Of hypoxanthine

Ibrahim and co-workers developed a new method for the quantitative analysis of hypoxanthine, a natural compound of some nucleic acids. " As part of their study they evaluated the method s selectivity for hypoxanthine in the presence of several possible interferents, including ascorbic acid. 

Xanthine oxidase, mol wt ca 275,000, present in milk, Hver, and intestinal mucosa (131), is required in the cataboHsm of nucleotides. The free bases guanine and hypoxanthine from the nucleotides are converted to uric acid and xanthine in the intermediate. Xanthine oxidase cataly2es oxidation of hypoxanthine to xanthine and xanthine to uric acid. In these processes and in the oxidations cataly2ed by aldehyde oxidase, molecular oxygen is reduced to H2O2 (133). Xanthine oxidase is also involved in iron metaboHsm. Release of iron from ferritin requires reduction of Fe " to Fe " and reduced xanthine oxidase participates in this conversion (133). 

Allopurinol is an analog of hypoxanthine and is converted to alloxanthine by XOD. Both allopurinol and hypoxanthine inhibit XOD (Fig. 1). Alloxanthine is a noncompetitive inhibitor of XOD as is allopurinol at high concentrations. At low concentrations, allopurinol is a competitive inhibitor of XOD. As a result of XOD inhibition, the formation of the poorly soluble... 

Hypouricemia and increased excretion of hypoxanthine and xanthine are associated with xanthine oxidase deficiency due to a genetic defect or to severe liver damage. Patients with a severe enzyme deficiency may exhibit xanthinuria and xanthine lithiasis. 

Xanthine dehydrogenase that mediates the conversion of hypoxanthine into xanthine and uric acid has been studied extensively since it is readily available from cow s milk. It has also been studied (Leimkiihler et al. 2004) in the anaerobic phototroph Rhodobacter capsulatus, and the crystal structures of both enzymes have been solved. Xanthine dehydrogenase is a complex flavoprotein containing Mo, FAD, and [2Fe-2S] redox centers, and the reactions may be rationalized (Hille and Sprecher 1987) ... 

The genes encoding the puc genes, which encode the enzymes in the aerobic pathway for the degradation of hypoxanthine, have been determined for Bacillus subtilis (Schultz et al. 2001), and involve the sequences (Figure 10.31) ... 

Although it had been assumed that only hypoxanthine dehydrogenase is required for the conversion of hypoxanthine (6-hydroxypurine) into uric acid, in Clostridium purinolyti-cum, two enzymes, both of which contain a selenium cofactor, are required. The enzymes differ in the molecular mass of their subunits, in their terminal amino acid sequences, in their kinetic parameters, and in their specific activities for purines (Self and Stadman 2000). Purine hydroxylase converts purine into hypoxanthine and xanthine (2,6-dihy-droxypurine), which is then further hydroxylated to uric acid (2,6,8-trihydroxypurine) by xanthine dehydrogenase (Self 2002). 

The effects of ROM on liver have already been discussed. The potential involvement of ROM in I/R injury is suggested by the finding that glycogen-depleted livers are more susceptible to ischaemic injury. This is thought to be related to accumulation of hypoxanthine during anaerobic metabolism, which serves as a substrate for XO and hence enhanced production of RDM (Younes and Strubelt, 1988). 

Most patients in the United States are treated with allopurinol, which usually is effective if the dosage is titrated appropriately. The drug and its primary active metabolite, oxypurinol, reduce serum uric acid concentrations by inhibiting the enzyme xanthine oxidase, thereby blocking the oxidation of hypoxanthine and xanthine to uric acid. 

The various methods that have been outlined in the previous sections are not exclusive and other analytical methods have been used for the determination of methylxanthines in food systems. One of the most widely used methods for food analysis is flow injection analysis (FIA). In a study by Numata,43 a flow injection analysis method for the determination of hypoxanthine in meat was described. 

Lartigue-Mattei, C., Chabard, J.L., Ristori, J.M., Bussiere, J.L., Bargnoux, H., Petit, J., and Berger, J.A., Kinetics of allopurinol and it s metabolite oxypurinol after oral administration of allopurinol alone or associated with benzbromarone in man. Simultaneous assay of hypoxanthine and xanthine by gas chromatogra-phy-mass spectrometry, Fund. Clin. Pharm., 5,621,1991. 

