Biochemical genetic analysis of 2',3'-dideoxyadenosine metabolism in human T lymphocytes

2',3'-dideoxyadenosine (ddAdo) has been shown to inhibit the infection of cultured human T lymphoblasts with the human immunodeficiency virus-1 (HIV-1). However, the pathways of ddAdo metabolism in T lymphocytes have not been well defined. We have studied the uptake and degradation of ddAdo in human CEM T lymphoblasts, in mutant CEM T cells deficient in adenosine kinase or deoxycytidine kinase, and in normal lymphocytes and monocytes. The results indicate that ddAdo may be phosphorylated in T cells by several different enzymes, although deoxycytidine kinase predominates. However, 99% of the ddAMP formed is deaminated by AMP deaminase and subsequently dephosphorylated. Thus, the ability of ddAdo to prevent HIV-1 infection may be limited in cells with high AMP deaminase activity.

Characterization of bifunctional adducts produced in DNA by trans-diamminedichloroplatinum(II)

The cancer chemotherapeutic drug cis-diamminedichloroplatinum(II) (cis-DDP) produces bifunctional reactions with DNA which appear critical to its toxic action. The relative inefficacy of the isomer trans-DDP results from its production of predominantly monofunctional adducts in DNA. However, trans-DDP is also toxic and this is presumed to result from bifunctional reaction. These reactions have been characterized by platinating pure DNA followed by enzyme digestion, HPLC separation and analysis by atomic absorption and nuclear magnetic resonance (NMR). Bifunctional adducts occur between deoxyguanosine (dG) and either deoxyadenosine (dA), deoxycytidine (dC) or another dG. Although dG-Pt-dG occurs in both double-stranded (approximately 40% of total adducts) and single-stranded DNA (approximately 60%) there is a marked preference for formation of dG-Pt-dC in double-stranded DNA (approximately 50%) and dG-Pt-dA in single-stranded DNA (approximately 35%). Only dG-Pt-dG forms rapidly; the other adducts derive from rapid formation of a monofunctional dG-Pt and further reaction with dA or dC over many hours.

2',3'-Dideoxynucleoside phosphorylation by deoxycytidine kinase from normal human thymus extracts: activation of potential drugs for AIDS therapy

As a first step toward improving dideoxynucleoside inhibition of human immunodeficiency virus replication in human lymphocytes, we examined the kinetics of 5'-phosphorylation of a series of 2',3'-dideoxynucleosides, using deoxycytidine kinase purified from human thymus extracts. Nucleosides with the 2'-deoxyribose moiety were activated 30 times faster than were 2',3'-dideoxynucleosides. The adenosine deaminase inhibitor, 2'-deoxycoformycin, showed an unexpected ability to inhibit purine and pyrimidine dideoxynucleoside phosphorylation; such inhibition was not competitive and was not observed when 2'-deoxycytidine was the substrate. 2'-Deoxycytidine, the natural substrate, inhibited dideoxynucleoside phosphorylation in a manner similar to that observed with 2'-deoxycoformycin. Thus, dideoxynucleosides are activated by deoxycytidine kinase through a different catalytic interaction than occurs in 5'-activation of 3'-hydroxynucleosides by this enzyme.

Distinct effects of adenine and guanine starvation on DNA synthesis associated with different pool sizes of nucleotide precursors

Guanine nucleotide depletion primarily causes a drastic inhibition of DNA synthesis, while adenine nucleotide depletion interferes with other vital functions before inhibiting DNA synthesis (M. B. Cohen and W. Sadée, Cancer Res., 43: 1589-1591, 1983). This study addresses the hypothesis that the presence of a large adenine nucleotide pool with direct access to DNA synthesis prevents immediate cessation of DNA synthesis under conditions of adenine starvation, while the small guanine-DNA precursor pool is readily exhausted under guanine starvation. Adenine, guanine, and deoxyadenosine tracers were incubated with asynchronized or synchronized S-49 cells, and tracer progression into cellular nucleotide pools and nucleic acids was measured. Compartmentation of the dATP pool into a functional DNA precursor pool and a general cellular pool could not be demonstrated with [3H]dAdo tracer experiments with S-phase cells. While guanine tracer was incorporated into DNA without delay (less than 5 min), consistent with a small functional guanine-DNA precursor pool, adenine tracer incorporation into DNA was associated with a substantial delay period (approximately 30 min) indicative of a large functional adenine-DNA precursor pool. These results suggest that the different size of the functional nucleotide precursor pools with rapid access to DNA synthesis accounts for the dramatic difference in the effects of purine antimetabolites that cause either adenine or guanine starvation.

5'-deoxy-5'-methylthioadenosine phosphorylase--IV. Biological activity of 2-fluoroadenine-substituted 5'-deoxy-5'-methylthioadenosine analogs

5'-Deoxy-5'-methylthioadenosine phosphorylase (MTAPase) phosphorolyzes 5'-deoxy-5'-methylthioadenosine (MTA) generated during polyamine biosynthesis to adenine and 5-methylthioribose-1-phosphate. Two doubly-substituted, 2-fluoroadenine-containing analogs of MTA, 5'-deoxy-2-fluoroadenosine (5'-dFAdo) and 5'-deoxy-5'-iodo-2-fluoroadenosine (5'-IFAdo), were synthesized and studied as substrates of MTAPase: their reaction with this enzyme resulted in the liberation of the cytotoxic base, 2-fluoroadenine, as well as potentially cytotoxic analogs of 5-methylribose-1-phosphate. The activities of these MTA analogs were compared to that of the singly-substituted analog, 5'-deoxy-5'-methylthio-2-fluoroadenosine (5'-MTFAdo). The cytotoxic action of these MTA analogs depended primarily on their conversion to 2-fluoroadenine-containing nucleotides, as a cell line that contains both MTAPase and adenine phosphoribosyltransferase (APRT) activity (HL-60 human promyelocytic leukemia) readily converted these MTA analogs to 2-fluoroadenine-containing nucleotides (especially 2-fluoroadenosine triphosphate) and was highly sensitive to the growth-inhibitory effects of all three compounds (IC50 values in the 10(-8) M range), whereas cell lines lacking MTAPase (CCRF-CEM human T-cell leukemia) or APRT (HL-60/aprt1 cells) did not form analog nucleotides and were relatively insensitive to these compounds (IC50 values in the 10(-5) M range). The doubly-substituted analogs were not more growth inhibitory than 5'-MTFAdo in wild type HL-60 cells as the potent effects of 2-fluoroadenine may mask the activity of the 5-methylthioribose-1-phosphate analogs generated in the reaction of these compounds with MTAPase. 5'-dFAdo and 5'-IFAdo also were irreversible inhibitors of S-adenosylhomocysteine hydrolase, which may explain in part the weak but observable growth inhibitory action of these compounds against MTAPase-deficient cell lines.

