Protein synthesis ribonuclease inhibition

Protein synthesis is inhibited by ribonuclease (cf. 376), whereas deoxyribonuclease generally, though not always, has little effect (379). 

Drugs/toxins Ricin (toxin) from castor oil beans (Ricinus communis) is a ribonuclease that inhibits protein synthesis by cleaving and thus inactivating 28S rRNA. A single molecule of ricin within a cell attacks all the ribosomes and kiUs the ceU Diphtheria toxin an exotoxin secreted by Corynebacterium diphtheriae that inactivates eukaryotic elongation factor 2. A single molecule can kill a ceU Aminoglycosides prevent the formation of the initiation complex Tetracyclines bind to the A site and block the attachment of aminoacyl-tRNA Chloramphenicol inhibits peptidyl transferase (ribozyme) Macrolides and clindamycin bind to the SOS subunit and block translocation... 

The ribonuclease activity was found in a sedimentable fraction known to contain the replication complex where synthesis of virus-coded MA takes place. Therefore, the antiviral action of interferon may be explained in this particular system by the ability of such a nuclease to degrade viral mRNA. Although the nuclease is not only active on mengo virus ENA its location within the cell would limit its action to the virus-coded single stranded ENA, the final consequence being some discrimination between viral and host cell messengers. This finding does not exclude the possibility that the inhibition of protein synthesis also occurs by other mechanisms such as impaired initiation of translation. 

From the scheme, we see immediately that the presence of sRNA is essential, since here we have a RNA molecule in actual chemical combination with an activated amino acid, presumably on its way to protein. Furthermore, as we have seen, the formation of the RNA-amino acid compound is extremely sensitive to ribonuclease. In principle, therefore, this step could account for the fact that protein synthesis does not take place in the absence of RNA, and that it is inhibited by ribonuclease. Nevertheless, all evidence to date strongly indicates that the cytoplasmic... 

A number of early observations pointed the way towards our current concepts of protein synthesis. Cells that are particularly active in protein synthesis are rich in cytoplasmic ribonucleic acid (RNA) and ribonucleoprotein (RNA-protein). Further, in a number of studies with intact cells it was found that their rate of protein synthesis varied directly as their cytoplasmic nucleic acid content and that their synthesis of protein was strongly inhibited by ribonuclease (which degrades RNA) and also by chloramphenicol. More recently, limited protein synthesis has been achieved in cell-free preparations obtained from bacterial and higher plant cells and reinforced by the addition of appropriate mixtures of the 20 protein amino acids and a suitable energy source such as ATP. Such in-vitro protein synthesis, most readily followed by studying the incorporation of C-labelled amino acids into protein, is prevented by adding ribonuclease or chloramphenicol. 

The incorporation of labeled amino acids by isolated nuclei from calf th3onus has been studied in detail by Allfrey and co-workers 1 4), and also by Breitman and Webster 66), and by Hopkins 1 4)- Nuclei from the thymus of puppies 67), from rabbit appendix 68), from chicken kidneys l 4)t from rat liver 69), from a lymphoma 64), and from wheat germ 70) have also been studied in this respect. The incorporation of labeled amino acids into the nuclear proteins is dependent on the presence of Na+ (the microsomal system requires K+) and upon oxidative phosphorylation. No effect of added amino acids or ATP have been demonstrated, but when the nuclei lose the ability to synthesize ATP from AMP, they also lose the ability to incorporate amino acids 71)-, however, they cannot utilize AMP supplied by the incubation medium, which means that they have to be prepared so as to retain their internal nucleotide pool. Amino acid incorporation also seems to be dependent upon a preliminary synthesis of RNA, since it is completely abolished by benzimidazole derivatives which inhibit the s3mthesis of the latter. However, intact RNA does not seem to be needed, and treatment with ribonuclease actually enhances amino acid incorporation into protein 7 ). 

Progress has been made in demonstrating synthesis of specific proteins by cell-free systems, e.g., on the synthesis of cytochrome c by isolated rat liver mitochondria (S3, cf. 383) and on the synthesis of serum albumin by the isolated microsome fraction of rat liver 34, cf. 335,336). Campbell et al. 34) concluded that while specific serum albumin is, indeed, synthesized on the ribonucleoprotein particle fraction of the microsomes, it is not readily released in soluble form. In other words, the isolated microsomes have lost their ability to promote substrate turnover. The same is true for the hemoglobin-synthesizing RNP particles from rabbit reticulocytes 35, cf. 138). Ogata and associates 36) have essentially confirmed the results of Campbell et al. 34) on the synthesis of serum albumin by liver microsomes they have also studied the relative effect of both stimulatory and inhibitory factors on the incorporation of different amino acids into total ribonucleoprotein and into serum albumin, and showed that the requirements for the two processes were generally the same it may be noted, however, that pretreatment of the pH 5 enzymes with ribonuclease, which caused a 95% inhibition of the ineorporation into ribonucleoprotein, inhibited the corresponding incorporation into serum albumin by only 55%. 

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