Salvage pathway purine

Pentostatin (deoxycoformycin Fig. 4) is a purine isolated from cultures of Streptomyces antibioticus. Its mode of action involves inhibition of adenosine deaminase, which plays a key role in purine salvage pathways and DNA synthesis. As a consequence, deoxyadenosine triphosphate (dATP) is accumulated, which is highly toxic to lymphocytes. This is associated with augmented susceptibility to apoptosis, particularly in T cells. 

Guanine Phosphoribosyl Transferase. Guanine phosphoribosyl transferase (GPRT) is one of the enzymes of the purine salvage pathway, which is needed by protozoa because they lack the ability to synthesize purine nucleotides. 

This rational approach to drug design has been adopted in developing a specific inhibitor of the human cellular enzyme, purine nucleoside phosphorylase (PNP). PNP functions in the purine salvage pathway, catalysing the reversible reaction shown below ... 

Mycophenolate mofetil (MMF, CellCept) is an ester prodrug of mycophenolic acid (MPA), a Penicillium-de-rived immunosuppressive agent (see Chapter 57) that blocks de novo purine synthesis by noncompetitively inhibiting the enzyme inosine monophosphate dehydrogenase. MPA preferentially suppresses the proliferation of cells, such as T and B lymphocytes, that lack the purine salvage pathway and must synthesize de novo... 

The enzyme has been isolated from both eukaryotic and prokaryotic organisms [2] and functions in the purine salvage pathway [1,3]. Purine nucleoside phosphorylase isolated from human erythrocytes is specific for the 6-oxypurines and many of their analogs [4] while PNPs from other organisms vary in their specificity [5]. The human enzyme is a trimer with identical subunits and a total molecular mass of about 97,000 daltons [6,7]. Each subunit contains 289 amino acid residues. 

Figure 25-17 Some purine salvage pathways and related reactions. Green lines indicate biosynthetic pathways.

Manfredi, J. P., and E. W. Holmes, Purine salvage pathways in myocardium. Ann. Rev. Physiol. 47 691-705, 1985. Although this review applies specifically to salvage in heart muscle, it is an excellent summary of purine salvage path-... 

Purine and pyrimidine nucleotides are essential components of many biochemical molecules, from DNA and RNA to ATP and NAD. In recent years, the pyrimidine and especially the purine metabolism of parasitic helminths have been investigated extensively, mainly because they are different from the pathways in the mammalian host such that they have potential as targets for chemotherapeutic attack. For a review of purine and pyrimidine pathways in parasitic helminths and protozoa, see Berens et al. (1995). Although parasitic helminths do not synthesize purines de novo, but obtain them from the host, they do possess elaborate purine salvage pathways for a more economical management of this resource. Pyrimidines, on the other hand, are synthesized de novo by all parasitic flat-worms studied so far and, as with mammalian... 

The most recent application of RPLC to the analysis of enzymes has been reported by Halfpenny and Brown (HI). An assay for purine nucleoside phosphorylase, a key mediator in the purine salvage pathway, has been developed and optimal conditions for the analysis determined. Figure 20 illustrates the simultaneous separation of the substrate, inosine, and products, uric acid and hypoxanthine. In another analysis. Halfpenny and Brown (H2) developed an assay for hypoxanthine-guanine phos-phoribosyltransferase. Deficiency of this enzyme has been associated with Lesch-Nyhan syndrome as well as primary gout. The activity of the enzyme is determined by measurement of the decrease of the substrate, hypoxanthine, and increase in the product, inosine-5 -monophosphoric acid. A major advantage of using HPLC for enzyme assays is that the simultaneous measurement of both substrate and product reduces the error due to interference from competing enzymes. 

Most of the free purines derived from the breakdown of DNA, RNA, and nucleotides in the diet are catabolized to xanthine and then to uric acid in the gut mucosa. The AMP and GMP biosynthesized in the body can also be bmken down to free purines, such as adenine, guanine, and hypoxanthine. These purines, in contrast to those derived frcim the diet, are largely reused for the synthesis of ATP and GTP- They are first converted back to AMP or GMP in a pathway of reutiliza-lion called the purine salvage pathway. For example, adenine phosphoribosyl-transferase (PRPP) catalyzes the conversion of adenine to AMP. Here, PRPP serves as the source of the phosphoribose group. Pyrophosphate is a product of the reaction. 

PRPP levels can also be increased as a result of underutilization in purine salvage pathways. Thus, HPRT deficiency (partial or complete) causes hyperuricemia as an X-linked recessive trait. In situations in which ATP is consumed more rapidly than it is synthesized or in which... 

The mechanism by which deficiency of HPRT causes central nervous system disorders remains unknown. Lesch-Nyhan patients do not show anatomical abnormalities in the brain. In normal subjects, HPRT activity is high in the brain and, in particular, in the basal ganglia where de novo purine biosynthesis is low. This suggests the importance of the purine salvage pathway in this tissue. However, the relationship between HPRT deficiency and... 

The synthesis of purine and pyrimidine nucleotides needed for the biosynthesis of nucleic acids, and in turn the processes of transcription and translation provide a number of suitable targets for therapeutic intervention. These have been subject of much interest for the discovery of new antiprotozoal drugs as well. Developments in two areas, viz. the purine salvage pathway and the pyrimidine biosynthesis have yielded useful drugs and are discussed. 

HGPRT Hypoxanthine-guanine phosphoribosyltransferase the enzyme that catalyzes the synthesis of inosine monophosphate (IMP) and guano-sine monophosphate (GMP) from hypoxanthine and guanine, respectively. It makes up part of the purine salvage pathway, a way of recycling purine bases back to the nucleotides. 

Purine salvage pathway The synthesis of purine nucleotides by the condensation of the purine bases with phosphoribosyl pyrophosphate. As the name suggests, it is a way in which purine bases can be recycled back to nucleotides. The purine salvage pathway consists of two enzymes, HGPRT and adenine phosphoribosyltransferase (APRT). 

Nucleotide C-N bond hydrolysis and phosphorolysis at the monomer level form part of purine salvage pathways and their mechanisms have been intensively investigated for pharmacological reasons. Humans can biosynthesise purine nucleosides de novo, whereas protozoal parasites such as those causing bilharzia and Chagas disease rely on hydrolysis of preformed nucleosides from the host. Finally, a series of ribosyl transfers from NAD are important in the modification of proteins by pathogens and have been studied extensively. 

In the purine salvage pathway, purine bases obtained from the normal turnover of cellular nucleic acids or (to a lesser extent) from the diet are reconverted into nucleotides. Because the de novo synthesis of nucleotides is metabolically expensive (i.e., relatively large amounts of phosphoryl bond energy are used), many cells have mechanisms to retrieve purine bases. Hypoxanthine-guaninephos-phoribosyltransferase (HGPRT) catalyzes nucleotide synthesis using PRPP and either hypoxanthine or guanine. The hydrolysis of pyrophosphate makes these reactions irreversible. 

The relative importance of the de novo and salvage pathways is unclear. However, the severe symptoms of hereditary HGPRT deficiency indicate that the purine salvage pathway is vitally important. In addition, investigations of purine nucleotide synthesis inhibitors for treating cancer indicate that both pathways must be inhibited for significant tumor growth suppression. 

Referring to Question 17, calculate the number of ATP molecules that are required to synthesize a purine. Referring to the purine salvage pathway, calculate the number of ATP molecules that are required to prepare the same molecule. How many ATP molecules are saved by the purine salvage pathway ... 

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