Hypoxanthine precursors

Inosine monophosphate (contains the purine base hypoxanthine) is the precursor for AMP and GMP. 

Allopurinol, in contrast to the uricosuric drugs, reduces serum urate levels through a competitive inhibition of uric acid synthesis rather than by impairing renal urate reabsorption. This action is accomplished by inhibiting xanthine oxidase, the enzyme involved in the metabolism of hypoxanthine and xanthine to uric acid. After enzyme inhibition, the urinary and blood concentrations of uric acid are greatly reduced and there is a simultaneous increase in the excretion of the more soluble uric acid precursors, xanthine and hypoxanthine. 

Autoradiographic analysis is the only method which can determine the proportion of cells incorporating a radioactive precursor and the site of that incorporation. Thus, tritiated thymidine is incorporated into DNA in the nuclei of those cells in S-phase and tritiated hypoxanthine appears first in the nucleus and later in the cytoplasm of cells with HPRT but not in mutants lacking the enzyme (see 13.2). 

Allopurinol [al oh PURE i nole] is a purine analog. It reduces the production of uric acid by competitively inhibiting the last two steps in uric acid biosynthesis, which are catalyzed by xanthine oxidase (see Figure 39.14). [Note Uric acid is less water-soluble than its precursors. When xanthine oxidase is inhibited, the circulating purine derivatives (xanthine and hypoxanthine) are more soluble and therefore are less likely to precipitate]. 

Allopurinoi, as well as its accumulating metabolite, oxypurinol ( alloxanthine ), inhibits xanthine oxidase, which catalyzes urate formation from hypoxanthine via xanthine. These precursors are readily eliminated via the urine. Allopurinoi is given orally (300-800 mg/day). Apart from infrequent allergic reactions, it is well tolerated and is the drug of choice for gout prophylaxis. Gout attacks may occur at the start of therapy but they can be prevented by concurrent administration of colchicine (0.5-1.5 mg/day). 

Application of HC to animal tissues was carried out for renal stones in kidneys. Rats were freely fed a laboratory ration containing 3% uric acid and 2% potassium oxonate (54). After 3 weeks on this diet, the rats were sacrificed to obtain the kidneys. The left kidney was frozen, and the right one was fixed in absolute alcohol. Both kidneys were sectioned to observe amorphous and crystalline deposits in the tubules and collecting tubes with the microscope. Amorphous and crystalline deposits in both kidneys were removed by the microaspiratoscope, separately, for analysis by HPLC (55). To determine the constituents of the deposits, uric acid, known as a potential component of kidney stones, xanthine and hypoxanthine as precursors, and potassium oxonate were used for reference on HPLC. Only uric acid, probably urate or both, was detected in both kidneys on HPLC. 

Ethyl acetate and ethyl propionate in ethanolic sodium ethoxide with 5-aminoimidazole-4-carboxamide and its riboside have provided a route to 2-methyl- and 2-ethyl-hypoxanthine arid corresponding 2-alkylinosines, respectively (67JOC3258,68JA2661). 2-Methylhypoxan-thine has also been prepared from the same imidazole and ethyl orthoacetate via an intermediate ethoxymethylene derivative which cyclized to the purine when heated (60JA3144). The same method has been applied to the synthesis of 2,8-dimethyl- and XS-diphenyl-hypoxanthine from appropriate imidazole precursors. 

Purine 9-oxides on the other hand are best made from imidazole precursors. Thus 9-hydroxy-8-methylxanthine (368) and hypoxanthine (369) have been most conveniently obtained by cyclization of 5-amino-l-benzyloxy-2-methylimidazole-4-carboxamide (370) with carbonate or formate esters, respectively, and debenzylation of the intermediate benzyloxy derivatives with hydrogen bromide in acetic acid (Scheme 156) (72JOC1867). 

Aminoimidazole-5(4)-carboxamides are the most common imidazole derived precursor molecules for oxopurines, such as hypoxanthine, xanthine, guanine, and isoguanine, using various condensation reagents. In particular, nucleosides are accessible by this route. 

Useful precursors for the preparation of substituted hypoxanthine derivatives are alkyl 4(5)-aminoimidazole-5(4)-carboxylates. Thus, reaction of ethyl 4-amino-l-arylimida7ole-5-carb-oxylates with thioamides in the presence of a catalytic amount of formic acid gives 2-substituted 7-arylhypoxanthines, e.g. formation of l. °... 

Tubule obstruction. Given certain physicochemical conditions, crystals can deposit within the tubular lumen. Methotrexate, for example, is relatively insoluble at low pH and can precipitate in the distal nephron when the urine is acid. Similarly the uric acid produced by the metabolism of nucleic acids released during rapid tumour cell lysis can cause a fatal urate nephropathy. This was a particular problem with the introduction of chemotherapy for leukaemias until the introduction of allopurinol it is now routinely given before the start of chemotherapy to block xanthine oxidase so that the much more soluble uric acid precursor, hypoxanthine, is excreted instead. Crystal-nephropathy is also a... 

