine triphosphate (dGTP), and deoxycytidine triphosphate (dCTP). The addition of dTTP, dGTP, and dCTP reversed the inhibition of DNA repair by 2'-deoxycoformycin plus deoxyadenosine, although the level of dATP was not affected. Reducing the phosphorylation of deoxy-adenosine to dATP by the addition of adenosine prevented the decrease in levels of dTTP, dGTP, and dCTP and the inhibition of DNA repair by 2'-deoxycoformycin and deoxyadenosine. In contrast, increasing the intracellular levels of dATP by the addition of 2'-deoxycoformycin together with dATP, deoxyadenosine diphosphate (dADP), or deoxyadenosine monophosphate (dAMP) had no effect on the levels of the other deoxynucleotide triphosphates and did not inhibit DNA repair. Moreover, DNA repair was not inhibited by the breakdown products of deoxyadenosine, adenine, or deoxyribose. These results suggest that inhibition of the repair of irradiation-induced DNA strand breaks by 2'-deoxycoformycin combined with deoxyadenosine requires the phosphorylation of deoxyadenosine and involves alterations in the levels of deoxynucleotide triphosphates.

Toxicity and metabolism of 3'-deoxyadenosine N1-oxide in mice and Ehrlich ascites tumor cells

The toxic effect of 3'-deoxyadenosine N1-oxide (3'-dANO) on mice, on their different organs, and on Ehrlich ascites tumor cells was studied. In both healthy and tumour-bearing animals, the lethal dose for 10% of the mice receiving i.p. injections (LD10) of 3'-dANO was estimated to be about 300 mg/kg x 4 days in one mouse strain (Theiller). In another mouse strain (NMRI), we obtained a markedly higher LD10 value (675 mg/kg x 5 days). At nonlethal doses (250 mg/kg x 4 days), we observed reversible neurological symptoms on days 4-12 after treatment, but no macroscopical or microscopical changes was detected in the brain, heart, thymus, lung, lymph node, spleen, liver, kidney, bone marrow, or gastrointestinal tract. At doses of 450 mg/kg x 4 days, severe neurological symptoms were observed, and atony of the gastrointestinal canal and damage to the kidney and liver were registered. Even at doses that were lethal to the mice, no histopathological change was observed in the bone marrow or in the gastrointestinal canal. Pharmacokinetics studies showed that after the i.p. injection of 3'-dANO, the maximal plasma concentration was reached after 10 min, after which it declined showing a half-life of about 40 min. A transient accumulation of 3'-deoxyadenosine triphosphate (3'-dATP) was observed within 24 h in the liver and kidney, with the maximal concentration being reached after about 2-3 h. 3'-dANO was excreted partly as the unchanged substance and partly as the metabolite 3'-deoxyinosine within 24 h. Flow-cytometric DNA analysis of Ehrlich tumor cells treated either in vitro or in vivo with 3'-dANO revealed no therapy-induced change in the cell-cycle perturbations, which indicates that cells were randomly killed during all phases of the cycle.

Effects of (R)-deoxycoformycin (pentostatin) on intrauterine nucleoside catabolism and embryo viability in the pregnant mouse

The viability of early mouse embryos is acutely sensitive to (R)-deoxycoformycin (pentostatin), a tight-binding inhibitor of adenosine deaminase (ADA). Previous studies have shown that a single 5-mg/kg dose on day 7 (plug = day 0) of gestation fully inhibits uteroplacental ADA activity within 0.5 h; causes massive cell death in the neural plate and primary mesenchyme by 6 h, major craniofacial anomalies by day 10, and resorption by day 12 (Knudsen et al., '89; Airhart et al., '91). The present study has examined further the developmental toxicity and early effects of this inhibitor on ADA metabolism. (R)-Deoxycoformycin was administered to pregnant CD-1 (ICR) mice as a single intraperitoneal dose of 0.5-10 mg/kg total body weight on days 6-11 of gestation. The major adverse effect, early resorption, was dose dependent and specific to day 7-8 exposure. Treatment with 5 mg/kg on day 7 resulted in 85% resorptions, 15% malformations, and a 24% reduction in mean fetal weight, whereas the same dose of (S)-deoxycoformycin had no effect. Levels of adenosine and 2'-deoxyadenosine, which are the endogenous substrates of ADA, were monitored in the embryo/decidual unit (E/D) by reversed-phase high-performance liquid chromatography (RP-HPLC). In response to the inhibitor, both nucleosides increased transiently in the antimesometrial compartment (antimesometrial decidua + embryo). Peak levels (Cmax) of adenosine and 2'-deoxyadenosine were dose dependent over the range tested (0.05-10 mg/kg). Exposure to 5 mg/kg on day 7 raised adenosine levels within 0.5 h to 42-fold over the basal level of 0.06 nmol/mg protein. There was an even stronger effect on 2'-deoxyadenosine levels, which were elevated 674-fold over the detection limit of 0.0005 nmol/mg protein. Direct exposure to the inhibitor in serum-free E/D culture produced similar results: 50 microM (R)-deoxycoformycin within 1 h raised adenosine levels 26-fold and 2'-deoxyadenosine levels 410-fold. In vivo studies also showed a general correlation between embryolethality and the length of adenine nucleoside pool expansion, apparent for exposure on day 7, 8, or 9 but not on day 6, suggesting that the embryo becomes sensitive to adenosine or 2'-deoxyadenosine once the neural plate has formed.

