The inhibition of nucleic acid synthesis by mycophenolic acid

1. Mycophenolic acid, an antibiotic of some antiquity that more recently has been found to have marked activity against a range of tumours in mice and rats, strongly inhibits DNA synthesis in the L strain of fibroblasts in vitro. 2. The extent of the inhibition of DNA synthesis is markedly increased by preincubation of the cells with mycophenolic acid before the addition of [(14)C]thymidine. 3. The inhibition of DNA synthesis by mycophenolic acid in L cells in vitro is reversed by guanine in a non-competitive manner, but not by hypoxanthine, xanthine or adenine. 4. The reversal of inhibition by guanine can be suppressed by hypoxanthine, 6-mercaptopurine and adenine. 5. Mycophenolic acid does not inhibit the incorporation of [(14)C]thymidine into DNA in suspensions of Landschütz and Yoshida ascites cells in vitro. 6. Mycophenolic acid inhibits the conversion of [(14)C]hypoxanthine into cold-acid-soluble and -insoluble guanine nucleotides in Landschütz and Yoshida ascites cells and also in L cells in vitro. There is some increase in the radioactivity of the adenine fraction in the presence of the antibiotic. 7. Mycophenolic acid inhibits the conversion of [(14)C]hypoxanthine into xanthine and guanine fractions in a cell-free system from Landschütz cells capable of converting hypoxanthine into IMP, XMP and GMP. 8. Preparations of IMP dehydrogenase from Landschütz ascites cells, calf thymus and LS cells are strongly inhibited by mycophenolic acid. The inhibition showed mixed type kinetics with K(i) values of between 3.03x10(-8) and 4.5x10(-8)m. 9. Evidence was also obtained for a partial, possibly indirect, inhibition by mycophenolic acid of an early stage of biosynthesis of purine nucleotides as indicated by a decrease in the accumulation of formylglycine amide ribonucleotide induced by the antibiotic azaserine in suspensions of Landschütz and Yoshida ascites cells and L cells in vitro.     

New hydrazone derivatives of adriamycin and their immunoconjugates--a correlation between acid stability and cytotoxicity

New N-substituted hydrazine linkers were synthesized and their hydrazone derivatives of adriamycin were prepared. These functionalized adriamycin derivatives were conjugated with a monoclonal antibody, 5E9. The release rate of adriamycin from the hydrazones and from some of the conjugates was studied, and their relationship to the IC50's of the conjugate against 5E9-positive Daudi cells was investigated.     

Chelation of divalent cations by lomofungin: role in inhibition of nucleic acid synthesis

Lomofungin inhibition of yeast growth and RNA synthesis is prevented by Cu⁺⁺ or Zn⁺⁺ ions which chelate with the antibiotic and prevent its uptake by the cells. EDTA potentiates the inhibition. Mg⁺⁺ ions do not protect $\text{in vivo}$ or against the inhibition of purified bacterial RNA and DNA polymerases. Lomofungin prevents formation of the RNA polymerase. DNA initiation complex, probably by chelation with the firmly bound Zn⁺⁺ of the enzyme.     

Evidence for the lack of a specific interaction between cholesterol and sphingomyelin

The putative specific interaction and complex formation by sphingomyelin and cholesterol was investigated. Accordingly, low contents (1 mol % each) of fluorescently labeled derivatives of these lipids, namely 1-palmitoyl-2[10-(pyren-1-yl)]decanoyl-sn-glycero-3-phosphocholine (PyrPC), n-[10-(1-pyrenyl)decanoyl]sphingomyelin (PyrSM), and increasing concentrations of cholesterol (up to 5 mol %), were included in large unilamellar vesicles composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or 1,2-dinervonoyl-sn-glycero-3-phosphocholine (DNPC), and the excimer/monomer fluorescence emission ratio (I(e)/I(m)) was measured. In DNPC below the main phase transition, the addition of up to 5 mol % cholesterol reduced I(e)/I(m) significantly. Except for this, cholesterol had only a negligible effect in both matrices and for both probes. We then compared the efficiency of resonance energy transfer from PyrPC and PyrSM to 22-(n-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-23,24-bisnor-5-cholen-3beta-ol (NBDchol). An augmenting colocalization of the latter resonance energy transfer pair with temperature was observed in a DMPC matrix below the main phase transition. In contrast, compared to PyrSM the colocalization of PyrPC with NBDchol was more efficient in the longer DNPC matrix. These results could be confirmed using 5,6-dibromo-cholestan-3beta-ol as a collisional quencher for the pyrene-labeled lipids. The results indicate lack of a specific interaction between sphingomyelin and cholesterol, and further imply that hydrophobic mismatch between the lipid constituents could provide the driving force for the cosegregation of sphingomyelin and cholesterol in fluid phospholipid bilayers of thicknesses comparable to those found for biomembranes.     

