Hairpin ribozyme cleavage catalyzed by aminoglycoside antibiotics and the polyamine spermine in the absence of metal ions.

The hairpin ribozyme is a small catalytic RNA that achieves an active configuration by docking of its two helical domains in an antiparallel fashion. Both docking and subsequent cleavage are dependent on the presence of divalent metal ions, such as magnesium, but there is no evidence to date for direct participation of such ions in the chemical cleavage step. We show that aminoglycoside antibiotics inhibit cleavage of the hairpin ribozyme in the presence of metal ions with the most effective being 5-epi-sisomicin and neomycin B. In contrast, in the absence of metal ions, a number of aminoglycoside antibiotics at 10 mM concentration promote hairpin cleavage with rates only 13-20-fold lower than the magnesium-dependent reaction. We show that neomycin B competes with metal ions by ion replacement with the postively charged amino groups of the antibiotic. In addition, we show that the polyamine spermine at 10 mM promotes efficient hairpin cleavage with rates similar to the magnesium-dependent reaction. Low concentrations of either spermine or the shorter polyamine spermidine synergize with 5 mM magnesium ions to boost cleavage rates considerably. In contrast, at 500 microM magnesium ions, 4 mM spermine, but not spermidine, boosts the cleavage rate. The results have significance both in understanding the role of ions in hairpin ribozyme cleavage and in potential therapeutic applications in mammalian cells.     

Inhibition of ribosomal translocation by aminoglycoside antibiotics.

The translocation of AcPhe-tRNA in a purified system and that of peptidyl-tRNA in a crude, complete polypeptide synthesizing system containing endogenous $\text{E. coli}$ polysomes are inhibited by antibiotics of the neomycin, kanamycin and gentamicin groups. The extent of inhibition varies with the different antibiotics, but it correlates well with the capacity of each antibiotic to inhibit polypeptide chain elongation. Thus, the inhibition of translocation by these antibiotics is clearly significant for their inhibitory effect on polypeptide synthesis.     

tRNA(Phe) binds aminoglycoside antibiotics.

Aminoglycoside antibiotics have recently been found to bind to a variety of unrelated RNA molecules, including sequences that are important for retroviral replication. We report the binding of neomycin B, kanamycin A, and Neo-Neo (a synthetic neomycin-neomycin dimer) to tRNA(Phe). Using thermal denaturation studies, fluorescence spectroscopy, Pb2+-mediated tRNA(Phe) cleavage, and gel mobility shift assays, we have established that aminoglycosides interact with yeast tRNA(Phe) and are likely to induce a conformational change. Thermal denaturation studies revealed that aminoglycosides have a substantial stabilizing effect on tRNA(Phe) secondary and tertiary structures, much greater than the stabilization effect of spermine, an unstructured polyamine. Aminoglycoside-induced inhibition of Pb2+-mediated tRNA(Phe) cleavage yielded IC50 values of: 5 microM for Neo-Neo, 100 microM for neomycin B, > 1 mM for kanamycin A, and > 10 mM for spermine. Enzymatic and chemical footprinting indicate that the anticodon stem as well as the junction of the TpsiC and D loops are preferred aminoglycoside binding sites.     

Stability of enzymes inactivating aminoglycoside antibiotics

Stability of aminoglycoside phosphotransferases, adenylyltransferases and acetyltransferases isolated from various sources was studied. The enzymes were characterized by different substrate profiles. They were stored at a temperature of -10 degrees C in the form of frozen solutions or at a temperature of 4 degrees C in the lyophilized form. It was shown that lyophilization markedly increased the stability of the enzymes inactivating aminoglycoside antibiotics. Aminoglycoside phosphotransferases and adenylyltransferases with streptomycin as substrate were less stable than aminoglycoside phosphotransferases with neomycin as substrate. Aminoglycoside acetyltransferases from Streptomyces fradiae 918 producing neomycin were least stable among the enzymes studied. Lyophilized enzymes as a possible stabilizer of ATP added to the preparations had no significant effect on their stability during storage.     

Study of the immunotropic activity of aminoglycoside antibiotics

The action of some aminoglycoside antibiotics on the immune system was studied on both intact mice and the animals with immune deficiency caused by administration of cyclophosphamide. The following tests were used: local hemolysis (the Herne test), lymphocyte transformation (LT), delayed hypersensitivity to sheep red blood cells and the local graft versus host reaction (GVHR). Amikacin was shown to have no significant action on the activity of lymphocytes in the intact mice and stimulated both cellular (LT and GVHR) and humoral (the Herne test) immunity in the animals with lowered immunological reactivity. Sisomicin had no significant action on the immune system of the animals. Gentamicin suppressed the immune response only in the intact mice. Kanamycin and streptomycin induced inhibition of humoral and cellular immunity in both the intact mice and animals with immune deficiency. On the basis of the results it was concluded that gentamicin, amikacin and sisomicin may be used in the treatment of diseases developing in the presence of immune deficiency whereas streptomycin and kanamycin should be recommended when inhibition of the immunity is needed.     

