Ribonucleic acids degradation

Studies on microbial RNases began in 1924 when Noguchi found ribonucleic acid degrading enzymes in Takadiastase (134). Since then extensive studies have been carried out on RNA degrading enzymes. It is rather surprising that guanyloribonuclease so widely distributed in microorganisms was found only in 1957. This is because earlier studies did not consider base specificity. Even quite recently studies on nucleases or ribonucleases do not consider base specificity or do not separate nuclease mixtures from each other thus, information available on microbial RNases is still scant. 

Although the 2, 3 -cyclic phosphates of adenosine, cytidine, guano-sine, and uridine appear to be stable in aqueous solution between, approximately, pH 4 and 9, they are hydrolyzed under more-strenuous conditions (for example, 0.1M hydrochloric acid for 1 hour at room temperature or 0.5 M sodium hydroxide for 12 hours at 37°) to give a mixture of 2 - and 3 -phosphates287 in approximately equal amounts. An appreciation of this aspect of the chemistry of cyclic phosphates was of considerable importance in elucidating the mechanism of the chemical degradation of ribonucleic acid.288... 

The discovery of a small proportion of a nucleoside containing thymine42 in the ribonucleic acid of two strains of Escherichia coli, in Aerobacter aero-genes, and in commercial, yeast-ribonucleic acid emphasizes the point made previously,26-28 namely, that the nucleic acids may contain constituents other than those heretofore identified. Alkaline hydrolysis of the ribonucleic acid from E. coli gave nucleotides42 (probably the 2- and 3-phosphate esters) which were converted to the nucleoside with prostatic phospho-monoesterase.62 Enzymic hydrolysis of the nucleic acid preparation also led to the nucleoside, which was degraded further to thymine by hydrolysis with perchloric acid.42 There can be little doubt that this carbohydrate derivative of thymine is intimately bound as part of the polynucleotide chain of this particular ribonucleic acid. 

Nucleosides are metabolites of ribonucleic acid (RNA). As a result of excessive turnover of RNA, which occurs in such pathological conditions as cancer, inflammation, and AIDS, nucleosides are excreted into urine in increased amounts. Normal nucleosides (adenosine, guanosine, cytidine, uridine) either undergo further degradation into uric acid, p-alanine, or p-aminoisobutyrate or are reutilized. On the other hand, modified nucleosides, especially methylated nucleosides, are excreted unchanged in urine. Therefore, it seems that elevated levels of modified nucleosides could be an indicator of the presence of cancer and used as a diagnostic tool in cancer prognosis [23]. 

Heme synthesis is controlled by a regulatory negative feedback loop in which heme inhibits the activity of fer-rochelatase and acquisition of iron fi om the transport protein transferrin. The decrease in iron acquisition leads to a decrease in iron uptake into the cell with subsequent decrease in 8-aminolevulinic acid and heme production. Iron deficiency and increased erythropoietin synthesis lead to the combination of the iron regulatory proteins with the iron-responsive elements in the transferrin receptor protein messenger ribonucleic acid (mRNA). This combination in turn leads to protection of the mRNA from degradation with subsequent increased uptake of iron into erythroid cells because of the increased expression of transferrin receptors on the cell membrane. 

Another process of degradation in muscle which appeal s also more rapid in fish than in warm-blooded animals have been described by Tarr (1952, 1953, 1954) and by Tarr and Bissett (1954). An important liberation of ribose occurs post-mortem in fish from ribonucleic acid, ribotides, ribosides, ATP, and ribose-5-phosphate under the influence of riboside hydrolases. A partial purification of these enzymes isolated from lingcod and rock cod muscle has been carried out recently (Tarr, 1955). As ribose plays a predominant role in the browning of heat-processed fish products, more work will certainly be done in this direction in the near future. 

Nucleoside 2 (or 3 ),5 -diphosphates have been isolated by degradation of certain coenzymes, as well as from hydrolyzates of nucleic acids. Adenosine 3, 5 -diphosphate (see p. 320) has been isolated by enzymic hydrolysis of coenzyme A and from active sulfate (adenosine 3 -phosphate 5 -phosphosulfate). Adenosine 2, 5 -diphosphate was shown to be present in the adenylic acid moiety of the coenzyme adenine-nicotinamide dinucleotide phosphate which, by treatment with a 5 -nucleotidase from potatoes, is converted into adenosine 2 -phosphate. Adenosine 3, 5 -di-phosphate is reported to play a role as a cofactor in the bioluminescence of Renilla reniformis (pansy) Ribonucleic acid carrying a terminal 5 -phos-phate group yields ribonucleoside 3, 5 -diphosphates on digestion with phosphoesterases. ... 

The carbohydrate component of ribonucleic acid and, therefore, of the corresponding purine nucleosides was identified as a pentose by Hammar-sten and, later, as D-ribose by degradation and then by synthesis. Because of the instability of 2-deoxy-D-erythro-peutose ( 2-deoxy-D-ribose ), its isolation from deoxyribonucleic acid was much more difficult. Levene and coworkers finally obtained the crystalline sugar from deoxyguanosine by brief treatment with dilute mineral acid. They established its identity by comparison with synthetic 2-deoxy-D-threo-pentose and 2-deoxy-L-er /[Pg.303]

Rabbit-muscle extracts contain the enzyme amylose isomerase, which also catalyzes the degradation of (1—>6)-interchain linkages. i -i i This reaction is not hydrolytic, but appears to be a reversible transglucosylation. Amylose isomerase preparations are heterogeneous, and, on electrophoresis, four components of various activity and ribonucleic acid content are obtained. The in vivo significance of this enzyme is not yet established. 

Kinetic studies generally do not employ ribonucleic acid itself as a substrate because the system becomes inhomogeneous as ribonucleic acid is degraded, thus making a detailed kinetic analysis impossible. Instead, relatively simple substrates such as dinucleosides, pyrimidine 2 3 -cyclic phos-... 

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