The 2',3'-dideoxyriboside of 2,6-diaminopurine and its 2',3'-didehydro derivative inhibit the deamination of 2',3'-dideoxyadenosine, an inhibitor of human immunodeficiency virus (HIV) replication

The 2',3'-dideoxyriboside of 2,6-diaminopurine (ddDAPR) and its 2',3'-didehydro derivative (ddeDAPR) are poor substrates for adenosine deaminase (ADA) but potent inhibitors of the enzyme. Their Km values for ADA are of the same order of magnitude as those of the natural adenosine (Ado) and 2'-deoxyadenosine (dAdo), but their Vmax values are 35-fold (ddDAPR) to 350-fold (ddeDAPR) lower than those of Ado and dAdo. The Ki/K values of ADA for ddeDAPR (as inhibitor) and Ado, 2',3'-dideoxyadenosine (ddAdo) and 9-beta-D-arabinofuranosyladenine (araA) as the substrates are 0.17, 0.05 and 0.06, respectively. ddDAPR is about 3-fold less potent as an inhibitor of ADA than ddeDAPR. The 2,6-diaminopurine derivatives ddeDAPR and ddDAPR [which is also a potent inhibitor of human immunodeficiency virus (HIV)], may hold great promise, from a chemotherapeutic viewpoint, in combination with other adenosine analogues such as ddAdo and araA, which have been recognized and/or being pursued as either anti-retrovirus or anti-herpesvirus agents.

Influence of prior dietary protein intake on metabolism, DNA binding and adduct formation of 7,12-dimethylbenz[a]anthracene in isolated rat mammary epithelial cells

These studies were designed to examine the influence of prior dietary protein intakes in rats on the ability of their isolated mammary cells to metabolize 7,12-dimethylbenz[a]anthracene (DMBA). Total metabolism of DMBA increased as dietary protein increased. After 6 h of incubation, water-soluble metabolites made only a minor (less than 12%) contribution to total DMBA metabolism. The binding of DMBA to isolated mammary cell DNA after 6 h of incubation from rats fed 15% dietary protein was 20% higher than binding from cells of rats fed 7.5% dietary protein. The increased binding in mammary epithelial cells from rats fed 15% protein was associated with an increase in the syn-dihydrodiol-epoxide adduct. The syn-dihydrodiol-epoxide:deoxyadenosine adduct was the major contributor to binding. The present studies are consistent with a decrease in carcinogen activation in tissues obtained from animals fed diets limiting in protein.

Resistance of 7,12-dimethylbenz[a]anthracene-deoxyadenosine adducts in DNA to hydrolysis by snake venom phosphodiesterase

In enzymatic hydrolyses of 7,12-dimethylbenz[a]anthracene (DMBA)-modified DNA isolated from fetal mouse cell cultures, a low concentration of venom phosphodiesterase was sufficient for complete release of DMBA-deoxyguanosine adducts. However, efficient release of DMBA-deoxyadenosine adducts required much higher levels of phosphodiesterase. If these adducts exhibit similarly differential susceptibilities to exonucleases involved in DNA metabolism or repair, each adduct could result in significantly different biological effects in vivo.

Ribonucleotide-induced helical alteration in DNA prevents nucleosome formation

Several polynucleotides that assume an A-form helical structure in solution are unable to form nucleosomes. We attempted to establish a relationship between the ease of the A-form----B-form helix transition and ease of nucleosome formation by reconstituting nucleosomes using ribosubstituted DNA containing various levels of ribonucleotides. Instead we discovered that, when riboadenosine is substituted for deoxyriboadenosine, even one ribonucleotide per 125 base pairs of DNA reduces nucleosome formation and that DNA containing greater than 5% ribonucleotide is completely unable to form nucleosomes. Ribosubstituted DNA restriction fragments exhibited altered mobility on native 6% polyacrylamide gels, indicating an altered helical structure (probably bending). The effects on both nucleosome formation and gel mobility are nucleotide specific and are correlated, being greatest for riboadenosine and decreasing in the order riboadenosine greater than riboguanosine greater than ribocytosine. The results are consistent with the hypothesis that the rate of nucleosome formation can be drastically reduced by isolated local perturbations, such as kinking or bending, in the helical structure of DNA.

Efficient retrovirus-mediated transfer and expression of a human adenosine deaminase gene in diploid skin fibroblasts from an adenosine deaminase-deficient human

Skin fibroblasts might be considered suitable recipients for therapeutic genes to cure several human genetic diseases; however, these cells are resistant to gene transfer by most methods. We have studied the ability of retroviral vectors to transfer genes into normal human diploid skin fibroblasts. Retroviruses carrying genes for neomycin or hygromycin B resistance conferred drug resistance to greater than 50% of the human fibroblasts after a single exposure to virus-containing medium. This represents at least a 500-fold increase in efficiency over other methods. Transfer was achieved in the absence of helper virus by using amphotropic retrovirus-packaging cells. A retrovirus vector containing a human adenosine deaminase (ADA) cDNA was constructed and used to infect ADA-fibroblasts from a patient with ADA deficiency. The infected cells produced 12-fold more ADA enzyme than fibroblasts from normal individuals and were able to rapidly metabolize exogenous deoxyadenosine and adenosine, metabolites that accumulate in plasma in ADA-deficient patients and are responsible for the severe combined immunodeficiency in these patients. These experiments indicate the potential of retrovirus-mediated gene transfer into human fibroblasts for gene therapy.

Deoxyadenosine deamination and phosphorylation in rat liver mitochondria

1. Deoxyadenosine metabolism was measured in freshly isolated mitochondria; these organelles took up the deoxynucleoside and formed three detectable products: deoxyinosine, dAMP and dIMP. 2. Enzyme extracts prepared from sonicated mitochondria exhibited deoxyadenosine deaminase, deoxyadenosine kinase, dAMP deaminase and deoxyinosine kinase activities. 3. These data suggest that deoxyadenosine was initially altered in mitochondria by at least two metabolic reactions--deamination and phosphorylation. Deoxyinosine and dAMP were produced. 4. These two products were subsequently phosphorylated and deaminated, respectively to produce dIMP.

Cordycepin disrupts the microtubule networks and arrests Nil 8 hamster fibroblasts at the onset of mitosis

The nucleoside analogue 3'-deoxyadenosine (cordycepin) arrests dividing cells at the onset of mitosis in prometaphase. The microtubules in the arrested prometaphase cells depolymerize to two small asters. A minimum of 80 micrograms/ml cordycepin or 20 micrograms/ml cordycepin in combination with 2 micrograms/ml of the deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl) adenosine (EHNA) to inhibit its degradation is required to see these effects. Analysis of cell extracts by high-pressure liquid chromatography indicates that cordycepin enters the cells rapidly and is phosphorylated to 3'-dATP. The intracellular concentration rises almost linearly from 0.7 mM after 15 min to 7 mM by 210 min. Concomitantly the ATP concentration shows a rapid drop from the 4 mM present in controls. However, the direct reduction of ATP levels does not mimic the same rapid effects of cordycepin on the microtubules. In addition, similar effects are not produced by a variety of other adenosine analogues with alterations in the 2' and 3' ribose positions. Although other pharmacological reagents arrest cells at the onset of mitosis, cordycepin is unusual because of the collapse of the microtubule networks to two small asters that radiate from the microtubule-organizing center. 3'-dATP can replace the requirement for ATP or GTP in the vitro polymerization of microtubules from microtubule protein: however, at limiting concentrations of nucleotide it requires approximately two times the concentration of 3'-dATP as ATP to support an equivalent level of microtubule polymerization. This suggests that the effects of cordycepin in vivo may be the result of the depletion of cellular ATP pools and the altered ability of 3'dATP to substitute for ATP-dependent reactions. Current experiments are testing this hypothesis.