DNA polymerase-mediated nucleotide incorporation adjacent to hydrocarbon-deoxyadenosine and hydrocarbon-deoxyguanosine adducts

To examine the effect of DNA adducts on nucleotide incorporation by DNA polymerase at 3' neighboring bases, synthetic oligonucleotides (16mers) containing a purine at position 13 from the 3' end and any one of the four possible bases at position 12 were prepared and reacted with 7-bromomethylbenz[a]anthracene. Using HPLC, unmodified oligonucleotide was separated from oligonucleotide containing a single adduct, at either an adenine or a guanine residue. These products were annealed with a 32P 5'-end labeled primer (11mer) and incubated with modified T7 DNA polymerase (Sequence, version 2.0) in the presence of deoxyribonucleoside 5'-triphosphates. Analysis by gel electrophoresis showed that unmodified oligonucleotide template allowed the primer to be rapidly extended to the entire length of the template. However, the presence of an adduct caused primer extension to stop at the base 3' to the adduct. While correct base pairing occurred at this termination site with most adducted templates, there was a high frequency of misincorporation of guanine opposite a thymine located 3' to an adenine adduct. This result suggest that some bulky carcinogen--DNA adducts may lead to base mismatches at neighboring bases.

Formation of C8-modified deoxyguanosine and C8-modified deoxyadenosine as major DNA adducts from 2-nitropyrene metabolism mediated by rat and mouse liver microsomes and cytosols

2-Nitropyrene, the geometric isomer of the most studied nitropolycyclic aromatic hydrocarbon (nitro-PAH), 1-nitropyrene, is an environmental contaminant detected in ambient air and a potent direct-acting mutagen. Its metabolic activation leading to the formation of DNA adducts was studied. The activated metabolite, N-hydroxy-2-aminopyrene, was prepared and reacted with calf thymus DNA. Upon enzymatic hydrolysis of the DNA, the resulting nucleosides were separated by HPLC, and the adducts were characterized by mass and proton NMR spectral analysis. Both N-(deoxyguanosin-8-yl)-2-aminopyrene and N-(deoxyadenosin-8-yl)-2-aminopyrene, in a 5:2 ratio, were identified. These adducts were then utilized as standards to identify the DNA adducts formed from reaction of [3H]2-nitropyrene with DNA mediated by liver microsomes and cytosols of mouse and rat. In all cases, both adducts were formed. The quantities of the two adducts formed in each system were: mouse liver microsomes (11.3 pmol [3H]2-nitropyrene/mg DNA), rat liver microsomes (23), mouse liver cytosol (11.4) and rat liver cytosol (5.1). Thus, these adducts were formed in highest yield from rat liver microsomes and the lowest from rat liver cytosol. The deoxyguanosine/deoxyadenosine adduct ratio was higher from rat and mouse liver microsomes (7.8:9.2) than from rat and mouse liver cytosols (2.5:3.1). Our results represent the first direct demonstration of a C8-deoxyadenosine adduct being formed as a major product from the reaction of a nitro-PAH metabolite with DNA.

Assay for nucleoside and nucleotide binding of a potent mutagen, 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone

Deoxyadenosine, deoxyguanosine, deoxycytidine; thymidine and a dinucleotide, 2'-deoxycytidylyl-(3'-5')-2'-deoxythymidine (dCdT), were reacted with 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX). Nucleoside HPLC analysis revealed that the major products are the degradation products of MX and no stable MX-adducts were found. The same result was obtained with sodium borohydride reduction. The fluorescence spectra of deoxyguanosine-MX showed no fluorescence at neutral or alkaline pH, indicating no binding products to the N2-groups of deoxyguanosine.

Stereoselective release of polycyclic aromatic hydrocarbon-deoxyadenosine adducts from DNA by the 32P postlabeling and deoxyribonuclease I/snake venom phosphodiesterase digestion methods

The restricted ability of deoxyribonuclease I/snake venom phosphodiesterase digestion to liberate deoxyadenosine (dA) nucleotide adducts of polycyclic aromatic hydrocarbons from DNA, first observed by Dipple and Pigott with the bay-region diol epoxide adducts of 7,12-dimethylbenz[a]anthracene, has been observed with the dA adducts of benz[a]anthracene and benzo[c]phenanthrene diol epoxides. The micrococcal nuclease/spleen phosphodiesterase digestion used in the original 32P postlabeling procedure developed by Randerath to determine DNA adducts also failed to liberate dA nucleotide adducts quantitatively. Thus either method can potentially lead to an underestimation of the extent to which dA has been modified in DNA. The two digestion procedures exhibit systematic and mostly opposite stereoselectivity in the pattern of which dA adducts are resistant to digestion, which suggest that these adducts may have preferred orientations within modified DNA that are determined by whether they have the R or S configuration at C-1, the point of attachment between the exocyclic amino group of dA and the hydrocarbon; this in turn is dictated by the configuration about the precursor benzylic epoxide carbon and the cis versus trans nature of epoxide opening during adduct formation.

