Purines adenosine deaminase

FIGURE 16.8 (a) Phosphoglycolohydroxamate is an analog of the enediolate transition state of the yeast aldolase reaction, (b) Purine riboside, a potent inhibitor of the calf intestinal adenosine deaminase reaction, binds to adenosine deaminase as the 1,6-hydrate. The hydrated form of purine riboside is an analog of the proposed transition state for the reaction. 

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. 

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. 

Humans convert adenosine and guanosine to uric acid (Figure 34-8). Adenosine is first converted to inosine by adenosine deaminase. In mammals other than higher primates, uricase converts uric acid to the water-soluble product allantoin. However, since humans lack uricase, the end product of purine catabofism in humans is uric acid. 

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. 

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. 

T. brucei is unable to synthesize purines de novo and, as such, is dependent upon salvage mechanisms from the host. A number of transporters and enzymes are used by T. brucei to accomplish this task, and inhibition of these targets offers promise for development of trypanocides [39]. This strategy has been validated by demonstration that cordycepin (34), a substrate for T. brucei adenosine kinase (TbAK), which terminates RNA synthesis and parasite growth, can cure stage 2 HAT infections in mice when coadministered with deoxycoformycin (35), an adenosine deaminase inhibitor [40]. 

Pentostatin -purine analogue antimetabolite inhibits adenosine deaminase -nephrotoxicity (including acute renal failure) -bone marrow suppression -neurotoxicity—lethargy, fatigue, seizures, coma -immunosuppression (lymphopenia) -nausea and vomiting -fever -anorexia -hepatotoxicity... 

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

Another early genetic disease for correction by gene therapy was SCID. One form of this disease is caused by a lack of adenosine deaminase (ADA) activity. ADA is an enzyme that plays a central role in the degradation of purine nucleosides (it catalyses the removal of ammonia from adenosine, forming inosine, which, in turn, is usually eventually converted to uric acid). This leads to T- and B-lymphocyte dysfunction. Lack of an effective immune system means that SCID sufferers must be kept in an essentially sterile environment. 

Singhal D, Ho NF and Anderson BD (1998) Absorption and Intestinal Metabolism of Purine Dideoxynucleosides and an Adenosine Deaminase-Activated Prodrug of 2/,3/-Dideoxyinosine in the Mesenteric Vein Cannulated Rat Ileum. J Pharm Sci 87 pp 569-577. 

Answer C. The child most likely has severe combined immunodeficiency caused by adenosine deaminase defidency. This enzyme deaminates adenosine (a nudeoside) to form inosine (another nucleoside). Hypoxanthine and xanthine are both purine bases, and the monophosphates are nucleotides. 

MTX also has several effects on the purine synthetic pathway. MTXPGs inhibit the enzyme aminoimidazole carboxamide ribonucleotide (AlCAR) transformylase, which in turn causes intracellular accumulation of AICAR. AICAR and its metabolites can then inhibit two enzymes in the adenosine pathway adenosine deaminase and adenosine monophosphate (AMP) deaminase, which leads to intracellnlar accumulation of adenosine and adenine nucleotides. Subsequent dephosphorylation of these nucleotides results in increased extracellular concentrations of adenosine, which is a powerful anti-inflammatory agent (11). 

A -Methyladenosine (XXV) (227, 239, 250], fV-methy 1-2 -deoxyadenosine (XXVI) [57], 3 -amino-V-methyl-3-deoxyadenosine [254], and 6-methoxy-purine ribonucleoside (LII) [63, 234, 241] are all good inhibitors of adenosine deaminase and fV-methyladenosine (XXV) has been used in combination with 9-/3-D-arabinofuranosyladenine (XIX) to increase its activity [255]. A number of 2-substituted A -methyladenosines [61] and 9-substituted adenines (see reference 256 and earlier papers by Schaeffer) are also inhibitors of the enzyme. 

The answer is D. Impaired immune function in severe combined immunodeficiency (SCID) is the direct result of blocked DNA synthesis due to inadequate supplies of de-oxyribonucleotides in B and T cells. This effect arises by dATP-induced allosteric inhibition of ribonucleotide reductase, which catalyzes reduction of the 2 -hydroxyl groups on ADP and GDP to form dADP and dCDP. The ultimate cause of many cases of SCID is adenosine deaminase deficiency, which leads to accumulation of dATP and consequent inhibition of ribonucleotide reductase. Although the other enzymes mentioned are also involved in purine nucleotide metabolism, their deficiencies do not lead to SCID. 

Severe combined immunodeficiency Adenosine deaminase, purine nucleoside phosphorylase... 

Vidarabine (adenine arabinoside, ara-A) is phos-phorylated in the cell to the triphosphate derivative which blocks DNA synthesis by inhibiting DNA polymerase. It is indicated for infections with herpes simplex virus and varicella-zoster however its use has to a large extend been surpassed by aciclovir. It is administered topically or intravenously. It is inactivated rapidly by adenosine deaminase which for systemic use necessitates constant infusion of the drug. Vidarabine is the least toxic of the purine analogues. Nausea and vomiting are the most frequent adverse effects and neurotoxicity may occur. 

In Fig. 1 various targets of some important cytostatic agents are depicted. Their main mechanisms of action can be briefly summarized as follows. Pentostatin blocks purine nucleotides by inhibiting adenosine deaminase. 6-Mercaptopurine and 6-thioguanine inhibit purine ring biosynthesis and they inhibit nucleotide interconversions. Methotrexate by inhibiting dihydrofolate reduction blocks thymidine monophosphate and purine synthesis. 5-Fluorouracil also blocks thymidine monophosphate synthesis. Dactinomycin, daunorubicin, doxorubicin and mitoxantrone intercalate with DNA and inhibit RNA synthesis. L-asparaginase deaminates... 

Adenosine deaminase (ADA) was the first therapeutic enzyme coupled to PEG with the aim of reducing clearance and thereby overcoming the short half-life of ADA. Patients deficient in ADA are unable to regulate purine metabolism. As a result purine metabolites (e.g., adenosine monophosphate) accumulate to cytotoxic levels in B-lymphocytes and lead to severe B-cell depletion that presents clinically as severe combined immunodeficiency syndrome (SCIDS). While intramuscular injection of unmodified ADA provides some relief, antibodies develop rapidly against the protein and prevent it from being useful as replacement therapy. Even in the absence of antibodies, unmodified ADA s plasma half-life is only a few minutes. 

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