2',5'-Phosphodiesterase activity depends upon the presence of a 3'-hydroxyl moiety in the penultimate position of the oligonucleotide substrate

3'-Deoxyadenosine (3'dA, cordycepin)-substituted analogs of 2-5A core 5'-monophosphate (p5'A2'p5'A2'p5'A) were examined for their sensitivity toward degradation by the 2'-phosphodiesterase activity in cytoplasmic extracts of mouse L cells. The analogs, p5'(3'dA)-2'p5'A2'p5'A, p5'(3'dA)2'p5'A2'p5'(3'dA) and p5'A2'p5'A2'p5'(3'dA) were degraded at a rate comparable to p5'A2'p5'A2'p5'A itself. On the other hand, under the assay conditions examined p5'A2'p5'(3'dA)2'p5'A, like p5'(3'dA)2'p5'(3'dA)2'p5'(3'dA), was completely resistant to degradation. The data imply that sensitivity to the 2',5'-phosphodiesterase activity of mouse L cells requires the presence of 3'-hydroxyl moiety in the penultimate nucleotide.

Changes in nucleoside transport of HL-60 human promyelocytic cells during N,N-dimethylformamide induced differentiation

The rate of nucleoside transport decreased profoundly in human promyelocytic leukemia HL-60 cells after myeloid differentiation was induced by 5-6 days of exposure to 0.8% N,N-dimethylformamide (DMF). The facilitated diffusion of 100 microM radiolabeled adenosine and 2'-deoxyadenosine, measured by rapid transport assays, decreased 10- to 20-fold. The transport of 2 microM coformycin or 2'-deoxycoformycin, which is mediated by the same mechanism and was monitored by the adenosine deaminase titration assay, decreased 29-fold. The reduction in nucleoside transport capacity after DMF treatment was confirmed by a 19-fold decrease in the number of specific binding sites per cell (from 24-30 X 10(4) to 1.2-1.7 X 10(4)) for [3H]-6-p-nitrobenzylthioinosine, a nucleoside transport inhibitor. The binding affinity of 6-p-nitrobenzylthioinosine was not altered significantly and nucleoside transport remained sensitive to the transport inhibitors, 6-p-nitrobenzylthioinosine, dipyridamole, and dilazep after DMF-induced maturation. Time-dependence studies showed that the rate of 100 microM deoxyadenosine transport was unchanged for the first 24 h of exposure to DMF but fell to about 36% of control rates at 24-26 h and then gradually decreased further to about 4-5% of control rates after 5-6 days. In contrast, transport rates of the purine bases were reduced only 2- to 3-fold in HL-60 cells after 5 days of DMF treatment. The rates of adenosine and deoxyadenosine transport were unchanged or reduced by no more than 2-fold after 5-6 days of exposure to 0.8% DMF in the following human tumor cell lines that are not inducible with DMF: ARH-77 (multiple myeloma), KG-1 (acute myelogenous), and K-562 (chronic myelogenous). Thus, changes in nucleoside transport may serve as an early, membrane-associated marker of differentiation of the HL-60 cell line.

Disturbances in DNA precursor metabolism associated with exposure to an inhibitor of poly(ADP-ribose) synthetase

3-Aminobenzamide (3AB) is widely used as an inhibitor of poly(ADP-ribose) synthetase to study the effect of protein ribosylation on various cellular processes, but the specificity of its inhibition has not been demonstrated. We found that 3AB has a wide range of effects on DNA precursor metabolism, as determined by high-performance liquid chromatographic separation of deoxynucleosides derived from enzymatic digestion of cellular DNA. 3AB (10-20 mM) significantly reduced cell growth in human lymphoblastoid cells. Furthermore, the incorporation of [3H]deoxycytidine into DNA was significantly enhanced relative to incorporation of [3H]deoxythymidine, [3H]deoxyguanosine, and [3H]deoxyadenosine. Incorporation of fragments of [3H]glucose into the pyrimidine fraction of DNA was significantly inhibited relative to incorporation into the purine fraction. At only 1 mM, 3AB had a major inhibitory effect on the incorporation of the methyl group from [3H]methionine into deoxyguanosine, deoxyadenosine, and deoxycytidine, with 50% inhibition into deoxyguanosine and deoxyadenosine and 90% inhibition into deoxycytidine. The specificity of 3AB inhibition to poly(ADP-ribose) synthetase is therefore doubtful in view of this variety of metabolic effects, involving pyrimidine synthesis and de novo synthesis via the one-carbon pool.

Differential composition of cytosol 5'-nucleotidases between T and B lymphoblasts

WI-L2 cells (a B-lymphoblastoid cell line) were more resistant than CEM cells (a T-lymphoblastoid cell line) to deoxyadenosine, ara-A (9-beta-D-arabinofuranosyladenine), or ara-C (1-beta-D-arabinofuranosylcytosine) inhibition. This was caused by a difference in the composition of cytosol 5'-nucleotidases between WI-L2 and CEM cells. In intact cells, the endogenous production of deoxyadenosine from WI-L2 cells deficient in adenosine kinase (EC 2.7.1.20) and deoxycytidine kinase (EC 2.7.1.74) was consistently high, despite changes in endogenous adenosine production. Endogenous production of deoxyadenosine from CEM cells deficient in adenosine kinase and deoxycytidine kinase was, however, coordinated with endogenous adenosine production. In broken cells, cytosol dAMPase (2'-deoxyadenosine 5'-monophosphate 5'-nucleotidase) activity of WI-L2 cells was 3-5-fold higher than that of CEM cells. dAMPase activity could be separated from ATP-activated IMPase (inosine 5'-monophosphate 5'-nucleotidase) by gel filtration (molecular weight: dAMPase; 39,000-46,000; ATP-activated IMPase, greater than 150,000). Cytosol ATP-activated IMPase and dAMPase were isolated by phosphocellulose or DEAE-Bio-Gel A chromatography from non-specific phosphatases. The ATP-activated IMPase showed only marginal activity towards dAMP (2'-deoxyadenosine 5'-monophosphate), ara-AMP (9-beta-D-arabinofuranosyladenine 5'-monophosphate), or ara-CMP (cytosine-beta-D-arabinofuranoside 5'-monophosphate), even in the presence of ATP. The activity of ATP-activated IMPase was similar in WI-L2 and CEM cells. dAMPase was separated into two peaks by DEAE-Bio-Gel A chromatography; one of these peaks degraded ara-AMP and ara-CMP. The activities of both peaks from WI-L2 cells were higher than those from CEM cells. These results show that the degradation of dAMP, ara-AMP or ara-CMP was more specific and rapid in WI-L2 than in CEM cells.