Activation of lysine 2,3-aminomutase by S-adenosylmethionine

Lysine 2,3-aminomutase, which catalyzes the interconversion of L-lysine and L-beta-lysine, is S-adenosyl-methionine-dependent, and the adenosyl-C-5' methylene group of this coenzyme mediates the transfer of hydrogen from C-3 of lysine to C-2 of beta-lysine. We here report experiments that address the mechanism by which S-adenosylmethionine activates lysine 2,3-aminomutase. We also describe an updated and improved purification procedure that produces enzyme with a specific activity substantially higher than that previously reported. Activation of the enzyme by less than 1 mol of S-adenosyl[1-14C]methionine/mol of subunits in the presence of lysine leads to the production of [14C] methionine in a kinetically biphasic process. After 1.8 min at 30 degrees C, 10% of the 14C is reisolated as [14C] methionine, and the cleavage increases to 19% after 10 min and to 51% after 40 min. Similar experiments with S-[8-14C]adenosylmethionine produce 5'-deoxy[14C]adenosine in amounts similar to the formation of methionine. The major radioactive products isolated in each case are [14C]methionine or 5'-deoxy[14C]adenosine, respectively, and unchanged 14C-labeled S-adenosylmethionine. These experiments support the hypothesis that activation of lysine 2,3-aminomutase involves a transfer of the 5'-deoxyadenosyl moiety from S-adenosylmethionine to another species associated with the enzyme, presumably another cofactor, to form an adenosyl cofactor that functions as the proximal, hydrogen abstracting species in the mechanism.

Incorporation of a complete set of deoxyadenosine and thymidine analogues suitable for the study of protein nucleic acid interactions into oligodeoxynucleotides. Application to the EcoRV restriction endonuclease and modification methylase

A complete set of dA and T analogues designed for the study of protein DNA interactions has been prepared. These modified bases have been designed by considering the groups on the dA and T bases that are accessible to proteins when these bases are incorporated into double-helical B-DNA [Seeman, N. C., Rosenberg, J. M., & Rich, A. (1976) Proc. Natl. Acad. Sci. U.S.A. 73, 804-808]. Each of the positions on the two bases, having the potential to interact with proteins, have been subject to nondisruptive, conservative change. Typically a particular group (e.g., the 6-NH2 of dA or the 5-CH3 of T) has been replaced with a hydrogen atom. Occasionally keto groups (the 2- and 4-keto oxygen atoms of T) have been replaced with sulfur. The base set has been incorporated into the self-complementary dodecamer d(GACGATATCGTC) at the central d(ATAT) sequence. Melting temperature determination shows that the modified bases do not destabilize the double helix. Additionally, circular dichroism spectroscopy shows that almost all the altered bases have very little effect on overall oligodeoxynucleotide conformation and that most of the modified oligomers have a B-DNA type structure. d(GATATC) is the recognition sequence for the EcoRV restriction modification system. Initial rate measurements (at a single oligodeoxynucleotide concentration of 20 microM) have been carried out with both the EcoRV restriction endonuclease and modification methylase. This has enabled a preliminary identification of the groups of the dA and T bases within the d(GATATC) sequence that make important contacts to both proteins.

Interaction of the EcoRV restriction endonuclease with the deoxyadenosine and thymidine bases in its recognition hexamer d(GATATC)

A set of dA and T analogues suitable for the study of protein DNA interactions have been incorporated into the central d(ATAT) sequence within d(GACGATATCGTC). The individual analogues have one potential protein contact (either a hydrogen-bonding group or a CH3 group capable of a van der Waals interaction) deleted. In general, the modified bases do not perturb the overall structure of the dodecamer, enabling results obtained to be simply interpreted in terms of loss of protein DNA contacts. We have used the modified oligodeoxynucleotide set to study the recognition of DNA by the EcoRV restriction endonuclease [recognition sequence d(GATATC)]. The kcat and Km values for the set have been determined, and a comparison with results seen with the parent oligodeoxynucleotide (containing no modified bases) has been carried out. Three classes of results are seen. First, some analogues lead to no change in kinetic parameters, meaning no enzyme contact at the altered site. Second (this is seen for most of the modified oligodeoxynucleotides), a drop in the kcat/Km ratio relative to the parent is observed. This comes mainly from a decrease in kcat, implying that the endonuclease uses the interaction under study to lower the transition-state barrier rather than to bind the substrate. Analyses of these results show that the drop in kcat/Km is what would be expected for the simple loss of a hydrogen bond or a CH3 contact between the enzyme and the oligodeoxynucleotide. This implies a contact of these types at these sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Mycoplasma contamination alters 2'-deoxyadenosine metabolism in deoxycoformycin-treated mouse leukemia cells

Deoxycoformycin-treated P388 and L1210 mouse leukemia cells salvage 2'-deoxyadenosine from the medium only inefficiently, because deoxyadenosine deamination is blocked and its phosphorylation is limited by feedback controls. Mycoplasma contamination at a level that had no significant effect on the growth of the cells increased the salvage of deoxyadenosine greater than 10 fold over a 90 min period of incubation at 37 degrees C, but in this case deoxyadenosine was mainly incorporated into ribonucleotides and RNA via adenine formed from deoxyadenosine by mycoplasma adenosine phosphorylase. Deoxyadenosine was an efficient substrate for this enzyme, in contrast to 2',3'-dideoxyadenosine which was not phosphorolyzed. Mycoplasma infection was confirmed by the presence of uracil phosphoribosyltransferase activity and by culture isolation. The contaminant has been identified as Mycoplasma orale. Mycoplasma infection had no effect on the deamination and phosphorylation of deoxyadenosine and adenosine, on the salvage of hypoxanthine and adenine, or on the degradation of dAMP and dATP by the cells or on their acid and alkaline phosphatase activities.

