Adenosine phosphorylase activity in a mutant Hep-2 cell line contaminated withmycoplasma hyorhinis

Summary Metabolic studies in HEp-2/MP,MIR cells (an adenosine kinase, hypoxanthine phosphoribosyltransferase negative mutant) indicated the presence of adenosine phosphorylase activity. This activity, unknown in established mammalian cell lines, resulted in the glycosidic cleavage of both adenosine and the antiviral drug arabinosyladenine. The activity was observed readily in the presence or absence of the adenosine deaminase inhibitor coformycin. Isopycnic separation of [³H]thymidine-labeled DNA species in CsCl density gradients resulted in the appearance of two distinct peaks. The heavier peak coincided with [¹⁴C]thymidine-labeled marker DNA of human origin, whereas the lighter peak was within the range associated with mycoplasmal DNA. Testing by commercial laboratories confirmed the presence of mycoplasma in HEp-2/MP,MIR cells. The contaminant was identified asMycoplasma hyorhinis, a porcine mycoplasma. Following γ-irradiation (3000 rads) to block cellular mitosis, the mycoplasma-contaminated HEp-2/MP,MIR cells were cocultivated with mycoplasma-free wild-type HEp-2 cells which did not exhibit adenosine phosphorylase activity. Following serial cocultivation in a medium designed to favor the survival of the wild-type cells, adenosine phosphorylase activity was found in the previously uninfected cells. Studies of this nature emphasize the need for investigators to carefully monitor their cell lines for mycoplasma. Presented at the 25th Annual Meeting of the Tissue Culture Association, Philadelphia, Pa., June 1976. This work was supported by Public Health Service Grants DE 02731 from the National Institute of Dental Research and CA 16219 from the National Cancer Institute.     

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

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

Mycoplasma in African green monkey kidney cell cultures: biochemical detection and effects in virus-infected cells.

Among a number of techniques for the detection of mycoplasmal contamination in African green monkey kidney (AGMK) cell lines, the assay of uridine phosphorylase activity is unsuitable because of the presence of high levels of endogenous enzymatic activity. A thymidine phosphorylase test, however, based on the chromatographic analysis of radiolabeled thymidine breakdown, turned out to be a simple and sensitive mycoplasma detection method. We found, using the latter technique, that 0.22-micrometer-filtered virus inocula could still transfer mycoplasma unless treated with diethyl ether. The effect of mycoplasmal contamination on the synthesis of simian virus 40 and adenovirus in AGMK cells was negligible under the conditions used (no depletion of arginine). Incorporation of radioactive thymidine in viral macromolecules, however, was inhibited severely by the presence of mycoplasma.     

Protease-sensitive transfection of Streptococcus pneumoniae with bacteriophage Cp-1 DNA.

The transfecting activity of pneumococcal phage Cp-1 DNA was destroyed by treatment with proteolytic enzymes, although these enzymes did not affect transfection with bacteriophage Dp-4 DNA. This transfection was stimulated by calcium ions. Protease-treated Cp-1 DNA competes for binding and uptake with transforming pneumococcal DNA as well as with transfecting Dp-4 DNA to approximately the same extent as does untreated Cp-1 DNA. In addition, [3H]thymidine-labeled Cp-1 DNA, treated with proteases or untreated, was absorbed with the same efficiency. These data suggest that uptake of Cp-1 DNA is not affected by protease treatment. [3H]thymidine-labeled Cp-1 DNA showed remarkable resistance against surface nuclease activity of competent wild-type cells. The monomeric form of the Cp-1 DNA-protein complex showed a linear dose response in transfection.     

Mycoplasma in african green monkey kidney cell cultures: Biochemical detection and effects in virus-infected cells

Summary Among a number of techniques for the detection of mycoplasmal contamination in African green monkey kidney (AGMK) cell lines, the assay of uridine phosphorylase activity is unsuitable because of the presence of high levels of endogenous enzymatic activity. A thymidine phosphorylase test, however, based on the chromatographic analysis of radiolabeled thymidine breakdown, turned out to be a simple and sensitive mycoplasma detection method. We found, using the latter technique, that 0.22-μm-filtered virus inocula could still transfer mycoplasma unless treated with diethyl ether. The effect of mycoplasmal contamination on the synthesis of simian virus 40 and adenovirus in AGMK cells was negligible under the conditions used (no depletion of arginine). Incorporation of radioactive thymidine in viral macromolecules, however, was inhibited severely by the presence of mycoplasma. This investigation was supported by a grant from theFonds voor Geneeskundig Wetenschappelijk Onderzoek (No. 20.298). F.V. R. is an Aspirant of the BelgianNationaal Fonds voor Wetenschappelijk Onderzoek.     

