Inhibition of nucleic acid and protein

Dacarbazine is activated by photodecomposition (chemical breakdown caused by radiant energy) and by enzymatic N-demethylation. Formation of a methyl carbonium ion results in methylation of DNA and RNA and inhibition of nucleic acid and protein synthesis. Cells in all phases of the cell cycle are susceptible to dacarbazine. The drug is not appreciably protein bound, and it does not enter the central nervous system. 

Inhibition of nucleic acid and protein synthesis and carcinogen-induced neoplastic transformation... 

This reaction has been widely used in heterocyclic chemistry for preparing certain types of nitrogeneous heterocycles, such as nitrobenzofuroxans, which have been used for the inhibition of nucleic acid and protein biosyntheses. Some interesting rearrangements are listed in experimental examples. 

The varied modes of antibiotic activity are presented in two reports whiph discuss the interaction of antibiotics with nucleic acld. and the inhibition of nucleic acid and protein synthesis. ... 

Vinblastine and vincristine are alkaloids isolated from plants of the periwinkle family (Vinca rosea). These compounds cause cells to stop at metaphase and inhibit assembly of microtubules, and likewise, failure of mitotic spindle formations. They inhibit synthesis of nucleic acids and proteins. 

Sulfonamides were the first group of chemotherapeutic agents used for the prevention or treatment of bacterial infections in humans. Sulfonamides (e.g., sulfisoxazole) act by inhibiting bacterial synthesis of folic acid, a chemical required for synthesis of nucleic acid and protein. These drugs competitively inhibit the first step in the synthesis of folic acid—the conversion of para-aminobenzoic acid into dihydrofolic acid. Because humans absorb preformed folic acid from food, sulfonamide inhibition has only a minimal effect on hiunan cells. 

The inhibition of biosyntheses of nucleic acids and proteins in rat spinal cord cell cultures were also studied to ascertain the mechanism of action of sea cucumber glycosides [137]. Addition of holotoxin Ai (33) to incubation media (2.5-5 p,g/ml) inhibited " C-alanine incorporation into the acid-insoluble cell fraction more... 

The antimetabolites discussed so far, are all compounds of relatively small molecular weight. In the majority of cases, they act by competitively inhibiting the metabolic transformations of the analogous normal metabolites which usually are intermediates (precursors) or cofactors in the biosyntheses of nucleic acids and proteins. In some other cases, the inhibitory action of the antimetabolites is a consequence of their incorporation into the macromolecules (see Section 2.3. B) however, also this type of action depends on their... 

Fast blot methods to minimize nucleic acid extraction and immobilization steps have been developed. Those with nylon as a solid phase can take advantage of the ability of NaOH to dissociate cells, denature DNA and immobilize DNA. Nitrocellulose membranes have a lower binding capacity and co-immobilization of nucleic acid and protein from neutral solutions can be a problem. Bresser et al. (1983) used hot concentrated Nal to inhibit protein immobilization, to denature DNA and to irreversibly bind the nucleic acid to nitrocellulose (no baking required). This method can also be used for RNA. About 10 cells are minimally required for a unique DNA sequence, whereas > 0.01% of total mRNA can be detected by the Nal methods. 

Sulfamethoxazole inhibits bacterial synthesis of dihydrofohc acid, and trimethoprim blocks the production of tetrahydrofolic acid by inhibiting the enzyme dihydrofolate reductase. Thus two consecutive steps ate blocked in the biosynthesis of nucleic acids and proteins essential to many bacteria. In vitro serial dilution tests have shown that the combination of sulfamethoxazole and trimethoprim [738-70-5] inhibits the growth of common urinary tract pathogens with the exception of Pseudomonas aeruginosa. Table 3 illustrates the enhanced effect of the combination over that of either agent alone. 

Early workers [29] found that, like benzylpenicillin, vancomycin, ristocetin and bacitracin, novobiocin caused an excessive accumulation of cell wall precursor, uridine diphosphate-7V-acetylmuramic acid-L-alanine-D-glutamic acid-L-lysine-D-alanine-D-alanine (UDP-MurNAc-L-ala-D-glu-L-lys-D-ala-D-ala) in Staph, aureus and it was thus considered that novobiocin was a specific inhibitor of peptidoglycan synthesis with an effect similar to that of penicillin. However, subsequent studies led to the withdrawal of this hypothesis [26], since novobiocin caused the accumulation of other precursor-type compounds and also strongly inhibited both nucleic acid and protein synthesis in this organism. Thus, accumulation of particular precursors does not necessarily reflect the site of action of an antibacterial agent [30]. 