Selective inactivation of the deoxyadenosine phosphorylating activity of pure human deoxycytidine kinase: stabilization of different forms of the enzyme by substrates and biological detergents

Deoxycytidine kinase, purified from human leukemic spleen to apparent homogeneity, is a multisubstrate enzyme that also phosphorylates purine deoxyribonucleosides [Bohman & Eriksson (1988) Biochemistry 27, 4258-4265]. In the present investigation we show that the stability and temperature dependence of dCyd kinase activity differed appreciably from the dAdo kinase activity of the same pure enzyme. Selective inactivation of dAdo activity was observed upon an incubation of the enzyme at both 4 and 37 degrees C. The half-life of dAdo activity at 4 degrees C increased from 36 to 84 h, when the protein concentration was increased by addition of bovine serum albumin. However, the half-life of dCyd activity increased from 72 h to more than 7 days under the same conditions. dCyd activity was stable for at least 6 h at 37 degrees C while the half-life of dAdo activity was 2 h. The presence of substrates like ATP, dTTP, or dAdo stabilized dAdo activity at both temperatures, and full maintenance of both activities at 37 degrees C was obtained by the addition of the zwitterionic detergent CHAPS. Furthermore, thermal inactivation of the dAdo activity occurred at a lower temperature (48 degrees C) as compared to the dCyd activity (54 degrees C). The presence of protease inhibitors had no effect on enzyme inactivation, nor was there a difference in the subunit structure of the selectively inactivated enzyme as compared to the fully active form, as revealed by size-exclusion chromatography.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of a mutated phage T6 receptor protein of Escherichia coli K 12

The tsx-206 allele encodes an altered Tsx protein, Tsx-206, that can no longer function as the T6 receptor. We show here that this allele also confers resistance to the Tsx-specific phages H1, H3, H8, K9, K18 and Ox1 but not to colicin K. The Tsx-206 protein still mediates the efficient permeation of deoxyadenosine across the outer membrane at low substrate concentration. A host-range mutant of phage T6, T6h3.1, was isolated which can use both the Tsx-206 and the Tsx wild-type protein as its receptor. Cloning and DNA sequence analysis of the tsx-206 allele showed that the phage resistant phenotype was associated with an Asn to Tyr substitution at position 254 of the 272-residue Tsx protein.

Adduct formation identification between phenyl glycidyl ether and 2'-deoxyadenosine and thymidine by chromatography, mass spectrometry and nuclear magnetic resonance spectroscopy

Thymidine and 2'-deoxyadenosine were reacted with phenyl glycidyl ether in order to study the formation of the corresponding 2'-deoxynucleoside adducts. Separation methods were elaborated using either reversed-phase high-performance liquid chromatography with photodiode-array detection, or centrifugal circular thin-layer chromatography. The adducts were isolated on a preparative scale and were fully characterized by UV spectroscopy, desorption chemical ionization and fast atom bombardment mass spectrometry and 270- and 360-MHz 1H NMR spectrometry. For thymidine the main adduct was characterized as N-3-(2-hydroxy-3-phenoxypropyl)thymidine. With 2'-deoxyadenosine, predominantly N-1-(2-hydroxy-3-phenoxypropyl)-2'-deoxyadenosine was formed. With longer reaction times, the formation of a minor amount of dialkylated 2'-deoxyadenosine was observed. These nucleoside adducts will be used as marker compounds for studies of DNA adduct formation.

Investigations on the relationship between DNA ethenobase adduct levels in several organs of vinyl chloride-exposed rats and cancer susceptibility

The levels of 1,N6-ethenodeoxyadenosine (epsilon dAdo) and 3,N4-ethenodeoxycytidine (epsilon dCyd) were measured in DNA of several target organs of vinyl chloride (VC)-exposed rats. Seven-day-old (group I) and 13-week-old (group II) BD VI rats were exposed during 2 weeks to 500 ppm VC in air (7 hr per day and 7 days per week). epsilon dAdo and epsilon dCyd were measured by a combination of prepurification of DNA hydrolysates by HPLC and competitive radioimmunoassay using specific murine monoclonal antibodies. Both ethenodeoxynucleosides were detected in liver, lungs and brain (levels ranging from 0.6 x 10(-7) to 1.3 x 10(-7) for epsilon dAdo/2'-deoxyadenosine and from 1.95 x 10(-7) to 4.92 x 10(-7) for epsilon dCyd/2'-deoxycytidine) but not in kidneys of group I rats. In group II rats, only liver DNA was analysed and the levels of each adduct were six times lower than in young (group II) rats. These findings are in good agreement with the organotropism and the age-related sensitivity of VC-induced carcinogenesis in rodents.

