Purine nucleoside phosphorylase, function

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. 

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 ... 

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. 

The conversions of inosine to hypoxanthine (Fig. 25-17, step e), of guanosine to guanine (step g), and of other purine ribonucleosides and deoxyribonucleo-sides to free purine bases are catalyzed by purine nucleoside phosphorylase.318 321b Absence of this enzyme also causes a severe immune deficiency which involves the T cells. However, B cell function is not impaired.312 315 322... 

Shi W, Ting LM, Kicska GA, Lewandowicz A, Tyler PC, Evans GB, Furneaux RH, Kim K, Almo SC, Schramm VL (2004) Plasmodium falciparum purine nucleoside phosphorylase crystal structures, immucillin inhibitors, and dual catalytic function. J. Biol. Chem. 279 18103-18106... 

Yokomatsu, T.. Hayakawa. Y.. Suemune, K., Kihara, T., Soeda, S., Shimeno, H., and Shibuya, S., Synthesis and biological evaluation of 1, l-difluoro-2-(tetrahydro-3-furanyl)ethylphosphonic acids possessing a N -purinylmethyl functional group at the ring. A new class of inhibitors for purine nucleoside phosphorylases. Bioorg. Med. Chem. Lett., 9, 2833, 1999. 

Erion, M.D., Takabayashi, K., Smith, H., Kessi, J., Wagner, S., Honger, S., Shames, S. and Ealick, S.E. (1997). Purine nucleoside phosphorylase. 1. Structure-function studies. 

I. Reactions of Hypoxanthine. The fact that hypoxanthine is an active intermediate in normal cells directs attention to the three chemical reactions hypoxanthine can undergo in the mammal (Fig. 2). It can be converted to inosine by reaction of the purine with ribose 1-phosphate catalyzed by purine nucleoside phosphorylase. This reaction is probably primarily a phosphorolytic reaction, in vivo, and converts inosine to hypoxanthine and probably does not function to convert hypoxanthine to inosine. There does exist a limited concentration of... 

In 1975, Giblett and co-workers (18) described a deficiency of the enzyme purine nucleoside phosphorylase in patients with severe combined immunodeficiency disease characterized clinically by a severe abnormality of T-lymphocyte function. ile these patients often exhibit hypouricemia as a result of the deficiency of purine nucleoside phosphorylase, they exhibit at the same time accelerated levels of purine biosynthesis de novo with increased excretion of adenosine, deoxyinosine,guanosine, and deoxyguanosine (19). In addition, the intracellular levels of PRPP are elevated in patients with purine nucleoside phosphorylase deficiency. It is assumed, therefore, that the accelerated rate of purine biosynthesis may be due, at least in part, to the elevated levels of PRPP. It has been suggested that a decreased availability of the substrates hypoxanthine and guanine leads to a decreased functional activity of the enzyme HPRT thus leading to decreased consumption of PRPP and hence the elevated levels observed. 

Although purine nucleoside phosphorylase is known to function in the... 

K.C.Rich,E.Mejians,I.H.Fox. Purine nucleoside phosphorylase deficiency improved metabolic and immunological function with erythrocyte transfusion. N.Eng.J.Med.303 973 (1980). 

Adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP) deficiency have been recognized as the primary cause of an associated immune deficiency syndrome. A number of mechanisms have been proposed to explain the predominant effect of these enzyme deficiencies on the development and function of the lymphoid system. One of the mechanisms concerns the phosphorylation of accumulated metabolic compounds i.e. deoxyadenosine (dAdo) in case of ADA-deficiency and deoxyguanosine (dGuo) in case of PNP deficiency in the lymphoid cells and particularly in thymocytes (1). Indeed increased deoxyATP and deoxyGTP levels have been found in the lymphocytes of ADA- and PNP-deficient patients respectively (2,3). These triphosphates may inhibit the enzyme ribonucleotide reductase which leads to a depletion of deoxyCTP and interference with lymphocytic DNA-synthesis (1). 

Thymidine phosphorylase can also use deoxyuridine as substrate [161-163], and the purine nucleoside enzyme can use either the ribonu-cleoside or the deoxyribonucleoside forms of adenine or guanine [115,164], Uridine phosphorylase (EC 2.4.2.3) is a separate entity and will not be considered here, since its regulation is not clearly understood. The four enzymes under consideration are interrelated in function and operate in concert in the regulation of nucleoside catabolism. The mechanisms of their regulation evolved from a number of independent and seemingly devious observations and events, the essence of which may be summarized as follows. 

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