Basis for prokaryotic specificity of action of aminoglycoside antibiotics.

The aminoglycosides, a group of structurally related antibiotics, bind to rRNA in the small subunit of the prokaryotic ribosome. Most aminoglycosides are inactive or weakly active against eukaryotic ribosomes. A major difference in the binding site for these antibiotics between prokaryotic and eukaryotic ribosomes is the identity of the nucleotide at position 1408 (Escherichia coli numbering), which is an adenosine in prokaryotic ribosomes and a guanosine in eukaryotic ribosomes. Expression in E.coli of plasmid-encoded 16S rRNA containing an A1408 to G substitution confers resistance to a subclass of the aminoglycoside antibiotics that contain a 6' amino group on ring I. Chemical footprinting experiments indicate that resistance arises from the lower affinity of the drug for the eukaryotic rRNA sequence. The 1408G ribosomes are resistant to the same subclass of aminoglycosides as previously observed both for eukaryotic ribosomes and bacterial ribosomes containing a methylation at the N1 position of A1408. The results indicate that the identity of the nucleotide at position 1408 is a major determinant of specificity of aminoglycoside action, and agree with prior structural studies of aminoglycoside-rRNA complexes.     

Characterization of Saccharomyces cerevisiae mutants supersensitive to aminoglycoside antibiotics.

We describe mutants of Saccharomyces cerevisiae that are more sensitive than the wild type to the aminoglycoside antibiotics G418, hygromycin B, destomycin A, and gentamicin X2. In addition, the mutants are sensitive to apramycin, kanamycin B, lividomycin A, neamine, neomycin, paromomycin, and tobramycin--antibiotics which do not inhibit wild-type strains. Mapping studies suggest that supersensitivity is caused by mutations in at least three genes, denoted AGS1, AGS2, and AGS3 (for aminoglycoside antibiotic sensitivity). Mutations in all three genes are required for highest antibiotic sensitivity; ags1 ags2 double mutants have intermediate antibiotic sensitivity. AGS1 was mapped 8 centimorgans distal from LEU2 on chromosome III. Analyses of yeast strains transformed with vectors carrying antibiotic resistance genes revealed that G418, gentamicin X2, kanamycin B, lividomycin A, neamine, and paromomycin are inactivated by the Tn903 phosphotransferase and that destomycin A is inactivated by the hygromycin B phosphotransferase. ags strains are improved host strains for vectors carrying the phosphotransferase genes because a wide spectrum of aminoglycoside antibiotics can be used to select for plasmid maintenance.     

Effect of aminoglycoside antibiotics on the autolytic enzyme of Streptomyces griseus

The isolated cell wall of Streptomyces griseus 52–1 strain labelled with fluorescein isothiocyanate (FITC) and containing wall-bound autolytic enzyme was lysed as a function of different cations. The autolysis was accelerated by aminoglycoside antibiotics (streptomycin and the structurally closely related neomycin) which have a polycationic character. Since this strain is a streptomycin producer it is suggested that streptomycin may have a regulatory function on autolysis.     

A molecular basis for human hypersensitivity to aminoglycoside antibiotics.

We have investigated the distribution of mitochondrial DNA polymorphisms in a rare maternally transmitted genetic trait that causes hypersensitivity to aminoglycoside antibiotics, in the hope that a characterization of its molecular basis might provide a molecular and cellular understanding of aminoglycoside-induced deafness (AGD). Here we report that the frequency of a particular mitochondrial DNA polymorphism, 1555G, is associated nonrandomly with aminoglycoside-induced deafness in two Japanese pedigrees, bringing the frequency of this polymorphism to 5 occurrences in 5 pedigrees of AGD, and in 4 of 78 sporadic cases in which deafness was thought to be the result of aminoglycoside exposure; both frequencies are significantly different from the occurrence of this mutation in the hearing population, which was 0 in 414 individuals surveyed. The 1555G polymorphism occurred in none of 34 aminoglycoside-resistant individuals. We propose a specific molecular mechanism for aminoglycoside hypersensitivity in individuals carrying the 1555G polymorphism, based on the three-dimensional structure of the ribosome, in which the 1555G polymorphism favors aminoglycoside binding sterically, by increasing access to the the ribosome cleft.     

Inhibition of polyphosphoinositide phosphodiesterase by aminoglycoside antibiotics

The calcium-activated phosphodiesteratic hydrolysis of³²P-labeled phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-phosphate in prelabeled nerve ending membranes is inhibited by the aminoglycosides neomycin and gentamicin, and to a lesser extent, by streptomycin. The inhibition is overcome by increasing concentrations of Ca²⁺, indicating that the aminoglycosides exert their effect by displacing Ca²⁺ from lipid.Dedicated to Professor Yasuzo Tsukada.     

