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  • 1
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Biochemistry 11 (1972), S. 1183-1188 
    ISSN: 1520-4995
    Source: ACS Legacy Archives
    Topics: Biology , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Molecular and cellular biochemistry 77 (1987), S. 187-194 
    ISSN: 1573-4919
    Keywords: ribavirin ; differentiation ; erythroleukemia ; IMP dehydrogenase ; mycophenolic acid
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology , Medicine
    Notes: Summary The synthetic nucleoside, ribavirin (1-β-D-ribofuranosyl-1,2,4-triazole-3-carboxamide), a broad spectrum antiviral agent currently being tested in clinical studies with AIDS patients; and mycophenolic acid, a non-nucleoside inhibitor of inosinate (IMP) dehydrogenase, are effective inducers of terminal differentiation of Friend virus transformed murine erythroleukemia cells. The inhibition of cell division and the induced maturation produced by these agents appears to be a consequence of inhibition of IMP dehydrogenase, since growth inhibition is reversed and differentiation is prevented by the simultaneous exposure of cells treated with the agents to exogenous guanine or guanosine, which circumvents the effects of blockage of IMP dehydrogenase. However, while the effects mycophenolic acid, a pure IMP dehydrogenase inhibitor with no other biochemical effects, were completely reversed by guanine salvage supplies, cells exposed to ribavirin responded in a different manner. At levels of guanine salvage supplies below 50 μM, growth inhibition and cell differentiation were partially reversed. At salvage supply concentrations greater than 50 μM, while differentiation was completely blocked, the toxicity of ribavirin was increased and cell division was greatly diminished. These results indicate additional biochemical effects for ribavirin unrelated to the inhibition of IMP dehydrogenase, which may be related to its antiviral properties.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Molecular and cellular biochemistry 92 (1990), S. 37-44 
    ISSN: 1573-4919
    Keywords: methylated oxypurines ; differentiation ; erythroleukemia
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology , Medicine
    Notes: Abstract Murine erythroleukemia cell lines derived from Friend virus infected mice can be induced to differentiate in vitro by numerous agents. Among these compounds are certain naturally occurring purines such as hypoxanthine or 1-methylhypoxanthine. We have extended these studies to other modified oxypurines and have identified some areas of cell regulation with which they may be interacting. Monomethylated derivatives of guanine, hypoxanthine or xanthine are active as inducers of differentiation. Excluding hypoxanthine, the parent oxypurines guanine and xanthine are ineffective in inducing differentiation. The dimethyl- and trimethylxanthine derivatives are also inactive as inducers. The methylated oxypurines are not metabolized to nucleotides by the cell and, therefore, probably do not interact with nucleic acid synthesis directly. We have investigated one cellular process of possible regulatory significance with which they do interact. ADP-ribosylation has been implicated in control of gene expression and differentiation. The methylated oxypurines inhibit this reaction, as measured in permeabilized cells, in the same concentration range at which they are effective as inducers of differentiation. Additionally, 1-methylguanine and 7-methylguanine decrease incorporation of mannose and glucosamine into glycoprotein and into dolichol-oligosaccharide precursors. These effects may be related to cell surface alterations observed during differentiation.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Cell & tissue research 241 (1985), S. 203-206 
    ISSN: 1432-0878
    Keywords: Methylated purines ; Transforming agents ; Surface modification ; Cell agglutination ; Chinese hamster
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary 1-Methylguanine and 7-methylguanine, both metabolic products of tRNA degradation, are known to induce transformation of Chinese hamster fibroblasts in culture. The effects of these compounds on the cell membrane have been studied by the method of Concanavalin A-mediated hemadsorption. 1-Methylguanine or 7-methylguanine induced a 50% increase of Con A-mediated hemadsorption within 20 hours of exposure of the cells to the agent at a concentration of 10-5 M. This alteration was reversed within 13 days when the cells were grown in the control medium. Prolonged treatment with 1-methylguanine or 7-methylguanine resulted in changes which were only slowly reversed during growth of the cells in the control medium. The effect of the methylated purines on the cell membrane could be completely inhibited by simultaneous addition of dibutyryl-cAMP at a concentration of 10-5 M. The possible mechanism of cell membrane alteration by methylated purines and its relevance to transformation in vitro are discussed.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 0021-9541
    Keywords: Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Medicine
    Notes: In quiescent confluent monolayers of WI-38 cells, the specific activity of the tRNA methyltransferases falls to 20% of the level found in log phase cells. When the resting cells are stimulated to proliferate by a change to fresh medium, the enzymes show a rapid rise in specific activity which correlates with early increases in the rate of tRNA synthesis. The specific activity of the enzymes continues to rise throughout the period of DNA synthesis, at the end of which it is somewhat higher than that of log phase cells. The increases in enzyme activity could be blocked by exposure of the stimulated cells to Actinomycin D (2μ/ml). The increases in activity were not equivalent for the different base-specific enzymes. The contribution of the N2-methylguanine specific enzyme remained relatively constant, while that of the N2,N2-dimethylguanine specific and 1-methyladenine specific enzymes doubled and tripled, respectively, by late S phase. The contributions of the 1-methylguanine and the 7-methylguanine specific enzymes fell to a few percent of the total by late S phase. This indicates non-coordinate variations in the expression of the different base-specific enzymes after stimulation of resting cells and may be related to altered isoaccepting tRNA profiles observed in resting and growing cells.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Cellular Physiology 103 (1980), S. 29-33 
    ISSN: 0021-9541
    Keywords: Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Medicine
    Notes: Mouse L-cells were enucleated by exposure to cytochalasin B followed by centrifugation. The resulting karyoplasts, nuclei surrounded by a thin shell of cytoplasm and an outer cell membrane, and cytoplasts, the enucleated cell cytoplasm, were assayed for tRNA methyltransferase activity. The bulk of the enzyme activity was found to be localized in the nuclei. Analysis of the methylated nucleosides produced by the enzyme from the two sources showed that all the base-specific enzyme activities which are found in whole cell extracts were present in the nuclear extracts.The cytoplast extracts retained a low but detectable enzyme activity, which was composed predominantly of only two base-specific activities. This may represent tRNA methyltransferases of the mitochondria or may be cytoplasmic enzymes for late modification reactions.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
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