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.     

Mycoplasma contamination greatly enhances the apparent transport and concentrative accumulation of formycin B by mammalian cell culture

S49 mouse leukemia cells exhibit both equilibrative and Na(+)-dependent, concentrative formycin B transport. The latter represents only a minor nucleoside transport component and is detectable only when equilibrative nucleoside transport is inhibited by dipyridamole or another transport inhibitor. Thus in uncontaminated S49 cells formycin B accumulated only to slightly above the intracellular-extracellular equilibrium level. In contrast, in suspensions of S49 cells contaminated with mycoplasma, formycin B accumulated in the intracellular water space in unmodified form to 40-50-times the extracellular concentration in a dipyridamole-independent manner during 90 min of incubation at 37 degrees C. The mycoplasma active formycin B transport system was inhibited by all nucleosides tested, including thymidine and deoxycytidine, which are not substrates for the concentrative nucleoside transporter of S49 cells. Mycoplasma contamination was detected by the presence of cell-associated adenosine phosphorylase activity.     

An ultramicrochemical test for mycoplasmal contamination of cultured cells.

A sensitive ultramicrochemical enzyme test for mycoplasmal contamination of cultured cells, based on the determination of the activity of adenosine phosphorylase, is described. The test was performed by assaying the enzymatic conversion of [8-14C]adenine and ribose-1-phosphate to [8-14C]adenosine by incubating a plastic leaflet carrying a counted number of cells (1 to 10). These leaflets were isolated from the bottom of the same plastic film dish in which the cells were cultured for experimental or diagnostic purposes, e.g. prenatal diagnosis or inborn errors of metabolism. The present test should be several 1000-fold more sensitive than the originally reported enzymatic method because (a) the adenosine-phosphorylase reaction is measured in the nucleoside forming direction which is by far the most active; and (b) the assay is performed with the cells and not with the culture medium. The latter is of special importance for the detection of those low-grade contamination in which most of the mycoplasma particles are attached to cell membranes.     

Single-stranded regions in Streptococcus pneumoniae chromosomal deoxyribonucleic acid and their relation to transformation.

Deoxyribonucleic acid (DNA) in lysates of both completent and noncompetent streptococcus pneumoniae cells was characterized by chromatography on benzoylated, naphthoylated diethylaminoethyl-cellulose columns, by sensitivity to Aspergillus oryzae S1 endonuclease, and by sucrose gradient analysis. The DNAs from both competent and noncompetent cells were found to contain similar extents of single-stranded regions. These single-stranded regions appeared to be intact, unpaired regions in double-stranded DNA rather than gaps, nicks, or unpaired ends in the DNA. Inhibition of cells with rifampin prior to lysis increased the amount of such single strandedness in the DNA. Lysates made at various times after [14C]thymidine-labeled cells had bound [3H]thymidine-labeled transforming DNA were also characterized by benzoylated, naphthoylated diethylaminoethyl-cellulose chromatography. Changes in the elution profiles of DNA from cells exposed to homospecific (S. pneumoniae) donor DNA were indicative of the formation of complexes between donor DNA and the single-stranded regions of recipient DNA. In contrast, profiles of DNA from cells exposed to heterospecific (S. sanguis) DNA did not show significant changes, indicating that few such donor-recipient complexes were formed during heterospecific transformation.     

Adenosine-resistant chinese hamster fibroblast variants with hyperactive adenosine-deaminase: An analysis of the protection against exogenous adenosine afforded by increased activity of the deamination pathway

The activity of purine salvage and interconversion enzymes was examined in two sublines of Chinese hamster cells–RA11 and RA41–isolated on the basis of their resistance to adenosine concentrations toxic to wild-type CCL39 cells. Adenosine deaminase (ADA) activity was found to be two times higher in RA11 and three times higher in RA41 than in CCL39. Inhibition of ADA activity by coformycin reduced the level of adenosine resistance but did not restore wild-type sensitivity, indicating that a second defect contributes to the adenosine-resistant phenotype of these variants; evidence was indeed obtained for the presence in both lines of additional alterations protecting them against the lethal depletion of phosphoribosylpyrophosphate (Ishii and Green, 1973) imposed by adenosine to wild-type cells. To gain better insight into the influence of ADA hyperactivity on adenosine resistance, a procedure was developed for the specific isolation of variants with increased levels of ADA activity. Cell lines with 3–5 times and then 100–500 times the wild-type ADA activity were stepwise recovered. These investigations confirmed that amplification of ADA can efficiently contribute in protecting cells against high concentrations of exogenous adenosine. The variants isolated by this procedure again manifested, in addition to amplification of ADA activity, another alteration decreasing their sensitivity to adenosine. A possible mechanism accounting for the frequent isolation of variants that coexpress ADA hyper-activity and a second defect contributing protection against adenosine toxicity are considered.     

