Xanthine uric acid and

Purines are metabolized in a series of reactions involving hypoxanthine, xanthine, uric acid, and allantoin as end products that are subsequently excreted in urine. Fig. 3 shows the metabolic pathways for xanthine. Measurement of urinary excretion of purine metabolites, primarily allantoin or, additionally, uric acid, xanthine, and hypoxanthine, has been proposed as a marker for microbial... 

Thermodynamic modeling of crystal deposition (particularly, that of xanthine, uric acid, and urates) in humans 01PAC785. 

Determination of the specific activities of the adenine nucleotides and summation of the radioactivity appearing in all catabolites (adenosine, inosine, hypoxanthine, xanthine, uric acid and allantoin) allowed the calculation of the rates of catabolism of the adenine nucleotides in the three experimental conditions. 

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). 

Purines and pyrimidines, 7.5 X 10"6 molar solution of adenine, guanine, xanthine, uric acid, uracil, cytosine, thymine. 

Gout is caused by the deposition of crystals of monosodium urate hydrate which are ingested by leucocyctes and trigger the inflammatory response. The biosynthesis of uric acid involves the oxidation of the more soluble compound xanthine (2,6-dihydroxypurine) by xanthine oxidase, and this enzyme is inhibited by allopurinol (187). The treatment of gout also relies on uricosuric drugs to accelerate urinary excretion of uric acid and antiinflammatory drugs to ease the pain and inflammation. 

When xanthine oxidase is inhibited by allopurinol, me plasma level of uric acid and me size of me urate pool in me body bom decrease. 

The cofactors of both xanthine and aldehyde oxidases belong to the LMoVI(S)(0) subfamily (see Section IV). However, inactive dioxo forms, LMovi(0)2, of both xanthine and aldehyde oxidase are known. These dioxo forms do not catalyze oxidation of the respective substrates of these enzymes. The Mov/Molv redox potential for the inactive bis(oxido) form of xanthine oxidase differs from the oxido-sulfido form by -30 mV (bovine xanthine oxidase) and -lOOmV (chicken liver xanthine oxidase) [91]. Although the difference is small, given the xanthine/uric acid reduction potential (-360 mV), it is possible that the Mov/MoIV couple (-433 mV) of the chicken-liver xanthine oxidase bis(ox-ido) form impedes the effective oxidation of xanthine for redox reasons alone. However, the bis(oxido) form of bovine xanthine oxidase (with a reduction potential of -386 mV) should be able to oxidize xanthine, since the redox potential, and hence the thermodynamic driving force, is sufficient for activity [91,92,99]. As substrate oxidation does not occur, the chemical differences between the bis(oxido) and oxido-sulfido (Movl) forms must be critical to the dramatic difference in activity (see Section VI.E.l). 

Understand the properties of purine and pyrimidine bases and nucleosides, and nucleotides with varying amounts of phosphate. Recognize the structures of the various xanthines, cyclic nucleotides, uric acid, and bases found in nucleic acids. 

A very small number of children have been reported who are unable to synthesize molybdopterin they show severe neurological abnormalities shordy after birth and fail to survive more than a few days. As expected from the metabolic roles of molybdopterin, they have low blood concentrations of uric acid and sulfate, and abnormally high levels of xanthine and sulfite. The neurological damage is probably caused by sulfite, because similar abnormalities are seen in children with isolated sulfite oxidase deficiency (Reiss, 2000). 

The inhibitory effects of tea polyphenols on xanthine oxidase (XO) were investigated. Theaflavins and EGCG inhibit XO to produce uric acid and also act as scavengers of superoxide. Theaflavin 3,3 -digaUate (TE-3) acts as a competitive inhibitor and is the most potent inhibitor of XO among these compounds. TE-3 also inhibited the superoxide production in HL-60 cells. Therefore, the antioxidative activity of tea polyphenols is due not only to their ability to scavenge superoxides but also to their ability to block XO and relative oxidative signal transducers. ... 

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. 

The resistance of purine derivatives to UV irradiation depends on the character and position of the substituents in the heterocyclic ring. The photochemical stability of purine derivatives decreases in the order adenine > hypoxanthine > guanine > xanthine > uric acid. 

Allopurinol inhibits xanthine oxidase, the enzyme that converts xanthine and h)q)oxanthine to uric acid. Patients taking allopurinol excrete less uric acid and more xanthine and hypoxanthine in the urine. These compovmds are more soluble than uric acid (renal stones are rarely xanthine) and are more readily excreted in renal failure. 

Xanthine dehydrogenase Tlie enzyme Li used in the eatebolism of the purine ring. It catalyzes the N A[>dependent owdation of anthine lo uric acid. The enzyme also contains t AD and molybdenunn. A fraction of the enzyme normaLly occurs in the body as xanthine axtdase, which represents an altered form tjf the enzyTne. Xanthine oxida.se uses O2 as an nxidant, rather than NAD. Xanthine oxida.se converts xanthine to uric acid, and O3 to 1 lOOl I and the hydroxyl rad teal. 

Traditionally, oxypurines aUantoin, uric acid, and, in some cases, xanthines have been analyzed in biofluids by colorimetric methods. 

Various analytical methods for the simultaneous determination of allantoin, uric acid, and xanthine in biological samples, such as urine, blood plasma, anc semm, have been described. These procedures are mainl) based on separation by HPLC using reversed-phase Cig columns, UV detection, and isocratic elution or gradient... 

Kochansky and Strein reviewed recent developments in chromatography and CE for the determination of creatinine, uric acid, and xanthine in biological fluids. 

RP-HPLC procedures for the determination of creatinine and purine metabolites, such as allantoin, uric acid, xanthine, and hypoxanthine in ruminant urine, were described. Chromatography was achieved with a Cig column under isocratic conditions, and detection at 218 nm without allantoin derivatization. The chromatographic conditions were a compromise between the sensitivity and specificity of the measurement of each analyte, analysis time, and resolution of all analyte peaks from interfering compounds.Uremic toxins creatine, creatinine, uric acid, and xanthine were simultaneously determined in human biofluids, simply after dilution, with UV detection at 200 nm. This method was compared, for creatinine and uric acid, with conventional routine methods and did not give significantly different results. 

Kochansky, C.J. Strein, T.G. Determination of uremic toxins in biofluids Creatinine, creatine, uric acid and xanthines. J. Chromatogr., B 2000, 747, 211-221. 

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