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Biomedical subjects

L Carrasco

Publications and source records attributed to L Carrasco.

At least 289 records · Page 16Linked to original sources
PubMed1976

Specific inhibition of translocation by tubulosine in eukaryotic polysomes.

The alkaloid tubulosine inhibits the process of peptide chain elongation by eukaryotic polysomes by specifically preventing the elongation-factor-2-dependent step of translocation. Tubulosine does not affect either the elongation-factor-1-dependent binding of aminoacyl-tRNA or peptide bond formation. The site of action of tubulosine appears to be independent from the ribosomal site involved in cycloheximide action, since the alkaloid is active in blocking polymerization and enzymic translocation by polysomes from a yeast mutant resistant to cycloheximide. Furthermore tubulosine does not affect the non-enzymic translocation which takes place in the presence of high potassium ion concentrations, whereas this reaction is strongly inhibited by cyclohemimede. The different steps of translocation are discussed to explain the reactions blocked by tubulosine and cycloheximide.

Alkaloids↗
PubMed1976

Antibiotics and compounds affecting tanslation by eukaryotic ribosomes. Specific enhancement of aminoacyl-tRNA binding by methylaxnthines.

The mode and site of action of inhibitors of translation (initiation, elongation and termination of protein synthesis) in eukaryotic systems is reviewed. The isolation and characterization of a factor is described that binds Ac-Phe-tRNA to form a complex made up of binding factors, Ac-Phe-tRNA, and ribosome. The binding of Ac-Phe-tRNA probably occurs at the ribosomal site involved in the binding of the initiator substrate Met-tRNAF. The effect of inhibitors of the intitiation phase of protein synthesis on the nonenzymic Ac-Phe-tRNA binding to ribosomes is investigated. The two sites translocation model for translation in eukaryotic cells is presented and the effects of inhibitors on the various steps of protein synthesis are determined empirically. The site of action of inhibitors of peptide bond formation at the ribosomal peptidyl transferase center is elucidated. The action of inhibitors of translocation is sutdied in model cell-free systems from human cells. In addition, a number of methylxanthines are shown to enhance the elongation phase in polypeptide synthesis by stimulating the enzymic binding of aminoacyl-tRNA. The effect of caffeine, theophylline and its derivatives are shown to be fairly specific and dependent on the ribosome concentration. Aminophylline is shown to have a similar effect but also enhances aminoacyl-tRNA synthetase activity at low Mg++ concentrations, probably displacing the optimal concentration of Mg++ in the reaction. This second effect of aminophylline appears to be due to the ethylenediamine moiety of aminophylline since it is also observed in the presence of different polyamines but not in the presence of caffeine or theophylline.

Animals↗
PubMed1976

Binding of aminoacyl-tRNA to rat liver ribosomal proteins.

Rat liver ribosome treatment with ethanol and 1 M NH4Cl releases some 31-33 ribosomal proteins. This split protein fraction binds Phe-tRNA, Ac-Phe-tRNA, Met-tRNAM and f-Met-tRNAF in the absence of K+ and Mg++ ions. When the split protein fraction is passed through Sephadex G-100 only six proteins are retained in the column: S10, S14, S15, S19, L35, and L36. The aminoacyl-tRNA binding activity of this protein fraction retained in the Sephadex G-100 column is similar to that of the total split protein fraction, suggesting that the above six proteins, or only some of them, are involved in the binding reaction.

Animals↗
PubMed1975

Ribosome inactivation by the toxic lectins abrin and ricin. Kinetics of the enzymic activity of the toxin A-chains.

A sensitive test system for toxin-treated ribosomes was worked out by treating rabbit reticulocyte ribosomes with abrin A-chain, ricin A-chain or ricinus agglutinin A-chain, adding neutralizing amounts of specific antitoxins and testing for polyphenylalanine-synthesizing activity in a system where the concentration of elongation factors and ribosomes were varied. The strongest inhibition was obtained in the presence of low concentrations of elongation factor (EF-2). The activity of the ribosomes decreased with time of incubation with the toxin A-chains. Addition of anti-toxins stopped further inactivation. In systems containing untreated and toxin-treated ribosomes the ability to polymerize phenylalanine was proportional to the concentration of untreated ribosomes. There was a linear relationship between toxin A-chain concentration and the number of ribosomes inactivated per minute. The inactivation rate increased with temperature, and the estimated activation energy was 10.6 kcal (44.3 kJ). Linewaver-Burk plots of the data obtained by incubating various ribosome concentrations with toxins indicated a molecular activity of about 1500 ribosomes/minute for abrin and ricin A-chains and 100 ribosomes/minute for ricinus agglutinin A-chain. The apparent Michaelis constant was 0.1-0.2 muM for all three A-chains. The activity of the A-chains in the intact cell is discussed.