Competitor internal standards for quantitative detection of mycoplasma DNA

Abstract Homologous internal controls were used as competitor DNA in the polymerase chain reaction for the quantitative detection of mycoplasma DNA. PCR primer sets were designed on the basis of the most conserved nucleotide sequences of the 16S rRNA gene of mycoplasma species. Amplification of this gene was examined in five different mycoplasma species: Mycoplasma orale, M. hyorhinus, M. synoviae, M. gallisepticum and M. pneumonias . To evaluate the primers, a number of different cell lines were assayed for the detection of mycoplasma infections. All positive cell lines showed a distinct product on agarose gels while uninfected cells showed no DNA amplification. Neither bacterial nor eukaryotic DNA produced any cross-reaction with the primers used, thus confirming their specificity. Internal control DNA to be used for quantitation was constructed by modifying the sizes of the wild-type amplified products and cloning them in plasmid vectors. These controls used the same primer binding sites as the wild-type and the amplified products were differentiated by a size difference. The detection limits for all the mycoplasma species by competitive quantitative PCR were estimated to range from 4 to 60 genome copies per assay as determined by ethidium bromide-stained agarose gels. These internal standards also serve as positive controls in PCR-based detection of mycoplasma DNA, and therefore accidental contamination of test samples with wild-type positive controls can be eliminated. The quantitative PCR method developed will be useful in monitoring the progression and significance of mycoplasma in the disease process.     

Adenosine metabolism in wild-type and enzyme-deficient variants of Chinese hamster ovary and Novikoff rat hepatoma cells

Variants of Chinese hamster ovary and Novikoff rat hepatoma cells resistant to tubercidin and 2,5-diaminopurine, or to both drugs, were isolated, and their ability to convert adenosine and various adenosine analogs to nucleotides was compared to that of wild-type cells, both in intact cells and cell-free extracts. Adenosine deamination, and thus its conversion to nucleotides via inosine-hypoxanthine-inosine monophosphate, was inhibited by pretreatment of the cells or cell extracts with 2-deoxycoformycin. Cell-free extracts of the tubercidin-resistant variants, as well as of two adenosine-resistant mutants of Chinese hamster ovary cells, phosphorylated adenosine, tubercidin, pyrazofurin, or tricyclic nucleoside in the presence of ATP at less than 1% of the rate of extracts of wild-type cells. However, addition of phosphoribosyl pyrophosphate stimulated the conversion of adenosine to nucleotides 40-fold. Similarly, intact adenosine kinase-deficient cells failed to phosphorylate the adenosine analogs, but still converted adenosine to nucleotides at 5-10% the rate observed with wild-type cells. Phosphorylation of adenosine and tubercidin in wild-type cells was inhibited by substrate at concentration above 5-10 microM. In contrast, the rate of conversion of adenosine to nucleotides by adenosine kinase-deficient cells increased linearly up to a concentration of 400 microM adenosine, with the consequence that, at this concentration, these cells took up adenosine almost as rapidly as wild-type cells. Adenosine uptake by these kinase-deficient cells was inhibited by adenine and 5'-deoxyadenosine, and was largely abolished in mutants devoid also of adenine phosphoribosyltransferase. We conclude that adenosine is converted to nucleotides in adenosine kinase-deficient cells via adenine. Indirect evidence implicates 5'-methylthioadenosine phosphorylase as the enzyme responsible for the degradation of adenosine to adenine.     

Interference in hybrid clone selection caused by Mycoplasma hyorhinis infection

Mycoplasma hyorhinis strains were isolated from Chinese hamster DON cells which lacked the ability to produce hybrid colonies in HAT medium. The mycoplasma isolates were virtually devoid of HGPRT activity in vivo and in vitro in the presence of excess co-enzyme, phosphoribosylpyrophosphate. Deliberate infection of mycoplasma-free cells caused no alterations in the HGPRT^? and TK^? phenotypes of the cells. Heterokaryon formation with infected cells was normal and the failure to produce hybrid colonies resulted from depletion, by nucleoside phosphorylase activity, of exogenous thymidine required for rescue of hybrid cells in HAT medium. Increasing the thymidine concentration and repeatedly replenishing HAT medium permitted hybrid clone formation.     

Participation of ATP in the binding of a yeast replicative complex to DNA.