N6-methyldeoxyadenosine residues at specific sites decrease the activity of the E1A promoter of adenovirus type 12 DNA

The activity of eukaryotic promoters is highly sensitive to site-specific modifications by DNA methylations. We have used the E1A promoter of adenovirus type 12 (Ad12) DNA to investigate the effects of methylations at different promoter sites on its activity. The chloramphenicol acetyltransferase gene has served as an activity indicator. Activity of the E1A promoter is lost or markedly decreased by deoxycytidine methylation of two HpaII (5'-C-C-G-G-3') or seven HhaI (5'-G-C-G-C-3') sites upstream from the 3' located T-A-T-A signal. There are two T-A-T-A signals in the E1A promoter of adenovirus type 12 DNA, one T-A-T-T-A-T sequence starting at nucleotide 276 (5' located), a second T-A-T-T-T-A-A sequence starting at nucleotide 414 (3' located). Deoxycytidine methylations at two AluI (5'-A-G-C-T-3') sites downstream from the 5' located T-A-T-A signal have no effect on promoter activity. When one EcoRI (5'-G-A-A-T-T-C-3') or one TaqI (5'-T-C-G-A-3') sequence at 281 base-pairs upstream or 61 base-pairs downstream from the 5' located E1A T-A-T-A signal, respectively, is deoxyadenosine methylated, the promoter becomes inactive. Deoxyadenosine methylation at one MboI (5'-G-A-T-C-3') site, which is located 127 nucleotides downstream from the 5' located T-A-T-A signal, fails to decrease E1A promoter activity. There is no conspicuous anatomical relation of any of these sites to the two presumptive enhancer sequences in the E1A promoter. We conclude that 5-deoxymethylcytidine or N6-methyldeoxyadenosine residues have to be introduced at highly specific promoter sites to inactivate the promoter. These sites are probably different for different promoters.

Increased resistance to the anticellular effect of interferon in an ultraviolet light-resistant human cell line, UVr-1

Interferon (alpha, beta and gamma) susceptibility was tested in a human cell line, UVr-1, a u.v. light-resistant variant of RSa cells; the latter have high sensitivity to both u.v. lethality and the cell proliferation inhibition (anticellular) effect of human interferon (HuIFN) preparations. UVr-1 cells were less sensitive than the parental RSa cells to the inhibitory effects of HuIFN preparations, as measured by cell proliferation and the incorporation of [3H]deoxythymidine and [3H]deoxyadenosine into acid-insoluble cellular material. Nevertheless, UVr-1 cells exposed to HuIFN showed almost the same enhanced levels of antiviral activity and pppA(2'p5'A)n synthetase activity as similarly treated RSa cells. Further, UVr-1 cells had much the same binding capacity for 125I-labelled HuIFN-alpha A. Thus, it seems likely that the variant has an increased resistance to the anticellular effect but not to the antiviral effect of HuIFN preparations. UVr-1 cells showed no significant difference from RSa cells in u.v.-induced DNA repair synthesis. However, when a comparison was made between the susceptibility of normal fibroblasts and fibroblasts from patients with Cockayne's syndrome, characterized by an altered u.v. sensitivity but no alteration of DNA repair replication synthesis, the Cockayne's syndrome fibroblasts, CCK-3 and CCK-4, were more susceptible to HuIFN-beta as judged by cell proliferation and deoxythymidine incorporation tests.

The binding of 1,N6-ethenoNAD to bovine liver glutamate dehydrogenase: studies using the time-correlated single photon counting fluorescence technique

The time-correlated single photon counting (TCPC) fluorescence technique has been used as a novel approach to investigate ligand-protein interaction, for the case of the binding of the fluorescent coenzyme analogue 1,N6-ethenoNAD (epsilon NAD) to bovine liver glutamate dehydrogenase in the presence of glutarate, a substrate analogue which stabilizes the complex. System calibration was performed using solutions of epsilon ADP and carefully purified epsilon NAD mixed at variable molar ratios (pH 7.0, 0.05 M sodium phosphate buffer, 20 degrees C). The fluorescence lifetimes obtained after deconvolution were 2.4 ns (for epsilon NAD) and 23 ns (for epsilon ADP), in good agreement with literature values obtained under similar conditions. epsilon NAD binds to glutamate dehydrogenase in the presence of 50 mM glutarate, with a fluorescence quantum yield enhancement factor, Q, of about 17-fold, as previously reported (Favilla, R. and Mazzini, A. (1984) Biochim. Biophys. Acta 48-57). For this system, fluorescence lifetime values were obtained after deconvolution as 2.4 ns for free epsilon NAD and 21 ns for bound epsilon NAD. These values did not vary appreciably with enzyme concentration nor with degree of saturation, thus reflecting the existence of only one spectroscopically relevant type of complex. Addition of either GTP or ADP did not affect the lifetime of epsilon NAD bound to the enzyme, but only its affinity, thus allowing calculations of binding strengths. In the case of a simple binding (i.e., in the absence of GTP) the dissociation constant of the complex could be derived from a simple relationship, in which only the ratio between the pre-exponential factors and the parameter gamma, which represents the molar fraction of epsilon NAD molecules free in solution in the open conformation, are to be taken into account. The results are in good agreement with those reported by some of us (reference above) using a steady-state fluorescence technique, which by itself is, however, unable to resolve the number of relevant species present in the system.

Kinetic control of the length of very short homopolymeric additions by terminal deoxynucleotidyltransferase

This work is an investigation of the practicality of kinetic control of the length of very short deoxynucleotide homopolymeric additions by terminal deoxynucleotidyltransferase. For such very short additions, the possibility that terminal deoxynucleotidyltransferase acts differently with each deoxytriphosphate, or shows interaction effects when presented with multiple deoxytriphosphates, was investigated. Different relative rates of priming and different relative rates of subsequent additions were found for each deoxytriphosphate. Each triphosphate reacted uniquely, and one case of interaction was found, with adenosine interfering with cytidine addition.

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

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

Reactivity of mutagenic propylene oxides with deoxynucleosides and DNA

In an extension of previous studies with deoxycytidine and thymidine reactivities, propylene oxide, glycidol, epichlorohydrin, and trichloropropylene oxide were reacted with deoxyguanosine as well as deoxyadenosine and, except for the trichloro compound, with DNA. Reactivity with the purine deoxynucleosides as well as the four deoxynucleosides in DNA were quantitated by HPLC methods. Correlations were found for the reactivity with individual deoxynucleosides in solution to Taft sigma electron-withdrawing values of the substituents on the epoxides and for reaction with model nucleophiles. In general, these correlations were not as pronounced for the reactivities of the propylene oxides with the nucleosides in DNA. Correlations for reactivity of the propylene oxides with the individual deoxynucleosides in solution and in DNA, except for dThd, were indicated for mutagenicity in TA100 in the liquid-preincubation Ames test. However, this was not the case for mutagenicities determined with the plate incorporation procedure nor with TA1535, where the relative mutagenicity of trichloropropylene oxide was the outstanding difference. Trichloropropylene oxide appeared to depend upon the error-prone system in TA100 for full expression of its mutagenicity.