Fig. 5.1. Two pathways in normal cells to synthesize DNA precursors. The mutant myeloma cell line, which has the capacity to grow indeflnitely in vitro and can confer this property on antibody-producing lymphocytes through cell fusion, lacks the salvage pathway (no thymidine kinase (TK) or hypoxanthine-guanine phosphori-bosyl transferase (HGPRT)). This cell line would, therefore, not grow in the presence of folic acid antagonists. Fused cells, however, may grow since the antibody-producing cells contribute the salvage pathway.

The answer is d. (Murray, pp 375-401. Scriver, pp 2513-2570. Sack, pp 121-138. Wilson, pp 287-320.) Xanthine oxidase catalyzes the last two steps in the degradation of purines. Hypoxanthine is oxidized to xanthine, and xanthine is further oxidized to uric acid. Thus, xanthine is both product and substrate in this two-step reaction. In humans, uric acid is excreted via the urine. Allopurinol, an analogue of xanthine, is used in gout to block uric acid production and deposition of uric acid crystals in the kidneys and joints. It acts as a suicide inhibitor of xanthine oxidase after it is converted to alloxanthine. Guanine can also be a precursor of xanthine. 

Purine nucleosides, with the exception of adenosine, are salvaged by converting them into the base followed by phosphoribosylation. Adenosine is phosphorylated directly by adenosine kinase or deaminated by adenosine deaminase to inosine. Adenine and adenosine deaminase are present in the sporozoite and merozoite forms (64) the former is not in extracts from unsporulated oocysts (11) but the latter has apparently not been looked for. The ability to deaminate both adenine and adenosine allows this parasite to synthesize guanine nucleotides in the absence of AMP deaminase. The ratio of labeled adenine nucleotides to guanine nucleotides is about 20% higher when both adenine and adenosine are the precursors compared to the ratio obtained when hypoxanthine or inosine was used (64). This indicates that although the major route of salvage for adenine and adenosine is by conversion into hypoxanthine, there is some direct conversion of these compounds into AMP. 

The pathophysiology of gout is understood poorly. While a prerequisite, hyperuricemia does not inevitably lead to gout. Uric acid, the end product of purine metabolism, is relatively insoluble compared to its hypoxanthine and xanthine precursors, and normal serum urate levels approach the limit of solubility. In most patients with gout, hyperuricemia arises from underexcretion rather than overproduction of urate. Urate tends to crystallize in colder or more acidic conditions. Neutrophils ingesting urate crystals secrete inflammatory mediators that lower the local pH and lead to further urate precipitation. 

The biosynthesis of uric acid from the immediate purine precursor xanthine that results from adenine, via the intermediate hypoxanthine, or from guanine is illustrated in Figure 36.32. The enzyme xanthine oxidase (a molybdenum hydroxylase enzyme) is involved in two steps, the conversion of hypoxanthine to xanthine, and the final step, the conversion of xanthine to uric acid. Allopurinol originally was designed... 

IMP is the product of the pathway for de novo purine biosynthesis and the precursor of AMP and GMP. With the de novo pathway for purines not working effectively, it would be helpful to stimulate the salvage pathway, perhaps with a diet that is rich in nucleotides that would then be a source of the preformed purine bases hypoxanthine, adenine, and guanine. 

Uric acid that is produced in man is essentially the product of the action of the enzyme xanthine oxidase on xanthine and hypoxanthine. A tiny amount of uric acid may be ingested as part of the diet, but the great bulk is the result of the action of this enzyme on these two purines. These purines are themselves produced either as a result of the breakdown of cellular material in toto, the turnover of nucleic acids in the cells, or as a result of the intermediary metabolism of various purine nucleotide derivatives. These latter compounds are active in the flow of energy, in methyl group transfer reactions, and as part of the functional molecule of many vitamins. There is direct and indirect evidence that some of the uric acid derives from all these sources. Essentially this evidence consists of the demonstration that other parts of the nucleie acids are found in the urine, such as pyrimidine breakdown products (P9) and methylated purines, which are found only in nucleic acids. There is also isotopic evidence that some labeled purines appear in the urine too quickly after administration of radioactive precursors... 

Not only is uric acid found in the urine and the serum, but small amounts of its precursors xanthine and hypoxanthine and a variety of other purines are found in the serum and in the urine. Many of these are breakdown products of the ribonucleic acids which contain methylated purines. When these macromolecules are broken down the methylated purines are not reutilized but are excreted (B13). In addition, a large number of purine derivatives such as theobromine and caffeine are found in various foods. 

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