Effect of cell density on the inhibition of tumor cell attachment and nucleic acid synthesis by selenite

The effect of cell density on the sensitivity of tumor celles to selenite has been examined. The inhibitory effect of selenite on cellular DNA and RNA synthesis was significantly greater in higher density cultures of HeLa cells and A2780 ovarian tumor cells. High-density cells were also more sensitive to the inhibitory effect of selenite on cell attachment. This difference could not be accounted for by a higher intracellular level of glutathione, since there was no significant difference between the cells at high or low density. The high-density cells were found to take up more selenium per cell during the exposure period; the resulting higher level of intracellular Se could explain their greater sensitivity to selenite. This hypothesis is supported by the observation that DNA synthesis in nuclei isolated from high-density cells did not exhibit higher sensitivity to inhibition by selenite than synthesis in nuclei isolated from low-density cells.     

Evidence for the lack of deoxyribonucleic acid dark-repair in Halobacterium cutirubrum.

1. Halobacterium cutirubrum does not perform dark-repair of DNA either after u.v. irradiation or during normal growth. 2. Cultures irradiated with u.v. are readily photoreactivated, but do not recover viability in the dark. 3. No increase in the rate of DNA synthesis is observed in the surviving cells after u.v. irradiation. 4. At early times during normal semiconservative replication, newly incorporated thymidine is found only in the hybrid DNA. 5. It is suggested that these bacteria may be useful in the study of DNA replication and photoreactivation.     

Evidence for sialidase activity in K 562 cells: inhibition by adriamycin treatment

Sialidase activity has been studied in the human erythroleukemia K 562 cell line grown in vitro. The total sialidase activity was determined using disialoganglioside GD1a and fetuin as exogenous substrates. The enzymatic activity was stimulated by 0.08% Triton X-100 and reached the highest level at pH 4.0. Results obtained showed that gangliosides are hydrolysed more extensively than glycoproteins by K 562 sialidases. This finding could suggest that endogenous gangliosides may be the main source of metabolically available sialic acid in K 562 cell line. After treatment of K 562 cells by Adriamycin (40 nM), a potent anticancer drug, sialidase activity decreased by 40% as compared to control cells. This decrease occurs early during the first day of incubation with Adriamycin. This inhibition of sialidase activity could explain previous results obtained in our laboratory which show an enhanced sialylation of the membrane glycoconjugates after Adriamycin treatment.     

The improbability of prebiotic nucleic acid synthesis

Many accounts of the origin of life assume that the spontaneous synthesis of a self-replicating nucleic acid could take place readily. Serious chemical obstacles exist, however, which make such an event extremely improbable.Prebiotic syntheses of adenine from HCN, of D, L-ribose from adenosine, and of adenosine from adenine and D-ribose have in fact been demonstrated. However these procedures use pure starting materials, afford poor yields, and are run under conditions which are not compatible with one another.Any nucleic acid components which are formed on the primitive earth would tend to hydrolyze by a number of pathways. Their polymerization would be inhibited by the presence of vast numbers of related substances which would react preferentially with them.It appears likely that nucleic acids were not formed by prebiotic routes, but are later products of evolution.     

The improbability of prebiotic nucleic acid synthesis

Many accounts of the origin of life assume that the spontaneous synthesis of a self-replicating nucleic acid could take place readily. Serious chemical obstacles exist, however, which make such an event extremely improbable. Prebiotic syntheses of adenine from HCN, of D,L-ribose from adenosine, and of adenosine from adenine and D-ribose have in fact been demonstrated. However these procedures use pure starting materials, afford poor yields, and are run under conditions which are not compatible with one another. Any nucleic acid components which were formed on the primitive earth would tend to hydrolyze by a number of pathways. Their polymerization would be inhibited by the presence of vast numbers of related substances which would react preferentially with them. It appears likely that nucleic acids were not formed by prebiotic routes, but are later products of evolution.     

The relationship between amyloid structure and cytotoxicity

Self-assembly of proteins and peptides into amyloid structures has been the subject of intense and focused research due to their association with neurodegenerative, age-related human diseases and transmissible prion diseases in humans and mammals. Of the disease associated amyloid assemblies, a diverse array of species, ranging from small oligomeric assembly intermediates to fibrillar structures, have been shown to have toxic potential. Equally, a range of species formed by the same disease associated amyloid sequences have been found to be relatively benign under comparable monomer equivalent concentrations and conditions. In recent years, an increasing number of functional amyloid systems have also been found. These developments show that not all amyloid structures are generically toxic to cells. Given these observations, it is important to understand why amyloid structures may encode such varied toxic potential despite sharing a common core molecular architecture. Here, we discuss possible links between different aspects of amyloidogenic structures and assembly mechanisms with their varied functional effects. We propose testable hypotheses for the relationship between amyloid structure and its toxic potential in the context of recent reports on amyloid sequence, structure, and toxicity relationships.