Xanthine formation from hypoxanthine

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). 

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. 

Fig. 5.7. Formation of hypoxanthine, xanthine and uric acid and their release from different avian tissues. The thicknesses of the arrows are indicative of the rates of release and uptake.

Adenosine formation is accelerated under hypoxic conditions (Rudolphi et al. 1992), but it is metabohsed only after reperfusion. Now superoxide dismutase and catalase are insufficient to serve detoxification of the reactive oxygen species resulting from the biotransformation of hypoxanthine to xanthine and of xanthine to uric acid. While xanthine oxidase inhibitors as oxypurinol do protect against ischaemic damage (Helfman and Phillis 1989, Lin and Phillis 1992), adenosine deaminase inhibitors as trazodone (Sheid 1985) are expected to reduce the formation of hypoxanthine and xanthine, the substrates for xanthine oxidase. 

Formation from Elementary Precursors. Pigeon liver has been an indispensable tissue in the study of purine synthesis by in vitro techniques because it lacks the enzyme xanthine oxidase. Owing to the absence of xanthine oxidase, a substance accumulates during incubation of pigeon liver slices which may be converted into uric acid upon addition of the above-mentioned enzyme.This compound, correctly identified by Edson, Krebs, and Model as hypoxanthine,was believed to be an important intermediary in the synthesis of purines de novo. 

Figure 34-8. Formation of uric acid from purine nucleosides byway of the purine bases hypoxanthine, xanthine, and guanine. Purine deoxyribonucleosides are degraded by the same catabolic pathwayand enzymes,all of which existin the mucosa of the mammalian gastrointestinal tract.

Dietary purines are not an important source of uric acid. Quantitatively important amounts of purine are formed from amino acids, formate, and carbon dioxide in the body. Those purine ribonucleotides not incorporated into nucleic acids and derived from nucleic acid degradation are converted to xanthine or hypoxanthine and oxidized to uric acid (Figure 36-7). Allopurinol inhibits this last step, resulting in a fall in the plasma urate level and a decrease in the size of the urate pool. The more soluble xanthine and hypoxanthine are increased. 

Figure 10.6 HPLC elution profiles of an incubation mixture made up of. 2 nmol of hypoxanthine/guanine phosphoribosyltransferase, SO fiM guanine (G), SO /xM hypo-xanthine (H), 100 fiM PRibPP, and 1 mM MgCl2 in potassium phosphate (pH 7.4). At time intervals of 0 to 5 minutes, aliquots of the mixture were injected onto the HPLC ion-exchange column and eluted. Inset Time-dependent utilization of H and G and formation of GMP and IMP as determined by the absorbance of each peak at 254 nm. (From Ali and Sloan, 1982.)...

Amino-4-imidazole carboxamide ribotide, a precursor only two steps removed (formylation and cycli-zation) from inosinic acid, can be synthesized by the direct condensation of the imidazole with 5-phosphori-bosyl pyrophosphate. The enzyme catalyzing this reaction was purified from an acetone powder of beef liver. The same enzyme (AMP pyrophosphorylase) catalyzes the condensation of adenine, guanine, and hypoxan-thine. Nucleoside phosphorylase is an enzyme that catalyzes the formation of a ribose nucleoside from a purine base and ribose-1-phosphate. Guanine, adenine, xanthine, hypoxanthine, 2,6-diaminopurine, and aminoimidazole carboxamide are known to be converted to their respective nucleosides by such a mechanism. In the presence of a specific kinase and ATP, the nucleoside is then phosphorylated to the corresponding nucleotide. 

Allopurinol therapy in hyperuricaemic man has been shown to be advantageous from two points of view. Firstly, it reduces urinary uric acid excretion and increases the excretion of the precursor purines xanthine and, to a lesser extent, hypoxanthine. In addition, total urinary purine excretion (the sum of these three) may be reduced by as much as 0 during allopurinol therapy (1). This latter effect has been attributed to the formation of nucleotides of either hypoxanthine (1) or allopurinol itself (2), which in turn exert a feed back inhibitary effect on the first enzyme of de novo purine synthesis. 

Uric Add Formation. In vertebrates purines are oxidized to uric acid. This reaction is catalyzed by xanthine oxidase (or dehydrogenase), which attacks both hypoxanthine and xanthine.Since adenine and guanine nucleotides can give rise to the hydroxylated purines either as the nucleotide, nucleoside, or free base, all of the naturally occurring purines of animals can be converted to uric acid. Adenine may also be oxidized to 2,8-dihydroxy-4-aminopurine, which is excreted in the urine. The formation of uric acid from any of its precursors is followed conveniently spectrophotometrically (Fig. 29). ... 

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