Studies of the effects of cortisol on nucleic acids and protein of thymocytes in vivo and in vitro have shown interference with penetration of nucleic acids and protein precursors inhibition of RNA, DNA, and protein synthesis decreases in RNA polymerase activity and reduction in ribosomal RNA synthesis. Interestingly enough, these inhibiting effects of cortisol depend upon the presence of an energy source in the medium. 

The liver and kidneys are subjected to severe damage from tannin ingestion and injection (Price and Butler, 1980 Vohra et al., 1966). At cellular and biochemical levels, injection of tannic acid can cause liver fibrosis and necrosis, polyribosome disaggregation, inhibition of microsomal enzymes, and synthesis of nucleic acid and protein (Badawy et al., 1969 Reddy et al., 1970 Oler et al., 1976). 

The third criterion is that suppression or inhibition of any phenomena in the cause-effect sequence of the model should produce suppression or inhibition of all the following phenomena of this sequence. In the case of ecdysone. inhibitors of nucleic acids and protein synthesis, such as actinomycin, mitomycin. F-deoxycytidine. puromycin, streptomycin, erythromycin, etc., inhibit in parallel the enhancement of DOPA decarbox ase activity, sclerotization and pupation. These results support the model, but they do not prove it Indeed, they suggest that intact nucleic acid and protein synthesis are required for the expression of the hormonal effect but they do not imply that the hormone acts through stimulation of the synthesis of some proteins and/or nucleic adds. Furthermore, inhibitors of nucleic acids and protein synthesis may have multiple sites of action, so that inhibition of a hormonal effect may be non-specific. 

Competitive Inhibition of enzymes. In the case of competitive inhibition, the structural analogs compete with the natural substrates for the active site of enzymes. The enzymes of nucleic acid, and protein, synthesis can also be inhibited in this way. Thus, the enzyme thymidilate synthetase is competitively inhibited by 5-fiuorodeoxyuridine, a derivative of 5-fluorouracil (Fig. 14). Thymidilate synthetase supplies d-thymidine-5 -phosphate (-thymidilate), a substance which is important to the synthesis of DNA. Inhibition of this enzyme thus brings DNA synthesis to a stop and, with it, all further development. We will discuss DNA synthesis and also thymidilate synthetase in more detail later (p. 165). 

The dicarboximides inhibit spore germination and cause increased branching, swelling and lysis of germ tubes and hyphal tips. Effects on cell division have been reported but no major inhibition of nucleic acid metabolism, respiration, protein or lipid synthesis has been observed. 

Mycelial growth is more sensitive to cymoxanil than early growth phases, including the release of zoospores from sporangia and their germination. Cymoxanil inhibits nucleic acid and protein synthesis in some fungi but may have to be activated to induce a fungicidal response. 

Low activities of orotidine phosphate decarboxylase and (usually) orotate phosphoribosyltransferase are associated with a genetic disease in children that is characterized by abnormal growth, megaloblastic anemia, and the excretion of large amounts of orotate. When affected children are fed a pyrimidine nucleoside, usually uridine, the anemia decreases and the excretion of orotate diminishes. A likely explanation for the improvement is that the ingested uridine is phosphorylated to UMP, which is then converted to other pyrimidine nucleotides so that nucleic acid and protein synthesis can resume. In addition, the increased intracellular concentrations of pyrimidine nucleotides inhibit carbamoyl phosphate synthase, the first enzyme in the. naibwav of aro-tate synthesis. 

Mechanism of Action. Pyrimethamine blocks the production of folic acid in susceptible protozoa by inhibiting the function of the dihydrofolate reductase enzyme. Folic acid helps catalyze the production of nucleic and amino acids in these parasites. Therefore, this drug ultimately impairs nucleic acid and protein synthesis by interfering with folic acid production. The action of sulfadoxine and other sulfonamide antibacterial agents was discussed in Chapter 33. These agents also inhibit folic acid synthesis in certain bacterial and protozoal cells. 

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