Purine phosphorylase

Purine nucleosides are cleaved by the action of purine nucleoside phosphorylase with the liberation of ribose 1-phosphate (Kl, PI). The enzyme is apparently specific for purines. The material from erythrocytes catalyzes the phosphorolysis of purine but not pyrimidine nucleosides (T6.) Purine phosphorylase activity is found widespread in nature and in many animal tissues (FIO). Friedkin and Kalckar investigated an enzyme capable of cleaving purine deoxynucleosides to the aglycone and deoxy-ribose 1-phosphate. They concluded that the enzyme was identical to that which splits purine ribonucleosides (F8, F9). This enzyme is capable of degrading inosine, xanthosine, and guanosine to forms readily attacked by other enzymes. In so doing, it permits living cells to retain the ribose and deoxyribose moieties. 

The susceptibilities of some of these fluorinated purine nucleosides to the action of enzymes are now described. In contrast to the inertness of the 2 -deoxy-2 -fluoro- and 3 -deoxy-3 -fluorocytidine analogs 739, 744, and 821 towards cytidine deaminase, the adenosine compounds 867, 883, and 906 are readily deaminated - by the adenosine deaminase in erythrocytes and calf intestine, but the resulting (deaminated) inosine compounds (from 867 and 883), as well as 888, are highly resistant - to cleavage by purine nucleoside phosphorylase (to give hypoxanthine base for the first two). The reason was discussed. Both 867 and 883 can form the 5 -triphosphates, without deamination, in human erythrocytes or murine sarcoma cells in the presence of 2 -deoxycoformycin, an adenosine deaminase inhibitor, but... 

The biosynthesis of purines and pyrimidines is stringently regulated and coordinated by feedback mechanisms that ensure their production in quantities and at times appropriate to varying physiologic demand. Genetic diseases of purine metabolism include gout, Lesch-Nyhan syndrome, adenosine deaminase deficiency, and purine nucleoside phosphorylase deficiency. By contrast, apart from the orotic acidurias, there are few clinically significant disorders of pyrimidine catabolism. 

Adenosine Deaminase Purine Nucleoside Phosphorylase Deficiency... 

Adenosine deaminase deficiency is associated with an immunodeficiency disease in which both thymus-derived lymphocytes (T cells) and bone marrow-derived lymphocytes (B cells) are sparse and dysfunctional. Purine nucleoside phosphorylase deficiency is associated with a severe deficiency of T cells but apparently normal B cell function. Immune dysfunctions appear to result from accumulation of dGTP and dATP, which inhibit ribonucleotide reductase and thereby deplete cells of DNA precursors. 

Purine nucleoside phosphorylase (PNP) deficiency engenders a combined immunodeficiency and neurologic abnormalities and is usually fatal in childhood (G4). Patients with PNP deficiency have profound lymphopenia and a small thymus with poorly formed Hassall corpuscles. Lymphocyte enumeration shows markedly decreased numbers of T cells and T-cell subsets, with normal percentages of B cells. Point mutations and a splicing mutation have been identified in some PNP-deficient patients (H4). 

H4. Hershfield, M. S., and Mitchell, B. S., Immunodeficiency diseases caused by adenosine deaminase deficiency and purine nucleoside phosphorylase deficiency. In Metabolic and Molecular Bases of Inherited Disease, 7th ed. (C. R. Scriver, A. L. Beaudet, W. S. Sly, and D. Valle, eds.), pp. 1725-1768. McGraw-Hill, New York, 1995. 

The review articles by Schramm (1998, 2003) provide a number of examples of the successful application of this protocol to the design of enzyme-specific transition state-like inhibitors. Among these, the transition state inhibitors of human purine nucleoside phosphorylase (PNP) are particularly interesting from a medicinal chemistry perspective, as examples of these compounds have entered human clinical trials for the treatment of T-cell cancers and autoimmune disorders. 

Table 1. H202-generating enzymatic systems for chemiluminescence-based optical fibre biosensors (Abbreviations OX = oxidase, PNPase = purine nucleoside phosphorylase).

Low-density protein receptor Adenosine deaminase Purine nucleoside phosphorylase Sphingomylinase Glucocerebrosidase... 

Habash, J. R. Helliwell, J. D. Stoeckler, R. E. Parks, Jr., S. F. Chen, and C. E. Bugg, Three-dimensional structure of human erythrocytic purine nucleoside phosphorylase at 3.2 A resolution, J. Biol. Chem. 265 1812 (1990). 

Shames, and S. E. Ealick, Purine nucleoside phosphorylase. 3. Reversal of purine base specificity by site-directed mutagenesis, Biochemistry 36 11725 (1997). 

A. Secrist, III, Application of crystallographic and modelling methods in the design of purine nucleoside phosphorylase inhibitors, Proc. Natl. Acad. Sci. USA 88 11540 (1991). 

Secrist, S. Y. Babu, C. E. Bugg, W. C. Guida, and S. E. Ealick, Structure-based design of inhibitors of purine nucleoside phosphorylase, 3,9-arylmethyl derivatives of a 9-deazaguanine substituted on the methylene group, J. Med. Chem. 36 3771 (1993). 

Like many reported A -transglycosylations, this reaction uses uncharacterized nucleoside phosphorylases from whole cells held at 50-60 °C, a temperature well above the range for viability of the parent microorganism. Remarkable temperature stability has been reported for three well-known NPs of E. coli purine nucleoside phosphorylase (PNP), uridine phosphorylase (URDP) and thymidine phosphorylase. ... 

Krenitsky, T.A., Koszalka, G.W. and Tuttle, J.V., Purine nucleoside synthesis, an efficient method employing nucleoside phosphorylases. Biochemistry, 1981, 20, 3615-3621. 

Nucleoside phosphorylases that catalyse the reversible cleavage of purine nucleosides to the free bases and ribose-1-phosphate are found in most cells, although a phosphorylase that will cleave adenosine has so far been identified only in bacteria. Recent studies have shown that ribo- and 2 -deoxyribofurano-syltransferase activity is associated with phosphorylase activity [19, 23., 222] and that both activities reside in one enzyme, which can be converted from one form to the other by substrate or product binding [20]. Upon crystallization of the enzyme from human erythrocytes a marked decrease in the ribosyl transfer reaction was observed [21b]. 

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