Mobility of nucleoside transporter of human erythrocytes differs greatly when loaded with different nucleosides

Time courses of transmembrane equilibration of 2-chloroadenosine, 2'-deoxyadenosine, 3'-deoxyadenosine, cytidine and 2'-deoxycytidine were measured by rapid kinetic techniques in human erythrocytes under equilibrium exchange and zero-trans conditions. The kinetic parameters for transport were computed by fitting appropriate integrated rate equations to the data pooled for seven concentrations and compared to the kinetic parameters for uridine, adenosine, thymidine and formycin B transport determined previously for human erythrocytes under comparable experimental conditions. The transport of all nucleosides conformed to the simple carrier model and was directionally symmetric. The Michaelis-Menten constants for equilibrium exchange (Kee) ranged from 22 microM for 2-chloroadenosine to about 4 mM for cytidine and the maximum velocities (Vee) differed in a similar manner, so that the first-order rate constants (Vee/Kee) were similar for all nucleosides. The kinetic parameters for 2'-deoxyadenosine transport were similar to those for adenosine transport, whereas the lack of the 3'-OH group greatly reduced the affinity of 3'-deoxyadenosine (cordycepin) for the carrier. 2', 3'-Dideoxynucleosides were transported less than 1% as efficiently as 2'- and 3'-deoxynucleosides. Thus, the 2'- and 3'-OH groups play an important role in nucleoside transport. The mobility of the carrier when loaded with pyrimidine nucleosides (reflected by Vee) was 5-10-times greater than that of the empty carrier, whereas the mobility of the adenosine-loaded or 2'-deoxyadenosine-loaded carrier was about equal to that of the empty carrier. Loading the carrier with 2-chloroadenosine or 3'-deoxyadenosine actually decreased its mobility. Thus, the differential mobility of the loaded and empty carrier differs greatly with the nucleoside substrate. The mobility of the loaded carrier as well as Kee increased with a decrease in lipid solubility of the nucleoside substrate, but the relationship was complex.

Ethanol increases extracellular adenosine by inhibiting adenosine uptake via the nucleoside transporter

Chronic exposure to ethanol results in heterologous desensitization of receptors coupled to adenylyl cyclase via Gs, the stimulatory guanine nucleotide regulatory protein. Ethanol-induced accumulation of extracellular adenosine is required for the development of heterologous desensitization (Nagy, L. E., Diamond, I., Collier, K., Lopez, L., Ullman, B., and Gordon, A. S., Mol. Pharmacol., in press). To understand the mechanism underlying ethanol-induced increases in extracellular adenosine, we examined the interaction of ethanol with the adenosine transport system in S49 lymphoma cells. We found that ethanol inhibited nucleoside uptake without affecting deoxyglucose or isoleucine transport. Inhibition of adenosine uptake was due to decreased influx via the nucleoside transporter. Thus, ethanol-induced increases in extracellular adenosine appear to be due to inhibition of adenosine influx. After chronic exposure to ethanol, cells became tolerant to the acute effects of ethanol, i.e. ethanol no longer inhibited uptake. Consequently, ethanol no longer increased extracellular adenosine concentrations. Taken together with our previous studies, these results suggest that ethanol inhibition of adenosine influx leads to an increase in extracellular adenosine which causes an initial increase in intracellular cAMP levels and subsequent development of heterologous desensitization of cAMP signal transduction.

Mechanisms of deoxyguanosine lymphotoxicity. Human thymocytes, but not peripheral blood lymphocytes accumulate deoxy-GTP in conditions simulating purine nucleoside phosphorylase deficiency

This study was designed to simulate purine nucleoside phosphorylase (PNP) deficiency by preincubating with guanosine (Guo) to minimize PNP activity while investigating the metabolism of [14C] deoxyguanosine (dGuo) at physiologic concentrations (10 microM) by unstimulated thymocytes, tonsil-derived T and B lymphocytes, and peripheral blood cells over short time periods. GTP was the principal metabolite formed from dGuo by all cell types with functional PNP and hypoxanthine-guanine phosphoribosyltransferase, confirming formation via degradation to guanine with subsequent salvage by hypoxanthine-guanine phosphoribosyltransferase. Thymocytes also formed a small amount of deoxyguanosine triphosphate (dGTP), presumably through direct phosphorylation by deoxycytidine kinase. Incorporation of dGuo into GTP was effectively inhibited in all instances under PNP deficiency conditions and dGTP levels increased up to 10-fold in thymocytes, but tonsil-derived B or T lymphocytes and unfractionated PBL still accumulated no detectable dGTP. E and platelets formed low amounts of dGTP under these conditions. Preincubation with adenine (50 microM) to reverse any Guo-induced toxicity reduced the incorporation of dGuo into GTP without inhibitor in all cell types with intact adenine phosphoribosyltransferase, but had no effect on dGTP accumulation in thymocytes, with or without inhibitor, thus excluding any indirect formation of dGTP via the de novo route. The rapid metabolism of dGuo to GTP, in the absence of PNP inhibition and subsequent effects of the altered GTP concentrations on cellular metabolism, may account for the differing responses reported by investigators with the use of low dGuo concentrations (enhancing), compared with high (inhibitory), concentrations in mitogen-stimulated lymphocyte studies. The exclusive ability of thymocytes to accumulate significant amounts of dGTP, and inability of B cells to do so, provides a logical explanation for the selective T cell immunodeficiency in PNP deficiency.

