Purine ribonucleotide interconversion

Having established the general routes of purine ribonucleotide interconversion, the six principal enzymes involved will be discussed in more detail. 

As mentioned in Chapter 7, adenylosuccinate lyase not only functions in purine ribonucleotide interconversion, but also converts phosphori-bosyl aminoimidazole succinocarboxamide to phosphoribosyl amino-imidazole carboxamide. Kinetic studies show that these compounds are alternative substrates and products, respectively, with adenylosuccinate and adenylate, and attempts physically to separate the two activities have consistently failed. Genetic studies have shown that both activities are governed by the same locus and that mutant enzymes behave similarly in both reactions 20, 21). 

The reactions of purine ribonucleotide interconversion may be arranged in two cycles which have inosinate as the common intermediate. One may ask exactly what is the function of these two cycles, into which there are so many points of entry the question may also be asked whether these pathways actually function in a cyclic manner. 

The activities of the enzymes of purine ribonucleotide interconversion can be both stimulated and inhibited in a variety of ways, and these potential control mechanisms may function not only to regulate the synthesis of ATP and of GTP, but also to maintain a balance in the relative intracellular concentrations of these two nucleotides. 

Nitcubotide Inhibitors of Enzymes of Purine Ribonucleotide Interconversion... 

This subject has been reviewed recently by Balls (4S). Three general classes of inhibitors of purine ribonucleotide interconversions may be considered the 6-thio and 6-chloro purines, amino acid analogues, and psicofuranine. 

The effect of 6-mercaptopurine on the incorporation of a number of C-labelled compounds into soluble purine nucleotides and into RNA and DNA has been studied in leukemia L1210, Ehrlich ascites carcinoma, and solid sarcoma 180. At a level of 6-mercaptopurine that markedly inhibited the incorporation of formate and glycine, the utilization of adenine or 2-aminoadenine was not affected. There was no inhibition of the incorporation of 5(or 4)-aminoimidazole-4(5)-carboxamide (AIC) into adenine derivatives and no marked or consistent inhibition of its incorporation into guanine derivatives. The conversion of AIC to purines in ascites cells was not inhibited at levels of 6-mercaptopurine 8-20 times those that produced 50 per cent or greater inhibition of de novo synthesis [292]. Furthermore, AIC reverses the inhibition of growth of S180 cells (AH/5) in culture by 6-mercaptopurine [293]. These results suggest that in all these systems, in vitro and in vivo, the principal site at which 6-mercaptopurine inhibits nucleic acid biosynthesis is prior to the formation of AIC, and that the interconversion of purine ribonucleotides (see below) is not the primary site of action [292]. Presumably, this early step is the conversion of PRPP to 5-phosphoribosylamine inhibited allosterically by 6-mercaptopurine ribonucleotide (feedback inhibition is not observed in cells that cannot convert 6-mercaptopurine to its ribonucleotide [244]. 

Table 5-III lists the glutamine amide transfer reactions of purine and pyrimidine biosynthesis de novo, and of purine and pyrimidine ribonucleotide interconversion, and several more that occur in other areas of metabolism. All have features in common 1, 3).

Studies of the interconversion of purine ribonucleotides began with inosinate. Because radioactive glycine labeled both nucleic acid. adenine and guanine, it was apparent that inosinate had to be converted to adenylate and guanylate. 

As a redox couple, proline and pyrroline-5-carboxylate provide a mechanism for the intercompartmental and intercellular transfer of redox potential. The transfer of redox potential alters the ratio of NADP /NADPH thereby activating certain metabolic pathways. Although the reduction of pyrroline-5-carboxylate is the central mechanism in the transfer of redox potential, the metabolic interconversions of proline, ornithine, and glutamate with pyrroline-5-carboxylate as the obligate intermediate also may play a role. The endpoint of this regulation appears to be the formation of purine ribonucleotides by both salvage and de novo mechanisms. Proline and pyrroline-5-carboxylate appear to be metabolic signals which can be fine-tuned by humoral factors to coordinate the metabolism of amino acids and ribonucleotides. When the transfer is from cell to cell, proline and pyrroline-5-carboxyl-ate can function as intercellular communicators. 

Giardia, Trichomonas and Entamoeba. These parasitic protozoans differ from the other protozoans discussed in this chapter in that they are all incapable of interconversion between their guanine and adenine nucleotide pools. They are dependent on their host environment to supply them with both guanine and adenine. With the exception of E. histolytica, these parasites lack ribonucleotide reductase. This requires that the host also supply purine and pyrimidine deoxynucleosides. 

Biosynthesis and function of RNA - Mcmy inhibitors of RNA synthesis, such as aotinomycin, act by complexing with DNA and inhibiting its template function in the biosynthesis of RNA.3 Base analogs, such as 6-mercapto-purine and 8-azaguanine, interfere with interconversions of ribonucleotide subunits and inhibit novo synthesis of RNA.3 Utilization of these biochemical pathways for the design of chemotherapeutic agents is limited by considerations similar to those discussed for inhibitors of DNA synthesis. 

Fig. 23.1. Pyrimidine pathways Pathways for the de novo synthesis, interconversion, and breakdown of pyrimidine ribonucleotides, indicating their metabolic importance as the essential precursors of the pyrimidine sugars and, with purines, of DNA and RNA. Note that in contrast to purines salvage takes place at the nucleoside not the base level in human cells and pyrimidine metabolism normally lacks any detectable end-product. The importance of this network is highlighted by the variety of clinical symptoms associated with the possible enzyme defects indicated. 23.10, Uridine monophosphate synthase (UMPS), 23.11a, uridine monophosphate hydrolase 1 (UMPHl), 23.12, thymidine phosphorylase (TP), 23.13, dihydropyrimidine dehydrogenase (DPD), 23.14, dihydropyrimidine amidohydrolase (DHP), 23.15, y -ureidopropionase (UP) (23.11b, UMPH superactivity specific to fibroblasts is not shown). CP, carbamoyl phosphate. The pathological metabolites used as specific markers in differential diagnosis are highlighted...

In summary, the biochemical function of folate coenzymes is to transfer and use these one-carbon units in a variety of essential reactions (Figure 2), including de novo purine biosynthesis (formylation of glycinamide ribonucleotide and 5-amino-4-imidazole carboxamide ribonucleotide), pyrimidine nucleotide biosynthesis (methylation of deoxyuridylic acid to thy-midylic acid), amino-acid interconversions (the interconversion of serine to glycine, catabolism of histidine to glutamic acid, and conversion of homocysteine to methionine (which also requires vitamin B12)), and the generation and use of formate. 

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