Purines enzymic oxidation

Oxidation of purine derivative, oxidation of allopurinol to alloxanthene FAD dependent oxidative deamination of monoamines, e.g. primaquine Pyridoxal dependent, copper containing enzyme. Conversion of allylamine to acrolein... 

Purine oxidation. The oxidation of purines and purine derivatives is catalyzed by xanthine oxidase. For example, the enzyme oxidizes hypoxanthine to xanthine and thence uric acid (Fig. 4.34). Xanthine oxidase also catalyzes the oxidation of foreign compounds, such as the nitrogen heterocycle phthalazine (Fig. 4.35). This compound is also a substrate for aldehyde oxidase, giving the same product. 

Pyrimidines and purines derivatives act as bases and can be acquired through the diet. In particular, organ meats such as liver are a rich source of DNA and RNA. Most dietary purines are oxidized by enzymes to uric acid in the intestinal mucosa that is their excretory product in humans. The desease known as gout is related to high levels of uric acid in serum and the result of deposition of urate salts in various tissues. 

Inosine formed by either route is then phosphorolyzed to yield hypoxanthine. Although, as we have previously seen, much of the hypoxanthine and guanine produced in the mammalian body is converted to IMP and GMP by a phosphoribosyltransferase, about 10% is catabolized. Xanthine oxidase, an enzyme present in large amounts in liver and intestinal mucosa and in traces in other tissues, oxidizes hypoxanthine to xanthine, and xanthine to uric acid (see fig. 23.20). Xanthine oxidase contains FAD, molybdenum, iron, and acid-labile sulfur in the ratio 1 1 4 4, and in addition to forming hydrogen peroxide, it is also a strong producer of the superoxide anion 02, a very reactive species. The enzyme oxidizes a wide variety of purines, aldehydes, and pteridines. 

Enzymic oxidation pathways vary with the substrate molecule, for purine-6-thione (60), although being fully oxidized to 6-thiouric acid (62), undergoes displacement first at the 8-carbon (61) rather than at... 

The bacterial formamidopyrimidine-DNA glycosylase (alias Fpg or MutM) is a bifunctional base-repair enzyme (DNA glycosylase/AP lyase) that removes a wide range of oxidized purines from oxidatively damaged DNA. The crystal structures of the Fpg (MutM) enzymes from Thermus thermophilus, Lactococcus lactis Escherichia coli Bacillus stearothermophilus and the sequence similar endonuclease VIII (Nei) from E. all share the same... 

In primary rat hepatocyte cultures 100 jiM ferric nitrilotriacetate induced five oxidation products of cellular DNA derived from both purines and pyrimidines (Abalea et al. 1999). Addition of increasing concentrations of myricetin (25-50-100 iM) simultaneously with iron prevented both lipid peroxidation and accumulation of oxidation products in DNA. Moreover, as an activation of DNA repair pathways, myricetin stimulated the release of DNA oxidation bases into culture media, especially of purine-derived oxidation products. The removal of highly mutagenic oxidation products from DNA of hepatocytes might correspond to an activation of DNA excision-repair enzymes by myricetin. This was verified by RNA blot analysis of DNA polymerase P gene expression, which was induced by myricetin in a dose-dependent manner. 

This enzyme, sometimes also called the Schardinger enzyme, occurs in milk. It is capable of " oxidising" acetaldehyde to acetic acid, and also the purine bases xanthine and hypoxanthine to uric acid. The former reaction is not a simple direct oxidation and is assumed to take place as follows. The enzyme activates the hydrated form of the aldehyde so that it readily parts w ith two hydrogen atoms in the presence of a suitable hydrogen acceptor such as methylene-blue the latter being reduced to the colourless leuco-compound. The oxidation of certain substrates will not take place in the absence of such a hydrogen acceptor. 

Mechanistic aspects of the action of folate-requiring enzymes involve one-carbon unit transfer at the oxidation level of formaldehyde, formate and methyl (78ACR314, 8OMI2I6OO) and are exemplified in pyrimidine and purine biosynthesis. A more complex mechanism has to be suggested for the methyl transfer from 5-methyl-THF (322) to homocysteine, since this transmethylation reaction is cobalamine-dependent to form methionine in E. coli. 

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. 

A hypothesis for the oxidation of purines in the presence of this enzyme has been elaborated by Bergmann and his colleagues. It postulates that the purine, often in one of its less prevalent tautomeric forms, is adsorbed on the protein, or the riboflavin coenzyme, of the enzyme then hydration occurs under the influence of the electronic field of the enz5rme, and this must involve a group that is not sterically blocked by the enzyme but which is accessible to the electron-transport pathway of the riboflavin moiety. Finally, the secondary alcohol is assumed to be dehydrogenated in this pathway to give a doubly... 

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-... 

Uric acid is one of the principal products of purine metabolism in man 12 13). However, in many other organisms further oxidative degradation of the purine molecule occurs. One of the most important enzymes involved in uric acid oxidation is uricase, which has been studied to some extent in vitro. 

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-... 

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