Pyrimidine oxidase

Pyrimidine oxidase Mammalian liver (MPT)Mo(0)(S) Unknown Unknown 255,256... 

While there are reports that pyrimidine oxidase has been isolated, it appears that this enzyme may be an isoform of aldehyde oxidase, which is known to have pyrimidine oxidase activity. 

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 >. 

Very recently (84) this specific binding of alloxanthine-like compounds to active xanthine oxidase has been made the basis of a method of separating the active from the inactivated enzyme, using a pyrazolo-(3,4) pyrimidine attached to agarose. 

Fig. 6. Difference spectra between xanthine oxidase inactivated with various pyra-zolo [3, 4-d] pyrimidines and the native enzyme. The spectra are believed to represent the increase in absorption occurring when Mo(VI) of native enzyme is converted to Mo(IV) complexed with the inhibitors. Spectra were obtained by treating the enzyme with inhibitors in the presence of xanthine, then admitting air, so as to re-oxidize the iron and flavin chromophores. The extinction coefficients, de, are expressed per mole of enzyme flavin. Since some inactivated enzyme was present, extinction coefficients per atom of molybdenum of active enzyme will be about 30% higher than these values. (Reproduced from Ref. 33, with the permission of Dr. V. Massey.)...

The 4-hydrazino-pyrazolo[3,4-rf]pyrimidine derivative 80 yields pyrazolo[4,3-e]-[l,2,4]triazolo[4,3-c]pyrimidinones 82 through hydrazone derivatives 81 <00JCS(P1)33>, these compounds being a new class of potential xanthine oxidase inhibitors. 

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 amino groups are replaced with oxygen. Although here a biochemical reaction, the same can be achieved under acid-catalysed hydrolytic conditions, and resembles the nucleophilic substitution on pyrimidines (see Section 11.6.1). The first-formed hydroxy derivative would then tautomerize to the carbonyl structure. In the case of guanine, the product is xanthine, whereas adenine leads to hypoxanthine. The latter compound is also converted into xanthine by an oxidizing enzyme, xanthine oxidase. This enzyme also oxidizes xanthine at C-8, giving uric acid. 

Although only a few condensed 5 6 or 5 5 aromatic pyrazole derivatives can be isolated from biological sources, the chemistry of condensed pyrazoles has received considerable interest. Condensed pyrazoles with an indene skeleton can be considered as purine analogues and, as such, are expected to have biological activity. The discovery of the xanthene oxidase inhibitory action of pyrazolo[3,4-fiT pyrimidine and the cAMP phos-phodiasterase inhibitory action of pyrazolo[l,5-a]pyrimidines has stimulated considerable interest in the synthesis of analogues of both ring systems. 

The similarity in the pyrimidine ring of pteridines and pterins, especially to the purines adenine and guanosine, undoubtedly makes them good templates for inhibitor design as these examples show. Another enzyme for which inhibition by pteridines has been established is xanthine oxidase <1999BBA387>. In this case, the limitation on structure for inhibitors in the pteridine series was that there should be no substituent on C-7 and that the pteridines should be fully conjugated. The best inhibitors (ICso 0.1 xM) were 6-formylpterin and 6-hydroxylumazine. 

Febuxostat is a potent and selective inhibitor of xanthine oxidase, and thereby reduces the formation of xanthine and uric acid. No other enzymes involved in purine or pyrimidine metabolism are inhibited. In clinical trials, febuxostat at a daily dose of 80 mg or 120 mg was more effective than allopurinol at a standard 300 mg daily dose in lowering serum urate levels. The urate-lowering effect was comparable regardless of the pathogenic cause of hyperuricemia—overproduction or underexcretion. 