Purine deoxynucleoside salvage in Giardia lamblia

Giardia lamblia is dependent on the salvage of preformed purines and pyrimidines, including deoxythymidine. Dependence on deoxynucleoside salvage is extremely unusual among eucaryotic cells (Moore, E. C., and Hurlbert, R. B. (1985) Pharmacol & Ther. 27, 167-196). The present study investigates the possibility that giardia lacks ribonucleotide reductase and depends entirely on deoxynucleoside salvage. A ribonucleotide reductase inhibitor, hydroxyurea, at concentrations up to 2 mM had no effect on the growth of giardia. This is 15-20 times the ED50 of hydroxyurea for the protozoans Trypanosoma cruzi, Trypanosoma gambiense, and Leishmania donovani. A lysate of giardia had no detectable ribonucleotide reductase. Although radiolabeled adenine, adenosine, guanine, and guanosine were readily incorporated into RNA by cultured cells, no adenine or adenosine and only trace amounts of guanine and guanosine were detectable in DNA. This is in contrast to deoxynucleosides, where 58% of deoxyadenosine and 10% of deoxyguanosine incorporated into nucleic acid were found in DNA. Phosphorylation of both deoxyadenosine and deoxyguanosine was catalyzed by a cell lysate of giardia when nucleoside kinase co-substrates were included in the assay but not when phosphotransferase co-substrates were present. The absence of detectable ribonucleotide reductase, the failure to incorporate purine nucleobases and nucleosides into DNA to any significant extent, the ready incorporation of deoxynucleosides into DNA, and the demonstration of a purine deoxynucleoside kinase suggest that giardia are dependent on the salvage of exogenous deoxynucleosides.

Biosynthesis and metabolism of 9-[5'-deoxy-5'-(methylthio)-beta-D-xylofuranosyl]adenine, a novel natural analogue of methylthioadenosine

The biosynthesis of 9-[5'-deoxy-5'-(methylthio)-beta-D-xylofuranosyl]adenine (xylosyl-MTA), a naturally occurring analogue of 5'-deoxy-5'-methylthioadenosine (MTA) recently characterized, was studied in the nudibranch mollusc Doris verrucosa. Experiments performed in vivo with putative labelled precursors such as [8-14C]adenine, [Me-14C]methionine and [Me-14C]MTA indicate that xylosyl-MTA originates from MTA. Experiments with MTA double-labelled at critical positions are consistent with a 3'-isomerization of the nucleoside through the formation of a 3'-oxo intermediate. In addition, experiments with the newly synthesized [3'-3H]xylosyl-MTA are indicative for a very low turnover rate of this molecule, which therefore accumulates in the mollusc.

Biochemical pharmacology of 2-chlorodeoxyadenosine in malignant human hematopoietic cell lines and therapeutic effects of 2-bromodeoxyadenosine in drug combinations in mice

Growth of human hematopoietic cell lines showed a 100-fold range of sensitivity to inhibition by 2-chloro-2'-deoxyadenosine (CldAdo), with highly sensitive lines in all three groups: T-lymphoblastic, B-lymphoblastic, and non-T, non-B. Formation of nucleotides from [8-3H]CldAdo was investigated in ten lines. In cells exposed to 0.15 microM CldAdo, CldAdo 5'-phosphate (CldAMP) reached 0.7-14 microM and CldAdo 5'-triphosphate (CldATP) reached 0.05-6 microM in 1 h. In most cases these nucleotide concentrations at 1 h were close to the steady-state concentrations, and the latter concentrations were approximately proportional to extracellular CldAdo concentration. On removal of extracellular CldAdo, intracellular CldAMP and CldATP declined rapidly with half times of 0.56-0.9 and 0.64-1.46 h, respectively. There was no correlation between these rates of catabolism and steady-state levels. The different sensitivities of the lines to CldAdo is explained only in part by the different steady-state concentrations of CldATP, and must be more directly related to differential effects on target enzymes. Mice inoculated with L1210 leukemia were treated with 2-bromo-2'-deoxyadenosine (BrdAdo) paired with one of 18 other therapeutic agents. Eight of the drugs paired with BrdAdo gave therapeutic responses from the combination greater than the sum of the responses of members of the pair. They included alkylating agents, antimetabolites blocking deoxyribonucleotide synthesis, and DNA polymerase inhibitors. Toxic dosages of CldAdo caused damage chiefly to the hemic-lymphatic systems and the kidneys.

Differential deoxyadenosine toxicity to immature rabbit cartilage in vitro. A model for the chondro-osseous dysplasia of adenosine deaminase deficiency

Deoxyadenosine metabolism was investigated in rabbit growth plate and articular cartilage to elucidate the biochemical basis for the chondro-osseous dysplasia observed in adenosine deaminase (ADA) deficiency. Models of ADA deficiency, the combination of deoxy-adenosine and either of 2 ADA inhibitors, were selectively toxic to immature cartilage, supporting the hypothesis that the chondro-osseous dysplasia of ADA deficiency is the consequence of the enzyme deficiency. Depletion of ATP may play a role in the altered chondrocyte viability and function observed in this model.