Modulation of RNA function by aminoglycoside antibiotics

One of the most important families of antibiotics are the aminoglycosides, including drugs such as neomycin B, paromomycin, gentamicin and streptomycin. With the discovery of the catalytic potential of RNA, these antibiotics became very popular due to their RNA-binding capacity. They serve for the analysis of RNA function as well as for the study of RNA as a potential therapeutic target. Improvements in RNA structure determination recently provided first insights into the decoding site of the ribosome at high resolution and how aminoglycosides might induce misreading of the genetic code. In addition to inhibiting prokaryotic translation, aminoglycosides inhibit several catalytic RNAs such as self-splicing group I introns, RNase P and small ribozymes in vitro. Furthermore, these antibiotics interfere with human immunodeficiency virus (HIV) replication by disrupting essential RNA-protein contacts. Most exciting is the potential of many RNA-binding antibiotics to stimulate RNA activities, conceiving small-molecule partners for the hypothesis of an ancient RNA world. SELEX (systematic evolution of ligands by exponential enrichment) has been used in this evolutionary game leading to small synthetic RNAs, whose NMR structures gave valuable information on how aminoglycosides interact with RNA, which could possibly be used in applied science.     

Inhibition of superoxide-dependent processes by aminoglycoside antibiotics

The ability of various antibiotics to inhibit superoxide anion(O-2)-mediated formation of adrenochrome from adrenaline and recovery of cytochrome c by xanthine oxidase was studied. In the adrenaline system (pH 10.2), aminoglycosides might be arranged, according to the inhibitory effect, in the following order: monomycin greater than gentamicin greater than kanamycin greater than lincomycin greater than streptomycin. In the xanthine oxidase system (pH 7.8), that order was the following: monomycin greater than gentamicin greater than lincomycin greater than greater than kanamycin. It was suggested that the antibiotic inhibition of the O-2-dependent processes at the essential sites of metabolism and/or the antibiotic involvement into the process of free radical oxidation initiated by O-2 in the cells might be one of the mechanisms of the drug action and toxicity with respect to the host.     

The electrochemical interactions of heparin and the aminoglycoside antibiotics

Various aminoglycoside antibiotics—tobramycin "analytical standard"; tobramycin, gentamycin, kanamycin, and amikacin, all containing preservatives added in pharmaceutical preparations; and streptomycin—without preservatives—yield in aqueous solutions a conductance peak when titrated against a heparin aqueous solution, indicating the formation of a charge transfer complex, which subsequently dissociates. At the volume ratio corresponding to the peak, a colloidal suspensoid forms which is possibly a micellar complex stabilized by the ions produced by the dissociation of the complex. The clinical significance of this interaction is suggested by preliminary microbiological tests. The possible effects of plasma protein binding and dissociation of the complexes here reported in tissues, however, were not studied.In the interaction with heparin, the aminoglycoside antibiotics appear to act as electron acceptors, though heparin is reported to act as an acceptor against chlorpromazine, which is well known to be a strong electron donor. There appears to be an interaction between nonpreserved tobramycin and the preservative added for the pharmaceutical preparation, which would explain the difference in titration behavior between the antibiotic alone and the antibiotic with preservative.Conductance titrations of heparin against penicillin and cloxacillin, which are not aminoglycosides, show no evidence of complexation or any other interaction. No suspensoid forms.     

The effect of aminoglycoside antibiotics on the thermodynamic properties of liposomal vesicles

Liposomes are ideal drug-delivery systems because they can alter the pharmacokinetic characteristics and biodistribution profile of the incorporated bioactive molecule. The effect of the aminoglycoside antibiotics, gentamicin (GN), tobramycin (TOB), and amikacin (AMI), on the thermodynamic properties of multilamellar vesicles composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was studied by using differential scanning calorimetry (DSC), electron paramagnetic resonance (EPR), and ³¹P nuclear magnetic resonance (NMR) spectroscopy. The relationship between the structure of aminoglycoside antibiotics and their effect on the physical properties of the liposomal bilayers was investigated. The incorporation of the drugs was achieved and an osmotic gradient created by controlling the mole ratio of the drug inside to that outside of the DPPC vesicles so that [drug_{inside DPPC}]/[drug_{outside DPPC}] was 1:0, 1:0.2, 1:1, or 1:2.5. Incorporation of the drugs into liposomes caused the T_m to shift to a higher temperature and the δH_m and δT_1/2 values to decrease. The 2A_max and the order parameter (S), obtained from the EPR spectra, indicated that the fluidity of the liposomal membrane was affected by the type of drug and by the concentration used; GN and TOB decreased the fluidity and disturbed chain packing at mole ratios of [drug_{inside DPPC}]/[drug_{outside DPPC}] ranging from 1:0 to 1:0.2, while AMI increased the fluidity and disrupted chain packing at an osmotic gradient of 1:2.5. In conclusion, the molecular organization and thermotropic properties of the multilamellar DPPC vesicles were dependent on the osmotic gradient and structure of the aminoglycoside.