As indicated in Fig. 25-18, free adenine released from catabolism of nucleic acids can be deaminated hydrolytically to hypoxanthine, and guanine can be deaminated to xanthine.328 The molybdenum-containing xanthine oxidase (Chapter 16) oxidizes hypoxanthine to xanthine and the latter on to uric acid. Some Clostridia convert purine or hypoxanthine to xanthine by the action of a selenium-containing purine hydroxylase.3283 Another reaction of xanthine occurring in some plants is conversion to the trimethylated derivative caffeine. 328b One of the physiological effects of caffeine in animals is inhibition of pyrimidine synthesis.329 However, the effect most sought by coffee drinkers may be an increase in blood pressure caused by occupancy of adenosine receptors by caffeine.330... 

These enzymes catalyze the two-electron oxidation of purines, aldehydes and pyrimidines, sulfite, formate and nicotinic acid in the general reaction shown in equation (49). These enzymes show some differences in properties. Xanthine oxidase, xanthine dehydrogenase and aldehyde oxidase all have relatively low redox potentials and a unique cyanolyzable sulfur atom, and so will be discussed together. 

Aldehyde oxidase catalyzes the oxidation of aldehydes to carboxylic acids by dioxygen, but also catalyzes the hydroxylation of pyrimidines. Despite its rather broad specificity for substrates, it may well be that aldehyde oxidase should be regarded primarily as a pyrimidine hydroxylase. Thus, xanthine oxidase and aldehyde oxidase catalyze the hydroxylation of purines and pyrimidines respectively. The oxygen incorporated into the product comes from water, not 02. The dioxygen serves as the electron acceptor and other oxidizing agents may be used. 

Phenyl-l,2,3-triazolo[4,5-d]pyrimidin-7(6//)-one has bactericidal activity against Bacillus subtilis and Staphylococcus aureus (94MI2). 1-Benzyl-4-ethoxycarbonylpiperazinyl-l//-l,2,3-triazolo[4,5-d]pyrimidine almost completely removed cytokinin-stimulated effects in betacyanin synthesis in Amaranthus caudatus cotyledons, growth of radish cotyledons, and retention of chlorophyll in leaf explants (94MI4). Analogs of 117 were used as effective inhibitors of xanthine oxidase (95FA257). 

Aldehyde Oxidase. This enzyme is usually found in similar locations to xanthine oxidase or dehydrogenase and has been isolated from insects, birds, and mammals (20, 21). Aldehyde oxidase seems to be a poor choice of name for this enzyme because, while it oxidizes aldehydes to carboxylic acids, it also accepts a variety of purines and pyrimidines as oxidizable substrates. For example, aldehyde oxidase catalyzes the conversion of 2-hydroxypyrimidine to uracil and of adenine to 8-hydroxy-adenine (25). It appears that xanthine oxidase and aldehyde oxidase are... 

It is obvious that x-ray cyrstallographic methods will be the final arbiter of the structural features of molybdoproteins, but until such structures are obtained, and even afterwards as far as dynamic features are concerned, spectroscopic methods must be used to gain insight into the nature of these catalysts. Electronic spectroscopy so far has been of little use here since molybdenum complexes in general appear to exhibit broad weak absorptions. In proteins these are always buried under absorptions from hemes, flavins, and iron-sulfur centers. Massey et al., (15) discovered that pyrazolo [3,4-d] pyrimidines will bind Mo (IV) in milk xanthine oxidase that had been reduced with xanthine... 

The major enzyme involved in the formation of ammonia in the liver, brain, muscle, and kidney is glutamate dehydrogenase, which catalyzes the reaction in which ammonia is condensed with 2-oxoglutarate to form glutamate (Sec. 15.1). Small amounts of ammonia are produced from important amine metabolites such as epinephrine, norepinephrine, and histamine via amine oxidase reactions. It is also produced in the degradation of purines and pyrimidines (Sec. 15.6) and in the small intestine from the hydrolysis of glutamine. The concentration of ammonia is regulated within narrow limits the upper limit of normal in the blood in humans is 70/tmol L-1. It is toxic to most cells at quite low concentrations hence there are specific chemical mechanisms for its removal. The reasons for ammonia toxicity are still not understood. The activity of the urea cycle in the liver maintains the concentration of ammonia in peripheral blood at 20/ molL. ... 

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