Abrin↗
PubMed1975

The involvement of sulphydryl groups in the peptidyl transferase centre of eukaryotic ribosomes.

Treatment of mammalian ribosomes with N-ethylmaleimide enhances up to 100% the ribosome efficiency in the "fragment reaction assay" for peptide bond formation by increasing the affinity of the substrate C-A-C-C-A-Leu-Ac for the donor site. This stimulation in peptidyl transferase activity was not observed when yeast ribosomes were treated in a similar manner. Stimulation of the peptidyl transferase activity of mammalian ribosomes was also observed by treatment with either p-chloromercuribenzoic acid or 5,5'-dithiobis-(2-nitrobenzoic acid) or 5,5'-dithiobis-(2-nitropyridine) or the maleimide-derived antibiotic showdomycin. N-Ethylmaleimide treatment also enhances C-A-C-C-A-Leu binding to the acceptor site of the peptidyl transferase centre. However, neither binding of N-Ac-Phe-tRNA in the presence of ethanol, nor binding of Phe-tRNA to the ribosomes is stimulated by N-ethylmaleimide. The antibiotic tenuazionic acid (a selective inhibitor of peptide bond formation by mammalian ribosomes) appears to require for its inhibitory effect the ribosome sulphydryl residues, since its inhibitory action on the fragment reaction is greatly decreased in ribosomes treated with N-ethylmaleimide.

Acyltransferases↗
PubMed1975

Effects of ricin on the ribosomal sites involved in the interaction of the elongation factors.

The effects of ricin on the different steps of the elongation cycle of protein synthesis in a rabbit reticulocyte cell-free system are studied in this paper. The toxin most probably acts by catalytically inactivating the ribosomes, since a single molecule of the toxin can inactivate 300 ribosomes for poly(U)-directed phenylalanine incorporation. The effect of the toxin on the ribosome is irreversible. Ricin specifically inhibits elongation-factor-1-dependent aminoacyl-tRNA binding to ribosomes but has no effect on the non-enzymic binding of aminoacyl-tRNA. Ricin also inhibits formation of the complex elongation-factor-2 - ribosome - nucleotide with GTP, GDP or GMP-P(CH2)P. However, the toxin has no effect on translocation. These apparently conflicting results are discussed in this study.

Animals↗
PubMedDate not supplied

Endogenous lipid pneumonia in a dog.

Endogenous lipid pneumonia has previously been diagnosed in dogs only once. This report describes a case in a dog with a persistent cough, in which the histological diagnosis was based on the presence of numerous foamy macrophages that filled the alveoli and contained small sudanophilic vacuoles. The appearance of endogenous lipid pneumonia in this animal was accompanied by Dirofilaria immitis infection and chronic bronchitis.

Animals↗
PubMedDate not supplied

Comparison of diagnostic techniques for porcine proliferative enteropathy (Lawsonia intracellularis infection).

In studying the post-mortem diagnosis of porcine proliferative enteropathy (PPE) a "double-blind" study was performed on 77 apparently healthy "finisher" pigs at the time of slaughter, to compare the results of a polymerase chain reaction (PCR) technique with those of (1) an indirect immunofluorescence assay (IFA), (2) examination for gross proliferative lesions at slaughter, (3) histopathological study of sections stained with haematoxylin and eosin (HE), and (4) Warthin-Starry (WS) silver staining for intracellular bacteria. The IFA, with a sensitivity of 89% and a specificity of 97% (positive "likelihood ratio"=30) and an agreement of 82% with PCR, is suggested as an alternative to the PCR in the post-mortem diagnosis of PPE. Histopathological examination was shown to be of little use as a principal diagnostic method, although it appeared to be effective in diagnosing infection by Lawsonia intracellularis in cases with proliferative-type lesions (positive likelihood ratio=29.6). Finally, the values obtained from an examination of gross lesions (k=0.075; positive likelihood ratio of 1.3) and WS staining (k=0.42; positive likelihood ratio of 5.3) demonstrated the lack of validity of these tests for the diagnosis of L. intracellularis infection.

Animals↗