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

The enzymes responsible for the phosphorylation of deoxyadenosine and nucleoside analogs are important in the pathogenesis of adenosine deaminase deficiency and in the activation of specific anticancer and antiviral drugs. We examined the role of adenosine kinase in catalyzing these reactions using an enzyme purified 4000-fold (2.1 mumol/min/mg) from human placenta. The Km values of deoxyadenosine and ATP are 135 and 4 microM, respectively. Potassium and magnesium are absolute requirements for deoxyadenosine phosphorylation, and 150 mM potassium and 5 mM MgCl2 are critical for linear kinetics. With only 0.4 mM MgCl2 in excess of ATP levels, the Km for deoxyadenosine is increased 10-fold. ADP is a competitive inhibitor with a Ki of 13 microM with variable MgATP2-, while it is a mixed inhibitor with a Ki and Ki' of 600 and 92 microM, respectively, when deoxyadenosine is variable. AMP is a mixed inhibitor with Ki and Ki' of 177 and 15 microM, respectively, with variable deoxyadenosine; it is a non-competitive inhibitor with a Ki of 17 microM and Ki' of 27 microM with variable ATP. Adenosine kinase phosphorylates adenine arabinoside with an apparent Km of 1 mM using deoxyadenosine kinase assay conditions. The Km values for 6-methylmercaptopurine riboside and 5-iodotubercidin, substrates for adenosine kinase, are estimated to be 4.5 microM and 2.6 nM, respectively. Other nucleoside analogs are potent inhibitors of deoxyadenosine phosphorylation, but their status as substrates remains unknown. These data indicate that deoxyadenosine phosphorylation by adenosine kinase is primarily regulated by its Km and the concentrations of Mg2+, ADP, and AMP. The high Km values for phosphorylation of deoxyadenosine and adenine arabinoside suggest that adenosine kinase may be less likely to phosphorylate these nucleosides in vivo than other enzymes with lower Km values. Adenosine kinase appears to be important for adenosine analog phosphorylation where the Michaelis constant is in the low micromolar range.

Differential sensitivity of leukemic cells to growth inhibition by deoxyadenosine and deoxycoformycin

Both established cell lines and human leukemic cells in circulating blood which were incubated in vitro with 2'-deoxyadenosine (AdR) plus adenosine deaminase inhibitor, 2'-deoxycoformycin (dCF), showed different metabolic responses depending upon the histologic and immunologic types of leukemia. The leukemic T-cell lines in tissue culture were 200-fold more sensitive than B-cell lines to the toxic effect of deoxyadenosine. The increased sensitivity of T-cell lines to AdR plus dCF was associated with the accumulation of deoxyadenosine triphosphate (dATP) in the cells. In established cell lines, an inverse correlation was observed between ED 50 of AdR plus dCF and the relative increase of dATP levels in the cells after the incubation of the cells with AdR plus dCF. In circulating leukemic cells that had been incubated with AdR and dCF, dATP arose in all groups but the correlation was not found between the sensitivity of AdR and the relative dATP accumulation. The failure to find the correlation in patients's leukemic cells may be attributed to the heterogeneity of the response of the blasts to AdR and dCF.

Effects of nucleoside transport inhibitors on the salvage and toxicity of adenosine and deoxyadenosine in L1210 and P388 mouse leukemia cells

Incubation of deoxycoformycin-treated L1210 leukemia cells with dipyridamole or nitrobenzylthioinosine, inhibitors of nucleoside transport, enhanced the long-term incorporation of 2'-deoxyadenosine and adenosine into the nucleotide pool and the toxicity of 2'-deoxyadenosine for the cells. In contrast, 2'-deoxyadenosine uptake in deoxycoformycin-treated P388 leukemia cells, which was about 10 times greater than that in L1210 cells, was inhibited by dipyridamole and nitrobenzylthioinosine, and 2'-deoxyadenosine toxicity was not significantly affected by the transport inhibitors. P388 cells also were about 6 times more resistant to 2'-deoxyadenosine than were L1210 cells, in spite of the greater uptake of the nucleoside. We found that purine nucleoside transport in L1210 and P388 cells exhibited similar kinetic properties and sensitivity to dipyridamole and nitrobenzylthioinosine (both influx and efflux) and that the stimulation of 2'-deoxyadenosine uptake by the inhibitors in L1210 cells is not mediated at the level of its transport into the cells but rather reflects an enhanced intracellular net accumulation of deoxyadenosine nucleotides.

The binding site for the adenosyl group of coenzyme B12 in diol dehydrase

The binding of cob(II)alamin (CblII) and 5'-deoxyadenosine to diol dehydrase was studied spectroscopically and with [U-14C]5'-deoxyadenosine. CblII was bound to this enzyme forming a tight 1:1 complex which was resistant to oxidation by O2 even in the presence of CN-. An irreversible 1:1:1 ternary complex was formed between enzyme, CblII, and 5'-deoxyadenosine, when the enzyme was incubated first with the nucleoside and then with CblII. When this order of addition of the constituents was reversed, no 5'-deoxyadenosine was bound to the enzyme-CblII complex. Hydroxocobalamin could also bind to the enzyme together with the nucleoside, while other cob(III)alamins bearing a bulkier Co beta ligand displaced the nucleoside upon binding to the enzyme. The binding of [U-14C]5'-deoxyadenosine was strongly inhibited by unlabeled 5'-deoxy-ara-adenosine, 4',5'-anhydroadenosine, adenosine, adenine, and 5',8-cyclic adenosine, in this order, but not by 5'-deoxyuridine. These results constitute direct evidence for the presence of the binding site for the adenosyl group of adenosylcobalamin, which is spatially limited to and highly specific for adenine nucleosides. The binding of 5'-deoxyadenosine to the apoenzyme was reversible.

Human B lymphocytes and thymocytes but not peripheral blood mononuclear cells accumulate high dATP levels in conditions simulating ADA deficiency

Inherited adenosine deaminase (ADA) deficiency is associated with a lymphospecific cytotoxicity affecting both dividing and non-dividing cells. The metabolic basis for this was investigated using different cell types and the potentially toxic metabolite 2'-deoxyadenosine (dAR) in short-term experiments under physiological conditions simulating ADA deficiency (1 mM Pi 8.7 microM dAR). In the uncultured cells, [8-14C] dAR alone was metabolized almost completely only by thymocytes and tonsil-derived B-lymphocytes. The greater percentage of counts (greater than 75%) were in the medium (deoxyinosine, hypoxanthine). Cellular counts were predominantly in adenine nucleotides, and to a lesser extent guanine nucleotides. Interestingly, both thymocytes and tonsil-derived B-lymphocytes, and a partially ADA deficient B lymphoblast line, accumulated detectable amounts of dATP even in the absence of ADA inhibition. Peripheral blood lymphocytes (PBMs) did not, and showed little dAR metabolism. In experiments simulating ADA deficiency varying amounts of 2'-deoxycoformycin (2'dCF) were needed to completely inhibit ADA (20-60 microM), with thymocytes requiring the highest amount. ADA inhibited thymocytes and tonsillar B-lymphocytes accumulated very high dATP levels, which were sustained to an equal extent by both over a 60-min period; PBMs accumulated the lowest values. Results in cultured cells reflected findings in previous studies. Some counts were also found in ATP by a route excluding ADA or PNP. These results again question the hypothesis that B-cells are more resistant than T-cells to the toxic effects of dAR because of an inability to accumulate and sustain elevated dATP levels and underline the lack of comparability between enzyme activity in intact as distinct from lysed cells. They cast doubt on the validity of cultured cells as a model for ADA deficiency and suggest the observed toxicity in some instances might result from altered ATP or GTP pools through inadequate ADA inhibition. They indicate that combined immunodeficiency in ADA deficiency could relate to an equal sensitivity of B-cells and T-cell precursors to the toxic effects of dATP accumulation.