Dam methylated and hemimethylated oriC plasmids are replicated symmetrically; a novel and general test of replication symmetry

Deoxyadenosine methylation (dam) of the numerous GATC sequences present in the Escherichia coli origin of chromosomal replication (oriC) has been shown to be important both in vivo and in vitro for efficient initiation of DNA synthesis. Recent in vivo data suggest that initiation is only inefficient when these sequences are hemimethylated. This raises the interesting possibility that initiation may be inefficient because it only takes place on one strand of the template, i.e., replication is asymmetric on hemimethylated DNA. We tested this possibility by a novel and rapid approach which relies on the specificities of the restriction endonucleases MboI, MboII and DpnI. Although we show that replication takes place equally well on both strands of methylated and hemimethylated oriC DNA templates, the method should be applicable to the analysis of replication symmetry on most DNA templates which contain methylated deoxyadenosine GATC sequences as part of MboII restriction sites.

HPLC-32P-postlabelling analysis of 1,N6-ethenodeoxyadenosine and 3,N4-ethenodeoxycytidine

A 32P-postlabelling procedure coupled with HPLC has been developed to detect and measure the cyclic nucleic acid adducts 1,N6-ethenodeoxyadenosine and 3,N4-ethenodeoxycytidine in DNA. Chloroacetaldehyde-modified DNA containing these adducts was enzymatically digested to 3'-monophosphates and adducts were separated by ion-pair reverse-phase HPLC on PL RP-S prior to 32P-postlabelling with carrier free [gamma-32P]ATP. Following 3'-dephosphorylation with nuclease P1 the resulting [5'-32P]monophosphate adducts were finally resolved by HPLC on PL RP-S and assayed by liquid scintillation counting.

Muscarinic receptor subtype specificity of 5'-(isobutylthio)-adenosine (SIBA) and its analogs

1. 5'(Isobutylthio)-adenosine (SIBA) and its analogs, at 100 microM, inhibited [3H]N-methyl-scopolamine binding to homogenates of whole brain and cortex (mainly M1 subtype receptors) by 11-30% and to cerebellum (mainly M2 subtype receptors) by 20-39%. 2. At 0.01-1.0 microM, stimulation of [3H]QNB and NMS-inaccessible [3H]QNB binding was observed, with the most induced by 1 microM 3-deaza-SIBA. 3. In contrast, [3H]pirenzepine ([3H]PZ) binding to whole brain and cortex was inhibited in a dose-dependent manner with Ki values in the microM range. 4. As antagonists of acetylcholine-induced contraction of guinea pig ileum (mainly M2 subtype receptors), the analogs were slightly more potent than pirenzepine, but several orders of magnitude less than atropine; the order of potency was opposite that determined for the binding of [3H]PZ to cortex. 5. Thus, SIBA and its analogs may have differential effects on muscarinic receptor subtypes and show some specificity for the M1 receptor subtype.

Ion microscopy: a new approach for subcellular localization of labelled molecules

Secondary ion mass spectroscopy (SIMS) was used to obtain images representing the intracellular distribution of molecules labelled with carbon 14. Deoxyadenosine labelled with carbon 14 was added to a cultured human fibroblast cell medium, and the intracellular distribution of this molecule was studied using three different SIMS instruments: the CAMECA IMS 3F and SMI 300 ion microscopes and the UC-HRL scanning ion microprobe. Carbon 14 distribution images obtained by this method show that deoxyadenosine U-C14 is present in the cytoplasm as well as the nucleus, with a higher concentration in the nucleoli. Our study clearly demonstrates that ion microscopy is well suited for carbon 14 detection and localization at the subcellular level, permitting a wide variety of microanalytical tracer experiments.

Recognition and transport of adenine derivatives with synthetic receptors

Several new synthetic agents show high affinity for binding adenine derivatives. The structures feature complementary hydrogen bonds that cause the molecular chelation of the purine nucleus. The high lipophilicity of the new agents permits the transport of adenosine and deoxyadenosine across organic liquid membranes. The use of synthetic receptors for small biological targets may have application in drug delivery.

On the mechanism of DNA-adenine methylase

Experiments were performed to determine whether EcoRI methylase catalyzes the transfer of the methyl group of S-adenosylmethionine (a) directly to the N6 of adenine in DNA or (b) initially to N1 to give N1-methyladenine followed by isomerization of the N1-methylamino and 6-NH2 to give N6-methyladenine (Dimroth rearrangement). A facile synthesis of highly enriched [6-15N]deoxyadenosine and a dodecamer substrate of EcoRI methylase with [6-15N]adenine in the methylation site are reported. In the product of EcoRI enzymatic methylation, all of the isotope remains at the N6 position of the N6-methyladenine product. It is concluded that, contrary to existing chemical precedent, the methylation occurs by direct transfer from S-adenosylmethionine to the N6 of adenine in DNA.

Effects of cytidine analogs on methylation of DNA and retrovirus induction

Several cytidine analogs with known mutagenic capability were tested for their effects on DNA methylation and on induction of endogenous murine retrovirus. For each of the compounds tested it was found that DNA methylation was inhibited at the same concentrations that were required to induce virus expression. With each compound it was observed that increased dose levels produced an increase in the ability to inhibit methylation and an increase in the ability to induce virus.

Metabolic effects of 3'-deoxyadenosine (cordycepin) and 2-halo-3'-deoxyadenosine on repair of X-ray-induced potentially lethal damage in Chinese hamster V79 cells

Using cultured Chinese hamster V79 cells, an attempt has been made to examine the phosphorylation of cordycepin and its 2-halo derivatives (2-chloro-3'-deoxyadenosine, 2-bromo-3'-deoxyadenosine, and 2-iodo-3'-deoxyadenosine) by adenosine kinase, within the cells, and to correlate it with their cytotoxicities and abilities to inhibit the repair of X-ray-induced potentially lethal damage (PLDR). Of all compounds, only cordycepin was found to be phosphorylated and showed both potent cytotoxicity and ability to inhibit PLDR.