The activity that replicates yeast DNA in vitro can be isolated from cells of the budding yeast Saccharomyces in a high-Mr (approximately 2 X 10(6] form. Several lines of evidence indicate that this fraction contains a multiprotein replicative complex. A functional assay has been developed for the analysis of the interaction of the replicating activity with DNA. Binding of the activity required Mg2+, but did not require the addition of ATP or the other ribo- or deoxynucleoside triphosphates. However, the ATP analogues adenosine 5'-[gamma-thio]triphosphate and adenosine 5'-[beta gamma-imido]triphosphate blocked the binding, suggesting that ATP participates in the interaction at some stage. The binding was template (origin)-specific in either the presence or the absence of ATP and the other nucleoside triphosphates; however, ATP stabilized the replicating activity. The preferential inhibition of binding that was observed in the presence of the DNA topoisomerase II inhibitor coumermycin suggests that the requirement for ATP may be at least partially accounted for by the involvement of this enzyme in the initial interaction of the replicating activity with DNA. Finally, the binding was rapid. In contrast, DNA synthesis displayed a lag when assayed directly without first allowing a period for the replicating activity to bind to the DNA. In addition, binding was 'tight', as judged by the resistance of the protein--DNA complexes to salt in comparison with the relative sensitivity of binding. The replicating activity was not readily displaced from the complexes by exogenous DNAs, either possessing or lacking yeast origins of replication. The results suggest that the interaction of the replicating activity with the DNA occurs in more than one stage.     

Radioenzymatic determination of adenosine

Adenosine has been measured at the nanomolar level by an enzymatic radioactive assay. The nucleoside is converted into [U-14C]ribose-labeled inosine via the following reactions: adenosine + H2O----adenine + ribose (adenosine nucleosidase); adenine + [U-14C]ribose 1-phosphate in equilibrium with T[U-14C]ribose-adenosine + Pi (adenosine phosphorylase); [U-14C]ribose-adenosine + H2O----[U-14C]ribose-inosine + NH3 (adenosine deaminase). The radioactivity of inosine, separated by thin-layer chromatography, is a measure of the adenosine initially present.     

Adenosine modulation of D-[3H]aspartate release in cultured retina cells exposed to oxidative stress

In this study we evaluated the role of adenosine receptor activation on the K+-evoked D-[3H]aspartate release in cultured chick retina cells exposed to oxidant conditions. Oxidative stress, induced by ascorbate (3.5 mM)/Fe2+ (100 microM), increased by about fourfold the release of D-[3H]aspartate, evoked by KCl 35 mM in the presence and in the absence of Ca2+. The agonist of A1 adenosine receptors, N6-cyclopentyladenosine (CPA; 10 nM), inhibited the K+-evoked D-[3H]aspartate release in control in oxidized cells. The antagonist of A1 adenosine receptor, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 50 nM), potentiated the release of D-[3H]aspartate in oxidized cells, and reverted the effect observed in the presence of CPA 10 nM. However, in oxidized cells, when DPCPX was tested together with CPA 100 nM the total release of D-[3H]aspartate increased from 5.1 +/- 0.4% to 11.4 +/- 1.0%, this increase being reverted by 3,7-dimethyl-1-propargylxanthine (DMPX; 100 nM), an antagonist of A2A adenosine receptors. In cells of both experimental conditions, the K+-evoked release of D-[3H]aspartate was potentiated by the selective agonist of A2A adenosine receptors, 2-[4-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadenosin e (CGS 21680; 10 nM), whereas the antagonist of these receptors, DMPX (100 nM), inhibited the release of D-[3H]aspartate in oxidized cells, but not in control cells. Adenosine deaminase (ADA; 1 U/ml), which is able to remove adenosine from the synaptic space, reduced the K+-evoked D-[3H]aspartate release, from 5.1 +/- 0.4% to 3.1 +/- 0.3% in oxidized cells, and had no significant effect in control cells. The extracellular accumulation of endogenous adenosine, upon K+-depolarization, was higher in oxidized cells than in control cells, and was reduced by the inhibitors of adenosine transporter (NBTI) and of ecto-5'-nucleotidase (AOPCP). This suggests that adenosine accumulation resulted from the outflow of adenosine mediated by the transporter, and from extracellular degradation of adenine nucleotide. Our data show that both inhibitory A1 and excitatory A2A adenosine receptors are present in cultured retina cells, and that the K+-evoked D-[3H]aspartate release is modulated by the balance between inhibitory and excitatory responses. Under oxidative stress conditions, the extracellular accumulation of endogenous adenosine seems to reach levels enough to potentiate the release of D-[3H]aspartate by the tonic activation of A2A adenosine receptors.     