Human placental nucleoside kinase activities

Our studies have indicated that normal human placental cytosol contains a complex mixture of nucleoside kinase enzymes, some of which conform to previously characterized activities. Deoxyadenosine is phosphorylated by deoxycytidine kinase, adenosine kinase, and two as yet uncharacterized activities. Deoxyguanosine phosphorylation is associated with deoxycytidine kinase. More complete and detailed studies will be necessary to characterize these enzymes fully.

Purinergic regulation of basal and arginine vasopressin-stimulated hydraulic conductivity in rabbit cortical collecting tubule

An extracellular adenosine responsive site that stimulates adenylate cyclase activity has been identified in several tissues. There is limited information on the presence and physiologic significance of adenosine receptors in well-defined segments of the mammalian nephron. We therefore examined the effect of adenosine and selected analogues on basal hydraulic conductivity in rabbit cortical collecting tubules (CCT) perfused in vitro. Adenosine and analogues with an intact ribose moiety produced a significant, sustained increase in hydraulic conductivity. No increase in hydraulic conductivity was seen in either time control CCT's or CCT's exposed to an adenosine analogue with an altered ribose moiety. These experiments are compatible with the presence of a functional adenosine receptor which requires an intact ribose moiety and acts to increase hydraulic conductivity in the mammalian CCT. An intracellular adenosine responsive site, termed the "P site," which inhibits adenylate cyclase activity, has also been described in several tissues. We therefore examined the effect of a P site agonist on hydraulic conductivity responses to arginine vasopressin, forskolin and cAMP. P site stimulation with 2'5' dideoxyadenosine inhibited the effect of AVP and of forskolin but not of cAMP to increase hydraulic conductivity. These results are compatible with a functional P site in the rabbit CCT which acts at the catalytic subunit of adenylate cyclase to inhibit hydraulic conductivity. Together, these results demonstrate purinergic modulation of basal and arginine vasopressin-stimulated water flux in the mammalian collecting tubule.

Concentrative transport of purine nucleosides in brush border vesicles of the rat kidney

Nephrotoxicity has been reported in humans and in animals during treatment with nucleosides. We described recently a Na+ gradient-dependent concentrative transport of adenosine in brush border vesicles isolated from the cortex of the rat kidney. The results of the present study suggest that that transport is shared by other purine nucleosides. The transport of adenosine is inhibited by several purines at micromolar concentrations. The transports of deoxyadenosine, of inosine and of guanosine are concentrative in the presence of a Na+ gradient and they are inhibited by adenosine. We conclude that there exists a concentrative system for reabsorption of various purine nucleosides in the proximal tubule. The transport of nucleosides is non-concentrative in all other cell types studied so far. The concentrative capacity of the transport of nucleosides in the kidney might explain the particular sensitivity of that organ during treatment with nucleosides.

An approach to a selection system for adenosine-deaminase-positive (ADA+) cells and detection of rat ADA+ "revertants"

We have substituted deoxyadenosine or adenosine for hypoxanthine in the standard HAT selection system in an attempt to select for ADA-normal (ADA+) cells. ADA- human lymphoid line cells could not utilize deoxyadenosine as an alternative to hypoxanthine as a purine source (DAT) and failed to grow but were only somewhat inhibited in growth when adenosine was substituted for hypoxanthine (AAT). In contrast, ADA+ cells utilized adenosine or deoxyadenosine as efficiently as hypoxanthine as a purine source. Growth in DAT, but not in HAT, of an artificial mixture of one ADA+ human lymphoid cells in 1,000 ADA- cells resulted in enrichment of ADA+ cells to 25-86% of total cells. When we grew a rat ADA- cell line in two variations of the DAT system, we detected at least three electrophoretically different ADA+ patterns, one of which corresponded to normal rat ADA. These could represent "revertants."

Immunodeficiencies associated with errors in purine metabolism

The genetic deficiencies of adenosine deaminase and purine nucleoside phosphorylase lead to blocks in the purine pathway. The intracellular accumulation of deoxynucleosides and deoxynucleotides is toxic to both dividing and nondividing lymphocytes via multiple mechanisms. T-lymphocytes are uniquely sensitive to purine-mediated cytotoxicity because of a functional imbalance of phosphorylating and dephosphorylating enzymatic activities. These inborn errors or purine metabolism are rare disorders. The study of these conditions, however, has uncovered unique enzymatic properties of lymphocytes and lymphocyte subclasses. A better understanding of the mechanisms of lymphocytotoxicity in these two purine enzyme defects may lead to better modes of therapeutic manipulation of the immune system.

EHNA is a poor inhibitor of deoxyadenosine catabolism in cultured human lymphocytes

Erythro-9-(2-hydroxy-3-nonyl)-adenine (EHNA) has been used by many workers as enzyme inhibitor in vitro to simulate the in vivo situation in inherited adenosine deaminase (ADA) deficiency. In this study the metabolism of 8-14C deoxyadenosine (dAR) has been followed in cultured lymphocytes from patients deficient in enzymes associated with the catabolism and salvage of dAR, in the absence and presence of 10 microM EHNA. The results show that EHNA, at these concentrations, does not prevent the catabolism of dAR and thus does not provide a valid model for investigating the toxicity to the immune system in inherited ADA deficiency.

5'-Deoxy-5'-methylthioadenosine phosphorylase--III. Role of the enzyme in the metabolism and action of 5'-halogenated adenosine analogs

5'-Deoxy-5'-halogenated adenosines are alternative substrates for 5'-deoxy-5'-methylthioadenosine phosphorylase (MTAPase), an enzyme responsible for the metabolism of 5'-deoxy-5'-methylthioadenosine (MTA), a by-product of polyamine biosynthesis. The relative reactivity of these nucleosides with MTAPase from HL-60 human promyelocytic leukemia cells is MTA greater than 5'-deoxy-5'-fluoroadenosine (5'-FlAdo) greater than 5'-chloro-5'-deoxyadenosine (5'-ClAdo) greter than 5'-bromo-5'-deoxyadenosine (5'-BrAdo) greater than 5'-deoxy-5'-iodoadenosine (5'-IAdo). In MTAPase-containing cells, the adenine released from the 5'-halogenated adenosine was incorporated into adenine nucleotide pools; cleavage by (MTAPase appeared to be the rate-limiting step in this process. 5'-BrAdo and 5'-IAdo were growth inhibitors (EC50 values less than 10 microM) of MTAPase-containing cell lines (HL-60 human promyelocytic leukemia and the L5178Y murine lymphoblastic leukemia) but were much less active (EC50 values greater than 65 microM) against MTAPase-deficient cell lines (the CCRF-CEM human T cell leukemia and the L1210 murine leukemia). The full cytotoxicity of these compounds, therefore, appeared to be related to their phosphorolysis by MTAPase. Indirect evidence suggests that 5-halogenated ribose-1-phosphate derivatives of 5'-BrAdo or 5'-IAdo produced by the MTAPase reaction were the active metabolites of these 5'-halogenated adenosines.

