Purine Xanthine oxidase

At the present time, the greatest importance of covalent hydration in biology seems to lie in the direction of understanding the action of enzymes. In this connection, the enzyme known as xanthine oxidase has been extensively investigated.This enzyme catalyzes the oxidation of aldehydes to acids, purines to hydroxypurines, and pteridines to hydroxypteridines. The only structural feature which these three substituents have in common is a secondary alcoholic group present in the covalently hydrated forms. Therefore it was logical to conceive of this group as the point of attack by the enzyme. 

Anti-gout Drugs. Figure 1 Xanthine oxidase-catalyzed reactions. Xanthine oxidase converts hypoxanthine to xanthine and xanthine to uric acid, respectively. Hypoxanthine and xanthine are more soluble than uric acid. Xanthine oxidase also converts the uricostatic drug allopurinol to alloxanthine. Allopurinol and hypoxanthine are isomers that differ from each other in the substitution of positions 7 and 8 of the purine ring system. Although allopurinol is converted to alloxanthine by xanthine oxidase, allopurinol is also a xanthine oxidase inhibitor. Specifically, at low concentrations, allopurinol acts as a competitive inhibitor, and at high concentrations it acts as a noncompetitive inhibitor. Alloxanthine is a noncompetitive xanthine oxidase inhibitor. XOD xanthine oxidase. 

Schumacher HR Jr (2005) Febuxostat a non-purine, selective inhibitor of xanthine oxidase for the management of hyperaricaemia in patients with gout. Expert Opin Investig Drugs 14 893-903... 

Xanthine oxidase (XOD) is the key enzyme in purine catabolism. XOD catalyses the conversion ofhypoxan-thine to xanthine and of xanthine to uric acid, respectively. The uricostatic drug allopurinol and its major metabolite alloxanthine (oxypurinol) inhibit xanthine oxidase. 

A series of annulated purines 114-6 have been synthesised as potential inhibitors of xanthine oxidase but, in general, they showed poor activity and the simple pyrimidines 117 were more effective in vitro < 96MI06 96CA(125)86586 96JMC2529 >. 

Adenine phosphoribosyltransferase (APRT) deficiency is an inherited disorder of purine metabolism and is inherited in an autosomal recessive manner (K18, V7). This enzyme deficiency results in an inability to salvage the purine base adenine, which is oxidized via the 8-hydroxy intermediate by xanthine oxidase to 2,8-di-hydroxyadenine (2,8-DHA). This produces crystalluria and the possible formation of kidney stones due to the excretion of excessive amounts of this insoluble purine. Type I, with virtually undetectable enzyme activity, found predominantly in Caucasians, is found in homozygotes or compound heterozygotes for null alleles. Type II, with significant APRT activity, found only in Japan, is related to a missense mu-... 

Most in vitro studies of xanthines have centered around the enzyme xanthine oxidase. Bergmann and co-workers 40-4)) have examined the main oxidative pathways in the xanthine oxidase catalyzed oxidation of purines. The mechanism proposed by these workers 41 > is that the enzyme binds a specific tautomeric form of the substrate, regardless of whether or not that form represents the major structure present in solution. It is then proposed that the purine, e.g., xanthine, undergoes hydration at the N7=C8 double bond either prior to or simultaneously with dehydrogenation of the same position. Accordingly, the process would involve either pathway a or b. Fig. 15. Route a would give a lactim form of the oxidized purine, while b would give the cor-... 

Fig. IS. Proposed generalized route for oxidation of xanthine (and other purines) by xanthine oxidase 411...

Mononuclear (pterin-bonded) I. Xanthine oxidase family Xanthine oxidase Purine or pyrinidine catabolism... 

R is an electron-donor substrate such as purine or xanthine and A is an electron acceptor such as 02 or NAD+. It is thought that the in vivo mammalian form of xanthine oxidase uses NAD+ as acceptor and is therefore, more appropriately named xanthine dehydrogenase. No evidence exists for a dehydrogenase form of aldehyde oxidase. The specificities of xanthine oxidase and aldehyde oxidase have been extensively catalogued (96), and the mechanism and properties of these enzymes have been reviewed (97, 98). 

AHopurinol inhibits xanthine oxidase and also can reduce purine synthesis by inhibiting PRPP amidotransferase (provided HPRT is active). Hyperuricemia and gout often accompany the following conditions ... 

Gout is one of the most ancient diseases its clinical characteristics have been known for at least 2000 years. It is now very effectively treated with drugs that decrease production of uric acid by inhibition of the enzyme xanthine oxidase in purine degradation (Figure 10.9) (allopurinol), and a drug that increases the excretion of uric acid (probenecid)... 

The literature on xanthine oxidase [84] and its companion catabolic enzyme uricase [87] has been extensively reviewed. Many purine analogues, with the exception of most 9-substituted purines [262], serve as substrates for xanthine oxidase both in vitro and in vivo, and if the product is a substrate for uricase, in species that possess this enzyme, the ultimate product is allantoin (LVIII). Thus 2-aminoadenine [5], A -methyladenine [122], and purine [129] are all catabolized... 

Methylation of some form of 6-mercaptopurine in man has been established by the identification of 6-(methylsulphinyl)-8-hydroxypurine (LXV), 6-(methylthio)uric acid (LX), and 6-(methylthio)-8-hydroxy-A -glucuronide (LXVll). The oxidation of 6-(methylthio)purine to 6-(methylthio)-8-hydroxy-purine (LXVl) is mediated much more rapidly by rabbit liver aldehyde oxidase than by xanthine oxidase, and the oxidation is not inhibited by 4-hydroxy-pyrazolo [3, 4-d] pyrimidine [269], which is known to be an effective inhibitor of xanthine oxidase, and consequently, of the oxidation of 6-mercaptopurine [12,268]. 

The pyrazolo[3, 4-d] pyrimidines are substrates for and inhibitors of xanthine oxidase [266, 267]. 4-Hydroxypyrazolo[3,4-d] pyrimidine was first investigated for its ability to protect 6-mercaptopurine and other analogues from oxidation by xanthine oxidase [384], but it also inhibits the oxidation of the natural purines, hypoxanthine, and xanthine. Its profound effect on uric acid metabolism made it an obvious choice for the treatment of gout and its utility in the control of this disease has been demonstrated [385, 386]. 

The reason for the selective toxicity of 6-mercaptopuiine remains to be established, but two factors may be of primary importance. 6-Mercaptopurine is anabolized primarily, if not exclusively, to the monophosphate level, and it is readily catabolized by xanthine oxidase, an enzyme that is low in most cancer cells compared to normal cells. Another factor that must be considered is the metabolic state of the target cells. Actively proliferating leukaemia cells are more sensitive to 6-mercaptopurine, as they are to all antimetabolites, than cells in the so-called Gq or stationary phase. Although this does not explain the difference between 6-mercaptopurine and other purine analogues, it may explain the ineffectiveness of 6-mercaptopurine against solid tumours, most of the cells of which are in the non-dividing state. 

In the most important degradative pathway for adenosine monophosphate (AMP), it is the nucleotide that deaminated, and inosine monophosphate (IMP) arises. In the same way as in GMP, the purine base hypoxanthine is released from IMP. A single enzyme, xanthine oxidase [3], then both converts hypoxanthine into xanthine and xanthine into uric acid. An 0X0 group is introduced into the substrate in each of these reaction steps. The oxo group is derived from molecular oxygen another reaction product is hydrogen peroxide (H2O2), which is toxic and has to be removed by peroxidases. 

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