A Kinetic Study of the Inhibition of Protein Synthesis by Several Antibiotics

The effects of several antibiotics on the initial reaction rates of the polyphenylalanine and polylysine syntheses were measured in cell-free systems from Escherichia coli MRE600, stimulated by polyuridilic and polyadenylic acid, respectively. lVIichaelis-Menten ··type of dependence on the tRNA concentration, and a sigmoidal (possibly cooperative) dependence on the polynucleotide concentration was obtained. Puromycin and chloramphenicol were competitive inhibitors with respect to tRNA, while erythromycin, streptomycin and chlorotetracycline were non-competitive inhibitors. On the other hand, erythromycin was apparently a competitive inhibitor with respect to the synthetic polynucleotide, while other above-mentioned antibiotics showed either uncompetitive, non-competitive or mixed type of inhibition. This kinetic approach enabled the calculation of inhibition constants for the examined antibiotics; they ranged from 2.5 x 10-7 mol dm·3 for erythromycin and streptomycin, to over 1 · 10·4 mol dm-a for puromycin

Activation of Ribonuclease I by Gamma Irradiation of 30 S Ribosomes from Escherichia coli

A sensitive assay. of ribonuclease activity was designed using polyuridylic acid as a substrate. The amount of the undigested polyuridylic acid was subsequently determined in a cell-free system for the polymerization of phenylalanine from Escherichia coli. This assay was used for the characterization of a nuclease activated by the irradiation of purified ribosomes from the same bacterium. The enzyme was identified as ribonuclease I by (1) its location on the 30 S ribosomal subunit, (2) by the absence of mononucleotides in the products of short incubation, as determined by paper chromatography, and (3) by identification of oligonucleotides as products of such an incubation using chromatography on dextran gel. Activated enzyme sedimented with 30 S ribosomes and exhibited its activity even at the concentrations of Mg++ which are inhibitory for the »latent« ribosomal ribonuclease

The Possible Roles of Known Enzymes in the Breakdown of RNA in X-Irradiated Escherichia coli

Breafodown of 14C-adenine-labelled RNA of X-irradiated Escherichia coli proceeds at the same rate in the presence and in the absence of chloramphenicol, .thus indicating that pre-existing enzymes are responsible for the process . Though considerable amounts of RNA are broken down upon the incubation of bacteria in some salt solutions, the breakdown is e nhanced by irradiation only when the incubation medium contains both K+ and phosphate. Nucleoside diphosphates were found among the breakdown products of 14 C-adenine and 14 -uracil-labelled RNA, together with larger quantites of nucleoside monophosphates and some tdphosphates. The radiation-induced breakdown of RNA in a strain lacking r:ibonuclease I, E. coli MRE 600, proceeds at the same rate as in E. coli B. On the basis of these and some earlier findings it is proposed that ribonuclease I does not take part in degradation, while both ribonuclease II and polynucleotide phosphorylase are responsible for the process

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Metodičko konstruiranje kalupa za injekcijsko prešanje plastomera

U ovome radu je prikazano metodičko konstruiranje kalupa za injekcijsko prešanje zadanog otpreska. Provedeni su reološki, toplinski i mehanički proračuni kalupa, kao i proračun koštanja zadanog otpreska. Provedena je i simulacija punjenja kalupne šupljine

Erythromycin Series. X. Inhibitory Activity of Several New Erythromycin Derivatives in Cell-Free Amino Acid Polymerization Systems

Erythromycin A (I), erythromycin A 9-oxime (11), 9(S)-erythromycylamine (V), and several new derivatives of these compounds, were assayed for their ability to inhibit the poly(A)-directed synthesis of polylysine and the poly(C)-directed synthesis of polyproline in cell-free systems from Escherichia coli. The rate of polypeptide synthesis was inhibited 500/o by concentrations between 0.5 and 1.5 ~tmol · dm-3 of the eight examined compounds, in the following decreasing order of activity: methylsuccinate of V (VI), I, V, II, methylsuccinate of II (111), p-toluenesulfonyl-V (VII), p- acetylamino-benzenesulfonyl-V (VIII), and ethylsuccinate of I (IV). The derivative of VII lacking cladinose (IX) showed lower but still significant activity. Hence, none of the substitutions in the position 9 of the macrolide ring, present in these compounds, impairs the ability of I to bind the prokaryotic ribosome and inhibit its function, which is the basis for antibacterial activity of erythromycines

Feđa Milivojević, Cezarov Ilirik, Hrvatski institut za povijest – Filozofski fakultet u Rijeci, Zagreb – Rijeka, 2021., 350 str.

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Influence of Modified tRNATyr on the Activation of Tyrosine Catalyzed by Tyrosyl-tRNA Synthetase from Saccharomyces cerevisiae

Yeast tyrosyl-tRNA synthetase (TyrRS, EC 6.1.1.1) is a homodimeric enzyme capable of binding only one molecule of its macromolecular substrate, tRNATyr. The reactive intermediate tyrosyl adenylate is formed from tyrosine and ATP in the first reaction step, which can be conveniently assayed by pyrophosphate exchange. In order to determine the number of active sites per homodimer, the kinetics of pyrophosphate exchange was measured in the presence of the tRNATyr analogue unable to accept the amino acid. The analogue was found to form the expected equimolar complex with dimeric enzyme. It was a competitive inhibitor of pyrophosphate exchange with respect to ATP and non-competitive with respect to tyrosine. Inhibition cannot exceed 50%, suggesting the simplest model in which yeast TyrRS is a symmetrical dimer, possessing two identical active sites, both capable of catalyzing the formation of tyrosyl adenylate