Differential modulation of 1-beta-D-arabinofuranosylcytosine metabolism by hydroxyurea in human leukemic cell lines

The ability of hydroxyurea (HU) to modulate 1-beta-D-arabinofuranosylcytosine (Ara-C) metabolism was investigated in human leukemic cell lines. Exposure of HL-60 cells to 1 mM HU enhanced the accumulation of Ara-CTP up to 2.5-fold, whereas HU did not have significant effects on Ara-C metabolism in CEM cells. In addition, two adenine nucleosides, deoxyadenosine (dAdo) and 9-beta-D-arabinofuranosyladenine (Ara-A), which are known to be activated by deoxycytidine (dCyd) kinase as Ara-C, were more effectively phosphorylated after the addition of HU only in HL-60 cells. However, the changes of intracellular dCTP and TTP pools induced by HU, i.e. decrease in dCTP and increase in TTP, were the same in both cell lines. Finally, dCyd production under normal culture conditions was at least 3- to 4-fold higher in HL-60 cells and was inhibited significantly by HU administration. These results suggest that the modulation of Ara-C metabolism by HU occurs at the level of dCyd kinase through the regulation of de novo dCyd generation.

Metabolism and anti-human immunodeficiency virus-1 activity of 2-halo-2',3'-dideoxyadenosine derivatives

Both 2',3'-dideoxyadenosine and 2',3'-dideoxyinosine have been shown (Mitsuya, H., and Broder, S. (1987) Nature 325, 773-778) to have in vitro activity against the human immunodeficiency virus-1 (HIV). However, these dideoxynucleosides may be catabolized by human T cells, even when adenosine deaminase is inhibited by deoxycoformycin. To overcome this problem, we have synthesized the 2-fluoro-, 2-chloro-, and 2-bromo-derivatives of 2',3'-dideoxyadenosine. The metabolism and anti-HIV activity of the 2-halo-2',3'-dideoxyadenosine derivatives and of 2',3'-dideoxyadenosine were compared. The 2-halo-2',3'-dideoxyadenosine derivatives were not deaminated significantly by cultured CEM T lymphoblasts. Experiments with 2-chloro-2',3'-dideoxyadenosine showed that the T cells converted the dideoxynucleoside to the 5'-monophosphate, 5'-diphosphate, and 5'-triphosphate metabolites. At concentrations lower than those producing cytotoxicity in uninfected cells (3-10 microM), the 2-halo-2',3-dideoxyadenosine derivatives inhibited the cytopathic effects of HIV toward MT-2 T lymphoblasts, and retarded viral replication in CEM T lymphoblasts. Experiments with a deoxycytidine kinase-deficient mutant CEM T cell line showed that this enzyme was necessary for the phosphorylation and anti-HIV activity of the 2-chloro-2',3'-dideoxyadenosine. In contrast, 2',3'-dideoxyadenosine was phosphorylated by the deoxycytidine kinase-deficient mutant and retained anti-HIV activity in this cell line. Thus, the 2-halo derivatives of 2',3'-dideoxyadenosine, in contrast to 2',3'-dideoxyadenosine itself, are not catabolized by T cells. Their anti-HIV and anti-proliferative activities are manifest only in cells expressing deoxycytidine kinase. The in vivo implications of these results for anti-HIV chemotherapy are discussed.

Investigations on the anti-HIV activity of 2',3'-dideoxyadenosine analogues with modifications in either the pentose or purine moiety. Potent and selective anti-HIV activity of 2,6-diaminopurine 2',3'-dideoxyriboside

Several 2',3'-dideoxyadenosine analogues with modifications in either the ribose or purine moiety were evaluated for their inhibitory effects on the replication of human immunodeficiency virus (HIV) in MT-4 cell cultures. The 2',3'-dideoxyriboside of 2,6-diaminopurine (ddDAPR) inhibited HIV antigen expression and HIV-induced cytopathogenicity at a 50% effective dose of 2.4-3.8 microM, as compared to 3-6 microM for 2',3'-dideoxyadenosine (ddAdo), whereas 50% inhibition of MT-4 cell viability was noted only at a concentration of 477 and 889 microM, respectively. Both ddDAPR and ddAdo were only weakly inhibitory to the proliferation of a number of T-lymphoblast and T-lymphocyte cell lines, pointing to the selectivity of these compounds as anti-HIV agents. In contrast to ddAdo, ddDAPR was found to be a poor substrate for adenosine deaminase, which may be advantageous from a chemotherapeutic viewpoint. Substitution of an azido or fluoro group at the 2' and 3'-position of the ribose moiety in either "up" or "down" configurations resulted in a decrease of the anti-HIV potency and selectivity of ddAdo. In addition to ddDAPR other purine-modified ddAdo analogues, i.e. several pyrrolo[2,3-d]pyrimidine 2',3'-dideoxynucleosides, were investigated for their anti-HIV activity, but none of these derivatives proved as potent or selective as ddDAPR.