Misincorporation of deoxyuridine in human cells: consequences of antifolate exposure

Treatment of B and T lymphoblastoid cell lines (SB and MOLT-4, respectively) and a promyelocytic leukemia cell line (HL-60) with the lipid soluble antifolate, 2,4-diamino-5-methyl-6-(2',5'-dimeth-oxybenzyl) -pyrido (2,4-d) pyrimidine (BW301U), led to drug dose-dependent inhibition of [3H]deoxyuridine (dU) incorporation into DNA as thymidine, and to misincorporation of [3H]dU as dUMP. After a 15 min preincubation with up to 50 microM BW301U and a further 15 min incubation after addition of [3H]dU, the number of alkaline labile apyrimidinic sites increased with increasing drug dose, as demonstrated by alkaline sucrose gradient analysis. Significantly, new replication of DNA was inhibited only approximately 50% by 50 microM BW301U when [3H]dU incorporation was greater than or equal to 97% inhibited. Additional preliminary findings suggest that newly replicated DNA containing misincorporated dUMP is rapidly degraded in vivo by extensive excision-repair processes.

Distinct sites for deoxyguanosine and deoxyadenosine phosphorylation on a monomeric kinase from Lactobacillus acidophilus

Base-line separation of two paired deoxynucleoside kinase activities (deoxycytidine/deoxyadenosine and deoxyguanosine/deoxyadenosine kinase), previously resolved as overlapping peaks from Blue Sepharose, has now been achieved. The improved separation and recovery in relatively small volumes were accomplished by eluting Blue Sepharose with a bisubstrate mixture: 0.5 mM dCyd plus 1 mM ATP released dCyd/dAdo kinase, and 1 mM dGuo plus 5 mM ATP eluted dGuo/dAdo kinase. The latter pair of activities showed copurification through UDP-Sepharose affinity chromatography and HPLC anion-exchange chromatography. The HPLC preparation appeared to be homogeneous, on the basis of nondenaturing polyacrylamide gel electrophoresis at several gel concentrations and pH values. Both dGuo and dAdo kinase activities coincided with the protein band. A single band of protein was also observed upon sodium dodecyl sulfate gel electrophoresis. The estimated molecular weight of the denatured protein (56 000) agrees closely with values obtained for native activity by sedimentation equilibrium or gel permeation chromatography. The rate of dAdo phosphorylation was found to be stimulated more than 3-fold by the presence of dGuo, and dGuo kinase was also slightly activated by the presence of dAdo. This mutual activation indicates that dGuo and dAdo kinase activities do not share a common site. Selective chemical inactivation of dGuo kinase by 5'-[p-(fluorosulfonyl)benzoyl]adenosine eliminated the ability of dGuo to stimulate dAdo kinase in parallel with the loss of dGuo kinase activity. These lines of evidence strongly suggest that dGuo and dAdo kinase activities are functions of separate sites on a monomeric polypeptide and that these sites may be in allosteric communication.

In vitro metabolism of deoxycoformycin in human T lymphoblastoid cells. Phosphorylation of deoxycoformycin and incorporation into cellular DNA

The biochemical and metabolic effects of deoxycoformycin, a potent inhibitor of adenosine deaminase, were investigated using two human T lymphoblastoid cell lines. A dose-response analysis demonstrated that the concentration of deoxycoformycin at which there was 50% inhibition of growth was greater than 1 X 10(-3) M in lymphoblastoid cells. Uptake of deoxycoformycin was biphasic and occurred much more slowly than for natural nucleosides, and lower saturation levels were reached. The intracellular concentration of deoxycoformycin achieved was 0.4 to 0.5 microM when the extracellular concentration was 1 microM. At 10 microM extracellular concentration, the intracellular concentration was 3-4 microM. Although deoxycoformycin at very low concentrations (1 or 10 microM) did not have any detectable effects on the growth of these cells, the nucleoside was found to be metabolized, and was phosphorylated to give the mono-, di-, and triphosphate derivatives. The triphosphate derivative was incorporated into cellular DNA with little incorporation into cellular RNA. Metabolism of deoxycoformycin in several mutant lymphoblastoid cells deficient in adenosine kinase and/or deoxycytidine kinase was found to be unchanged from wild-type cells, indicating that these major nucleoside kinases do not play a significant role in the phosphorylation of deoxycoformycin. These results may account, at least in part, for the differences that are observed between the pharmacologic inhibition of adenosine deaminase, and the inherited deficiency of adenosine deaminase.

Effects of allosteric activation on the primary and secondary kinetic isotope effects for three AMP nucleosidases

Kinetic isotope effects (V/K) were measured with AMP nucleosidases isolated from Azotobacter vinelandii, from a Vmax mutant enzyme of A. vinelandii and from Escherichia coli. Specifically labeled AMP substrates were used to measure 3H secondary and 14C primary kinetic isotope effects on the N-glycosidic bond hydrolysis of AMP in the presence and absence of the allosteric activator, MgATP. Use of the three enzymes, variable MgATP concentration, a poor substrate (dAMP), and variable pH has allowed determination of the isotope effects over a 5000-fold range in the catalytic turnover number. The primary kinetic isotope effects were 1.025 +/- 0.004 and 1.041 +/- 0.006 for the native A. vinelandii enzyme and mutant enzyme, respectively, and were independent of MgATP concentration. The E. coli AMP nucleosidase had a primary isotope effect of 1.019 +/- 0.003 which was also independent of MgATP concentration. The secondary kinetic isotope effect decreased from 1.066 +/- 0.003 to 1.045 +/- 0.002 for the native enzyme from A. vinelandii as the concentration of MgATP increased from 0 to 500 microM. The secondary isotope effect of the mutant enzyme remained constant at 1.088 +/- 0.005 as the MgATP concentration increased from 0 to 500 microM. The secondary isotope effect of the E. coli enzyme showed a similar pattern to that of the native enzyme, decreasing from 1.087 +/- 0.003 to 1.050 +/- 0.003 as the enzyme was saturated with MgATP at a constant concentration of AMP. Saturation with AMP in the absence of MgATP gave similar results and suggested that AMP can cause the allosteric transition. Both the primary and secondary isotope effects for the native enzyme from A. vinelandii remained constant as the pH was varied in the absence of MgATP. Secondary isotope effects with a poor substrate, dAMP, were 1.08 for both the mutant and wild type enzymes from A. vinelandii in the presence of allosteric activator. In the native enzyme, this isotope effect was independent of MgATP concentration. The relative insensitivity in the magnitude of observed isotope effects to pH, allosteric activator, the mutant enzyme, and a poor substrate (dAMP) indicate that intrinsic isotope effects are being expressed. The data are interpreted in terms of a single rate-limiting transition state for hydrolysis of the N-glycosidic bond, although other mechanisms cannot be eliminated. Using this model, the transition states of the native A. vinelandii and E. coli enzymes exhibit properties of both dissociative and associative mechanisms but become more associative as the allosteric activator becomes saturating.(ABSTRACT TRUNCATED AT 400 WORDS)

