Hypoxanthine degradation

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.

The genes encoding the puc genes, which encode the enzymes in the aerobic pathway for the degradation of hypoxanthine, have been determined for Bacillus subtilis (Schultz et al. 2001), and involve the sequences (Figure 10.31) ... 

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. 

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. 

Since chemistry of pterines and purines has been already reviewed (42,48,491), only recent studies will be described here. Guanine (93) was prepared by reaction of 4-hydroxy-2,5,6-triaminopyrimidine sulfate (588 see Scheme 73) with HCONH2 with removal of H2O from the reaction system in an excellent yield (492). Also, irradiation of oxygenated aqueous solutions of 6-mercaptopurine with near-UV light gave hypoxanthine (92) as a minor product (< 10%) together with purine-6-sulfinate (589). It also arises from degradation of purine-6-sulfonate obtained from photooxidation of the sulfinate (589) (493). 

A method similar to K value is the Kj value index that is used for freshness evaluation of fish that accumulate more inosine than hypoxanthine during spoilage (24). The K value is illustrated in Figure 3 (18,19,20,24). K is used with salmon, halibut, yellowtail, and numerous other tropical fish, while K value is reliable for catfish, white flounder, ocean perch, English sole, and various other species (24). The Kj value can be utilized for species specified for K value but not vice versa due to sensitivity of Kj value to the build up of hypoxanthine and not the total sum of degradation products as needed with K value (24). 

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 degraded by a pathway in which they lose their phosphate through the action of 5 -nucleotidase (Fig. 22-45). Adenylate yields adenosine, which is deaminated to inosine by adenosine deaminase, and inosine is hydrolyzed to hypoxanthine (its purine base) and D-ribose. Hypoxanthine is oxidized successively to xanthine and then uric acid by xanthine oxidase, a flavoenzyme with an atom of molybdenum and four iron-sulfur centers in its prosthetic group. Molecular oxygen is the electron acceptor in this complex reaction. 

Hypoxanthine is oxidized by xanthine oxidase to xanthine, which is further oxidized by xanthine oxidase to uric acid, the final product of human purine degradation. Uric acid is excreted in the urine. 

Purines that result from the normal turnover of cellular nucleic acids can be reconverted into nucleoside triphosphates and used by the body. Thus, they are "salvaged" instead of being degraded to uric acid. PRPP is the source of the ribose-phosphate, and the reactions are catalyzed by adenine phosphoribosyltransferase, and hypoxanthine-guanine phosphoribosyltransferase (HPRT). 

An important role is played by adenosine triphosphate (ATP), involved in energy exchange relatively large amounts of free energy are released when ATP is hydrolyzed. A consequence of the loss of ATP in muscle postmortem is its conversion to hypoxanthine. Some 5 -mononucleotides, intermediates in the production of hypoxanthine and with the ribose component hy-droxylated at position 6, are flavor enhancers in muscle foods. Compounds of this kind are, for example, inosine 5 -monophosphate (IMP) and guanosine 5 -monophosphate (GMP). The ATP is first converted to ADP and then to AMP by a disproportionation reaction. The AMP is then de-aminated to IMP. The IMP can degrade to inosine and eventually to hypoxanthine. Hypoxanthine... 

In addition to analyzing compounds, enzyme sensor has been used to determine the freshness of meats. Xanthine oxidase has been used to determine the levels of xanthine and hypoxanthine that are accumulated from purine degradation during muscle aging so as to monitor fish freshness for a long time. Traditional methods including the automated colorimetric method (54) were time consuming. Jahn et al (55) developed a dipstick test by... 

Preparation of AIC from Hypoxanthine. A search of the chemical literature revealed a paper by Friedman and Gots,21 who found that hypoxanthine was stable to heating in 1 N sulfuric acid. However, the same authors showed that when hypoxanthine was heated in 1.5 N sulfuric acid, with zinc dust added, extensive degradation occurred. The imidazole moiety of hypoxanthine was shown to be stable to the reducing conditions and the product of degradation was found to be a mixture of AIC and a structurally related compound. However, the mixture could not be separated, nor was the unknown compound identified. 

With oxygen-free atmospheres, attack at the 8-position is followed by degradation to 5-formamidopyrimidines (71).126,127 The complexity of this reaction is shown by the fact that adenine gives, in addition to the 5-formamidopyrimidine, some 8-oxoadenine128 and small amounts of hypoxanthine, produced by oxo displacement of the amino group. 

The major bases found in nucleic acids are adenine and guanine (purines) and uracil, cytosine, and thymine (pyrimidines). Thymine is found primarily in DNA, uracil in RNA, and the others in both DNA and RNA. Their structures, along with their chemical parent compounds, purine and pyrimidine, are shown in Figure 10.1, which also indicates other biologically important purines that are not components of nucleic acids. Hypoxanthine, orotic acid, and xanthine are biosynthetic and/or degradation intermediates of purine and pyrimidine bases, whereas xanthine derivatives—caffeine, theophylline, and theobromine—are alkaloids from plant sources. Caffeine is a component of coffee beans and tea, and its effects on metabolism are mentioned in Chapter 16. Theophylline is found in tea and is used therapeutically in asthma, because it is a smooth muscle relaxant. Theobromine is found in chocolate. It is a diuretic, heart stimulant, and vasodilator. 

A close look at this reaction reveals that in the opposite direction, the reaction is of the phosphorolysis type. For this reason, the enzymes catalyzing the reaction with ribose-l-phosphate are called phosphorylases, and they also participate in nucleic acid degradation pathways. Purine nucleoside phosphorylases thus convert hypoxanthine and guanine to either inosine and guanosine if ribose-l-phosphate is the substrate or to deoxyinosine and deoxyguanosine if deoxyribose-1-phosphate is the substrate. Uridine phosphorylase converts uracil to uridine in the presence of ribose-l-phosphate, and thymidine is formed from thymine and deoxyribose-l-phosphate through the action of thymidine phosphorylase. 

In some diseases, excessive amounts of purines are produced in the body, leading to accumulation of urate. Patients with Lesch-Nyhan syndrome lack the enzyme hypoxanthine-guanine phosphoribosyltransferase (HG-PRTase). Children born with this disorder are mentally retarded and prone to self-mutilation. They produce excessive amounts of purines due to accumulation of P-Rib-PP which stimulates the first enzyme of the pathway, amido PRTase (Fig. 15-16). The excess purines are degraded via the reactions... 

Adenosine deaminase catalyzes the deamination of adenosine to form inosine and ammonia. The inosine (Ino) can be degraded further to hypoxanthine (Hyp) by nucleoside phosphorylase, an activity often present in extracts. Therefore, in many cases, the assay involves a determination of either the loss of adenosine (Ado) or the formation of both inosine and hypoxanthine. An early study by Uberti et al., 1977, was followed by another by Hartwick et al., 1978. 

Purines are degraded to urate in human beings. Gout, a disease that affects joints and leads to arthritis, is associated with the excessive accumulation of urate. The Lesch-Nyhan syndrome, a genetic disease characterized by self-mutilation, mental deficiency, and gout, is caused by the absence of hypoxanthine-guanine phosphoribosyltransferase. This enzyme is essential for the synthesis of purine nucleotides by the salvage pathway. 

Enz5une sensors or assays exist for the determination of adenosine triphosphate (ATP) and its degradation products inosine 5 -monophosphate (IMP), inosine (HxR), hypoxanthine (Hx) and xanthine (X) as ATP is used as an indicator of the presence of microorganisms and the concentrations of its degradation products are used as indicators for fish and meat freshness in food industry [118]. 

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