Polyomavirus major capsid protein VP1 is capable of packaging cellular DNA when expressed in the baculovirus system.

Using the p2Bac dual multiple cloning site transfer vector, the polyomavirus major capsid protein gene VP1 was cloned for expression in the baculovirus-insect cell expression system. The 5-day-infected cellular lysate from this recombinant preparation was purified by cesium chloride density gradient centrifugation. Capsid-like particles were observed in the resulting preparation. The purified particle preparation was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and was shown to have accurately expressed the polyomavirus VP1 protein as cloned. It was found that the preparation revealed the presence of host histones in the stained gels, which is indicative of DNA packaging. To determine if cellular DNA was being packaged in the particles, Sf9 insect cells were prelabeled with [3H] thymidine. The label was removed, and the cells were subsequently infected with a recombinant Autographa californica multiple nuclear polyhedrosis virus (AcMNPV) carrying the polyomavirus VP1 gene. Upon purification through three cesium chloride gradients and DNase I treatment, capsid-like particles, containing [3H]thymidine-labeled DNA, were isolated which were found to coincide with hemagglutination activity. Studies have indicated that the AcMNPV appears to have the ability to fragment Sf9 cellular DNA. When infected with the recombinant AcMNPV carrying the VP1 gene of polyomavirus, these host DNA fragments are being packaged by the VPI major capsid protein; further, these DNA fragments have been shown to be approximately 5 kb in size, which corresponds to the size of the native polyomavirus genome. These studies demonstrate that the recombinant polyomavirus VP1 protein has the ability to package DNA in the absence of the minor structural proteins VP2 and VP3 and independently of the polyomavirus T antigens.     

Adenosine, deoxyadenosine, and deoxyguanosine induce DNA cleavage in mouse thymocytes

When thymocytes were cultured with adenosine, deoxyadenosine, or deoxyguanosine at 1 mM for 24 h, DNA cleavage at internucleosomal sites with multiples of approximately 180 bp was induced, followed by lactate dehydrogenase release into the medium. In the presence of coformycin, an adenosine deaminase inhibitor, or formycin B, a purine nucleoside phosphorylase inhibitor, DNA cleavage was induced by these nucleosides at concentrations of less than 50 microM. Other purine and pyrimidine ribo- and deoxyribonucleosides did not induce DNA cleavage or LDH release. Because thymocyte nuclei contain a Ca2+,Mg2+-dependent endonuclease, which preferentially cuts DNA in its linker regions, DNA fragmentation induced by the three purine nucleosides was suggested to occur through increased activity of the endonuclease. The DNA cleavage induced by the nucleosides required protein phosphorylation and synthesis, inasmuch as it was inhibited by an inhibitor of protein kinases, H-7, and by an inhibitor of protein synthesis, cycloheximide. The inhibition of DNA cleavage was accompanied by a reduction in lactate dehydrogenase release, suggesting a causal relationship between DNA cleavage and cell death. The DNA cleavage and subsequent cell lysis might be related to the selective thymocyte deletion observed in patients with adenosine deaminase or purine nucleoside phosphorylase deficiency.     

A post-alpha gene function turns off the capacity of a host protein to bind DNA in cells infected with herpes simplex virus 1.

HEp-2 cell proteins electrophoretically separated in denaturing polyacrylamide gels and electrically transferred to nitrocellulose sheets contain a polypeptide which efficiently binds linear native DNA end labeled with 32P but not denatured DNA. The polypeptide has an apparent molecular weight of ca. 130,000. The activity of the protein was stable, and no appreciable turnover was observed after exposure of uninfected cells to inhibitory concentrations of cycloheximide for intervals of up to 24 h. However, the activity was absent from lysates of cells harvested 6 h or later postinfection with wild-type viruses. To identify the viral function involved in the loss of DNA-binding activity, we tested the lysates of cells infected with several mutants. Thus, the DNA-binding activity was unaffected in cells infected with a temperature-sensitive mutant (herpes simplex virus 1 tsLB2) in the alpha 4 gene and was maintained at a nonpermissive temperature (39 degrees C). Experiments involving (i) temperature shift-down of cells infected with tsLB2 in the presence of cycloheximide, (ii) withdrawal of cycloheximide in the presence and absence of actinomycin D from cells infected with wild-type virus, (iii) infection of cells at 33 and 39 degrees C with herpes simplex virus 1 tsHA1 carrying a temperature-sensitive lesion in the beta 8 gene, and (iv) infection of cells in the presence of inhibitory concentrations of phosphonoacetate led to the conclusion that the viral functions responsible for the loss of DNA-binding capacity were specified by either beta or gamma genes not dependent on viral DNA synthesis for their expression.