It is usually accepted that the augmentation of the XO activity in ischemic tissues undergoing reperfusion is a consequence of the formation of hypoxanthine from degradation of ATP in the presence of dioxygen. It has been confirmed by Xia and Zweier [55] who studied the mechanism of stimulation of the XO-catalyzed superoxide production in postischemic tissues. It was found that an increase in superoxide production in isolated rat hearts after reperfusion was triggered by the enhancement of hypoxanthine and xanthine levels due to the degradation of ATP during ischemia. 

Puig, J. G., Torres, R. J., Mateos, E A. et al. The spectrum of hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency. Clinical experience based on 22 patients from 18 Spanish families. Medicine (Baltimore) 80 102-112,2001. 

Allopurinol and its major metabolite, oxypurinol, are xanthine oxidase inhibitors and impair the conversion of hypoxanthine to xanthine and xanthine to uric acid. Allopurinol also lowers the intracellular concentration of PRPP. Because of the long half-life of its metabolite, allopurinol can be given once daily orally. It is typically initiated at a dose of 100 mg/day and increased by 100 mg/day at 1-week intervals to achieve a serum uric acid level of 6 mg/dL or less. Serum levels can be checked about 1 week after starting therapy or modifying the dose. Although typical doses are 100 to 300 mg daily, occasionally doses of 600 to 800 mg/day are necessary. The dose should be reduced in patients with renal insufficiency (200 mg/day for CLcr 60 mL/min or less, and 100 mg/day for CLcr 30 mL/min or less). 

Fig. 2.2 Schematic diagram showing XOR-catalysed oxidation of hypoxanthine and xanthine (also most reducing substrates) at the molybdenum (Mo) site, and ofNADH at the FAD site. Reduction of NAD+ or of molecular oxygen takes place at FAD. Adapted from Harrison [73],...

FIGURE 4.36 Xanthine oxidase-mediated oxidation of hypoxanthine to uric acid. 

Patients with Lesch-Nyhan syndrome have hyperuricemia, indicating an increased biosynthesis of purine nucleotides, and markedly decreased levels of hypoxanthine phbs-phoribosyl transferase (HPRT). The hyperuricemia can be explained on the basis of a decrease in which regulator of purine biosynthesis ... 

Answer D. IMP is a feedback inhibitor of PRPP amidophosphoribosyl transferase, the first reaction in the biosynthesis of purines. IMP is formed by the HPRT reaction in the salvage of hypoxanthine. 

Lesch-Nyhan syndrome A deficiency of hypoxanthine-guanine phosphoribosyltransferase results in accumulation of purine bases (Chapter 10). This causes a marked increase in the plasma level of uric acid, and hence can give rise to gout, but it also causes a severe neurological disorder, known as Lesch-Nyhan syndrome, the symptoms of which include... 

In this connection, a specific loss of hypoxanthine-quanine phosphoribosyltransferase activity has been observed in cases of gout associated with the overproduction of uric acid [305] and in a neurological disorder, the Lesch-Nyhan syndrome [306]. Although a causal relationship has not been established, these disorders could be indicative of the importance of these enzymes. 

Figure 38 Schematic of the active site of hypoxanthine phosphoribosyltransferase with bound phosphor-...

This enzyme [EC 2.4.2.8] (also known as hypoxanthine phosphoribosyltransferase, IMP pyrophosphorylase, transphosphoribosidase, and guanine phosphoribosyltransferase) catalyzes the purine salvage reaction of hypoxanthine with 5-phospho-a-D-ribose 1-diphosphate to... 

Pharmacology Allopurinol inhibits xanthine oxidase, the enzyme responsible for the conversion of hypoxanthine to xanthine to uric acid. Allopurinol acts on purine... 

Rare hereditary deficiency Avoid in patients with rare hereditary deficiency of hypoxanthine-guanine phosphoribosyl-transferase (HGPRT), such as Lesch-Nyhan and Kelley-Seegmiller syndromes. 

Among the methods used, the determination of selected of analytes in vitreous humor (and of potassium in particular, based on the observation that its concentration progressively increases in this substrate after death) has been often adopted in the attempt to reduce imprecision of the estimate. Recently, for example, Munoz et al. [142] have developed an HPLC method for the determination of hypoxanthine, another substance whose concentration has been found to increase after death in vitreous humor. Separation was carried out under RP conditions using a mobile phase of KH2PO4 0.05M (pH 3) containing 1% (v/v) methanol at a flow rate of 1.5mL/min. UV spectra were recorded in the range 200-400 nm. Based on the analysis of samples collected at different PMIs, the authors found that about 53% of the variation in the data is explained by PMI. 

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. 

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