The mechanism of inhibition and "reversal" of mitogen-induced lymphocyte activation in a model of adenosine deaminase deficiency

The biochemical mechanism of lymphocyte dysfunction with adenosine deaminase deficiency has been investigated using cultured phytohemagglutinin stimulated normal peripheral blood lymphocytes and the adenosine deaminase (ADA) inhibitor 2'-deoxycoformycin. The addition of deoxyadenosine to ADA-inhibited (but not to uninhibited) cells generated increased dATP pools (up to 50-fold greater than controls) and depressed the mitogen response. dATP Accumulation was accompanied by depletion of the other three deoxynucleoside triphosphate (dNTP) pools (dTTP, dCTP, and dGTP). Suppression of the mitogen response could be prevented ("reversed") to 90% of control levels by the addition of deoxynucleoside precursors for the depleted dNTPs at the initiation of mitogen stimulation. "Reversal" restored the dTTP and possibly the dGTP pools. Thus the mechanism of toxicity in this model appears to be inhibition of ribonucleotide reductase by massive accumulation of dATP, resulting in starvation for the other three deoxyribonucleoside triphosphates. "Reversibility" of this toxicity by providing sources for the missing three deoxynucleoside triphosphates argues for ribonucleotide reductase inhibition rather than other mechanisms of deoxyadenosine toxicity in this model.

Modulation of adenine nucleoside excretion and incorporation in adenosine deaminase deficient human lymphoma cells

The availability of a human lymphoma cell line deficient in adenosine deaminase, adenosine kinase and methylthioadenosine phosphorylase enabled us to compare the effects of nucleoside transport inhibitors on the excretion of endogenously generated adenosine, deoxyadenosine and 5'-methylthioadenosine. The nucleoside transport inhibitors nitrobenzylthioinosine and dipyridamole blocked the efflux of adenosine, but not deoxyadenosine or 5'-methylthioadenosine. The inhibitors also prevented the uptake of exogenous adenosine, but not deoxyadenosine or 5'-methylthioadenosine, by human lymphoblasts. The results show (i) that the transport inhibitors modify adenine nucleoside efflux and influx similarly, and (ii) that the effects of the compounds on the excretion and uptake of these three physiologically important adenine nucleosides are distinctly different.

Deoxyadenosine modulates human suppressor T cell function and B cell differentiation stimulated by Staphylococcus aureus protein A

Adenosine deaminase (ADA) deficiency and the resultant accumulation of deoxyadenosine (AdR) are associated with profound T cell dysfunction and variable B cell dysfunction in vivo. We examined the effects of AdR on the in vitro function of normal human peripheral blood B and T lymphocytes whose ADA activity was inhibited by 2'-deoxycoformycin. We found that OKT8+ T cell-mediated suppression of SPA-induced Ig production was markedly reduced by concentrations of AdR (3 to 10 microM) that did not affect helper T cell function. Because the lectin-induced proliferative responses of OKT8+ T cells and OKT8- T cells were equally susceptible to AdR, modulation of in vitro immune responses by low-dose AdR probably reflected different proliferative requirements for the expression of T cell helper or suppressor functions. Although low doses of AdR did not inhibit Ig production in SPA-stimulated cultures, we found that T cell-dependent, SPA-stimulated B cell proliferation was blocked by 3 to 10 microM AdR. Therefore, it appeared that B cell proliferation was not required for the induction of Ig synthesis in this system. Higher doses (30 to 100 microM) of AdR did block the induction of Ig synthesis, presumably by interfering with T-helper functions via a mechanism other than inhibition of proliferation and/or by inhibiting B cell differentiation events.

B cells as well as T cells form deoxynucleotides from either deoxyadenosine or deoxyguanosine

Enzyme inhibitors used to simulate the inherited immunodeficiency diseases, adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP) deficiency, have been assessed in cultured human lymphocytes. Only 2'-deoxycoformycin (dCF) completely inhibited ADA in T and B cells at concentrations in excess of 5 microM. Erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA) and 8-amino guanosine (8-NH2GR) did not inhibit ADA or PNP completely at any concentration. Detailed metabolic experiments comparing viability and deoxynucleotide accumulation showed that B cell lines of malignant origin also accumulated high levels of dATP from 2'-deoxyadenosine (dAR), and dGTP from 2'-deoxyguanosine (dGR) as effectively as T cells--even without inhibitors, however, dAR reduced cell viability only when ADA was inhibited by dCF, whilst dGR was equally toxic with or without inhibitor, even to a line which accumulated no dGTP. These experiments indicate that cultured lymphocytes, using either EHNA or 8-NH2GR as enzyme inhibitor, are not valid models of the toxicity to the immune system in inherited ADA or PNP deficiency. They demonstrate that the ability to accumulate high levels of dATP or dGTP is not exclusive to T cells and that the in vitro toxicity of dAR or dGR could relate to the use of excess substrate and/or accumulation in different nucleotide, not deoxynucleotide pools.

Adenine nucleotide levels and adenosine metabolism in cultured calvarial bone

Adenine nucleotide levels were measured in extracts of murine calvaria after different periods of culture with or without parathyroid hormone (PTH; 10(-8) M) or PGE2 (10(-7) M). In control calvaria the energy charge, (ATP + 1/2 ADP)/(ATP + ADP + AMP), remained at close to 0.7 over a 24 hour culture period. However, bones cultured with PTH or PGE2 showed a transient fall in the energy charge down to 0.5. This was not associated with a fall in total adenine nucleotides. The rate of adenosine metabolism in cultured bone in vitro was studied by determining the contents of adenosine, inosine, 2-deoxyadenosine, 2-deoxyinosine and hypoxanthine in the culture medium. There was a continuous increase in adenosine, inosine and hypoxanthine as well as a disappearance of medium 2-deoxyadenosine that was accounted for by appearance of 2-deoxyinosine. The deaminating activity could only partly be accounted for by activity in the medium and thus probably mainly resides in the bone cells. PTH (10(-8) M) did not alter the rate of disappearance of 2-deoxyadenosine or adenosine deaminase activity determined in bone extracts. The results demonstrate that two substances that increase calcium mobilization from bone alter ATP utilization and/or synthesis without significantly influencing adenosine production or metabolism.

Spectrophotometric studies of the interaction of S-adenosylhomocysteinase with adenosine, adenine and cordycepin

The spectral changes observed on interaction of S-adenosylhomocysteinase with adenine and cordycepin are approximated by the addition of dimethylsulfoxide to the aqueous solutions of these compounds, but not by protonation of the compounds. Although adenosine when bound to the enzyme undergoes partial reactions, it gives a spectral change similar to those obtained with adenine and cordycepin, except for the occurrence of a peak at 327 nm due to the reduction of the enzyme-bound NAD. From these results, it is suggested that S-adenosylhomocysteinase binds the nucleoside substrates mainly through hydrophobic interactions.