Xanthines, 9-amino oxidation

Adenine (6-amino purine) and guanine (2-amino-6-oxy purine), the two common purines, are found in both DNA and RNA (Figure 11.4). Other naturally occurring purine derivatives include hypoxanthlne, xanthine, and uric acid (Figure 11.5). Flypoxanthine and xanthine are found only rarely as constituents of nucleic acids. Uric acid, the most oxidized state for a purine derivative, is never found in nucleic acids. 

Bergmann has suggested that oxidation is ruled out at positions (where hydration occurs readily) which are not accessible to the enzyme after the pteridine is adsorbed on it. Alternatively, the destruction of co-planarity by hydration may prevent adsorption of the pteridine on the enzyme. The case of xanthopterin (2-amino-4,6-dihydroxypteridine) may be relevant. The neutral species of this substance exists as an equilibrium mixture of approximately equal parts of the anhydrous and 7,8-hydrated forms (in neutral aqueous solution at 20°). Xanthine oxidase cataljrzes the oxidation of the anhydrous form in the 7-position but leaves the hydrated form unaffected and about two hours is required to re-establish the former equilibrium. 

The aldehyde oxidoreductase from Desulfovibrio gigas shows 52% sequence identity with xanthine oxidase (199, 212) and is, so far, the single representative of the xanthine oxidase family. The 3D structure of MOP was analyzed at 1.8 A resolution in several states oxidized, reduced, desulfo and sulfo forms, and alcohol-bound (200), which has allowed more precise definition of the metal coordination site and contributed to the understanding of its role in catalysis. The overall structure, composed of a single polypeptide of 907 amino acid residues, is organized into four domains two N-terminus smaller domains, which bind the two types of [2Fe-2S] centers and two much larger domains, which harbor the molybdopterin cofactor, deeply buried in the molecule (Fig. 10). The pterin cofactor is present as a cytosine dinucleotide (MCD) and is 15 A away from the molecular surface,... 

Recently, the possibility to use C60 as anti-inflammatory compound has been reported (Huang et al., 2008). Fullerene-xanthine hybrids have been studied to determine if nitric oxide (NO) and tumor necrosis factor-alpha (TNF-a) production in lipopolysaccharide (LPS)-activated macrophages can be inhibited by hybrid administration, finding positive results. The presence of xanthine moiety seems to be essential for the inhibition of LPS-induced TNF-a production, while the fullerene portion ameliorates the efficiency in LPS-induced NO production blockage, leading to a new promising class of potent anti-inflammatoiy agents. It is necessary to mention also the opposite results obtained by an amino acid fullerene derivative tested on human epidermal keratinocytes at concentration from 0.4 to 400 pg/mL. 

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. 

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. 

Purine nucleotides are converted to uric acid by a pathway that removes amino groups and ribose 1-phosphate from the nucleotide, then uses xanthine oxidase to oxidize the carbon rings to uric acid. Allopurinol, a drug that inhibits xanthine oxidase, is used to treat gout. 

The interrelationship between the 2- and 8-positions is exemplified by 2-oxo-103 and 8-oxopurine, which are both converted by xanthine oxidase to 2,8-dioxopurine. Some unexpected results of oxidation can best be explained by assuming stereospecific factors. Thus, whereas purine, its 2-methyl, and its 2-amino derivatives give the 6-oxo derivatives, the 2-phenyl- and 2-oxopurines mentioned above oxidize at the 8-position.112... 

IMP is the key intermediate of purine nucleotide biosynthesis. IMP can react along two pathways that yield either GMP or AMP. Oxidation of the 2 position makes xanthine monophosphate, which is transamidated to GMP. Alternatively, the a-amino group of aspartate can replace the ring oxygen of IMP to make AMP. (Note again how this reaction is similar to the synthesis of arginine fromcitrulline.)... 

Condensation of 6-amino-5-nitrosouracils with ethanethiol and phenyl-methanethiol leads to formation of 8-substituted xanthines and 1,2,5-thiadiazolo[3,4-t/]pyrimidines [75JCS(P1) 1857]. Oxidation with lead tetraacetate forms furazano[3,4-with sodium nitrite or potassium nitrate and subsequent heating in DMF [73JHC415, 73JHC993 76JCS(P 1) 1327] (Scheme 65). 

Sensors have also been constructed from some oxidases directly contacted to electrodes to give bioelectrocatalytic systems. These enzymes utilize molecular oxygen as the electron acceptor for the oxidation of their substrates. Enzymes such as catechol oxidase, amino acid oxidase, glucose oxidase, lactate oxidase, pyruvate oxidase, alcohol oxidase, xanthine oxidase and cholesterol oxidase catalyze the oxidation of their respective substrates with the concomitant reduction of O2 to H2O2 ... 

Several important mammalian enzymes, such as sulfite oxidase, xanthine dehydrogenase, and aldehyde oxidase, require molybdenum as a cofactor. This organic component is a molybdopterin complex.Sulfite oxidase is probably the most important enzyme in relation to human health. This enzyme catalyzes the last step in the degradation of sulfur amino acids, oxidizing sulfite to sulfate and transferring electrons to cytochrome c. Xanthine dehydrogenase and aldehyde oxidase hydroxylate a number of heterocyclic substances, such as purines, pteridines, and others. ... 

Xanthine oxidase is a rather nonspecific enzyme it not only catalyzes the oxidation of hypoxanthine and xanthine, but also the conversion of adenine to 2,8-dihydroxyadenine (B23, K9) as well as the oxidation of many unusual purines, such as 2-azaadenine (S14). It also acts on xanthopterin (K4), and catalyzes the oxidation of a variety of aldehydes and NADH. Several pterins, notably 2-amino-4-hydroxy-6-formyl- (K3), 2-amino-4-hydroxy-6-carboxy-, 2-amino-4-hydroxy-, and 6-hydroxymethyl- (P3) pterins, inhibit xanthine oxidase. A variety of purines are both substrate and inhibitor of the enzyme. Antabuse (tetraethylthiuram disulfide) has a considerable inhibitory effect on xanthine oxidase in rat... 

Inhibition studies using xanthine oxidase also suggest that cationic substrates are oxidized at the same enzymic site as xanthine [ 179]. In this case, however, initial dissociation of an essential amino acid within the active site as a prerequisite is indicated before quaternary compounds bind, as these substrates are only very slowly oxidized at pH 7 and investigations have to be performed at pH > 9.6 [58, 179, 180]. Nevertheless, analogous substituent effects are observed with both molybdenum hydroxylases Table 3.7) and a relatively large hydrophobic binding site is again indicated with xanthine oxidase. 

Neither methotrexate nor its microbial breakdown product, APA, is a substrate for xanthine oxidase [6, 215], although this may be more a function of the 2,4-diaminopteridine moiety, which itself is refractory to xanthine oxidase [204], rather than the glutamic acid residue. In fact, methotrexate is a potent competitive inhibitor of this enzyme, with a K value of around 25 fiM [218,219]. There is considerable controversy as to whether folic acid, a substituted 2-aminopteridin-4-one, is also an inhibitor of xanthine oxidase. It is not oxidized at carbon 7, unlike the parent compound, which is a poor substrate [204]. However, some workers have shown that folic acid is an extremely potent competitive inhibitor of xanthine oxidase, some 10-times more effective in vitro than allopurinol, whereas other reports claim that the inhibition is due to the contaminant 2-amino-4-oxopteridine-6-aldehyde (27), which is a photolytic breakdown product of folic acid [4, 171, 172, 218-220]. 

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