27 Guanosine Hypoxanthine

Figure 1 Influence of pH on migration times of purine compounds. Conditions capillary, 44 cm (37 cm to detector) x 75 pm i.d. buffer, 20 mM borate voltage, 20 kV temperature, 37°C detection, UV, 254 nm. Key = adenine, + = adenosine, = guanine, x=guanosine, = hypoxanthine, A=xanthine, X = uric acid. (Reprinted from Ref. 1 3 with permission of Elsevier Science Publishers.)...

Uric acid is the major product of catabolism of purine nucleosides adenosine and guanosine. Hypoxanthine and xanthine are intermediates along this pathway (Fig. 2). Under normal conditions, they reflect the balance between the synthesis and breakdown of nucleotides. Levels of these compounds change in various situations (e.g., they decrease in experimental tumors) when synthesis prevails over catabolism, and are enhanced during oxidative stress and hypoxia. Uric acid serves as a marker for tubular... 

Nitrogen compounds acetylcholine, aminopyrine, aniline, bilirubin, cadaverine, caprolactam, epinephrine, guanosine, hypoxanthine, inosine, lecithin, monoamines, penicillin, spermine, spermidine, theophylline, urea, uric acid, xanthine. 

Figure 12 Gradient separation of bases, nucleosides and nucleoside mono- and polyphosphates. Column 0.6 x 45 cm. Aminex A-14 (20 3 p) in the chloride form. Eluent 0.1 M 2-methyl-2-amino-l-propanol delivered in a gradient from pH 9.9-100 mM NaCl to pH 10.0-400 mM NaCl. Flow rate 100 ml/hr. Temperature 55°C. Detection UV at 254 nm. Abbreviations (Cyt) cytosine, (Cyd) cytidine, (Ado) adenosine, (Urd) uridine, (Thyd) thymidine, (Ura) uracil, (CMP) cytidine monophosphate, (Gua) guanine, (Guo) guanosine, (Xan) xanthine, (Hyp) hypoxanthine, (Ino) inosine, (Ade) adenosine, (UMP) uridine monophosphate, (CDP) cytidine diphosphate, (AMP) adenosine monophosphate, (GMP) guanosine monophosphate, (IMP) inosine monophosphate, (CTP) cytidine triphosphate, (ADP) adenosine diphosphate, (UDP) uridine monophosphate, (GDP) guanosine diphosphate, (UTP) uridine triphosphate, (ATP) adenosine triphosphate, (GTP), guanosine triphosphate. (Reproduced with permission of Elsevier Science from Floridi, A., Palmerini, C. A., and Fini, C., /. Chromatogr., 138, 203, 1977.)...

A nucleoside consists of a purine or pyrimidine base linked to a pentose, either D-ribose to form a ribonucleo-side or 2-deoxy-D-ribose to form a deoxyribonucleoside. Three major purine bases and their corresponding ribo-nucleosides are adenine/adenosine, guanine/guanosine and hypoxanthine/inosine. The three major pyrimidines and their corresponding ribonucleosides are cytosine/ cytodine, uracil/uradine and thymine/thymidine. A nucleotide such as ATP (Fig. 17-1) is a phosphate or polyphosphate ester of a nucleoside. 

The purine ring-numbering scheme, 1, and structures of some simple purines, viz. adenine 2, guanine 3, caffeine 4, theophylline 5, adenosine 6, 2 -deoxyadenosine 7, guanosine 8, 2 -deoxyguanosine 9, xanthine 10, and hypoxanthine 11, are shown. 

Thymine, hypoxanthine, xanthine, uridine, thymidine, adenine, inosine, adenosine, and guanosine are purchased from Calbiochem. Orotic acid, pseudouridine,... 

Purine nucleoside phosphorylase converts inosine and guanosine into their respective purine bases, hypoxanthine and guanine. 

The conversions of inosine to hypoxanthine (Fig. 25-17, step e), of guanosine to guanine (step g), and of other purine ribonucleosides and deoxyribonucleo-sides to free purine bases are catalyzed by purine nucleoside phosphorylase.318 321b Absence of this enzyme also causes a severe immune deficiency which involves the T cells. However, B cell function is not impaired.312 315 322... 

The sugar specificity of RNase Tx appears to require a 2 -hydroxyl group for the substrate because DNA is not attacked by RNase Tx. This is consistent with the intermediary formation of 2, 3 -cyclic phosphate and also with the finding that 2 -0-methylated guanylyl bonds in tRNA is resistant to the enzyme (48)- Holy and Sorm (49) found that RNase Tx did not attack L-guanosine 2, 3 -cyclic phosphate and L-inosine 2, 3 -cyclic phosphate. They found further that RNase Tx split 9-(a-L-lyxo-furanosyl)-hypoxanthine 2, 3 -cyclic phosphate but not the D-lyxofura-nose derivative, and they concluded that the substrate molecule was fixed at least to three regions of RNase Tx (50). 

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... 

CoQ coenzyme Q (ubiquinone) HGPRT hypoxanthine-guanosine phosphoribosyl... 

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. 

Hypoxanthine guanosine phosphoribosyltransferase (HGPRT) catalyzes the formation of IMP and pyrophosphate (PPj) from hypoxanthine (Hyp) and phosphoribosylpyrophosphate (PRibPP) as shown in reaction (1) ... 

Fig. 11. Control sample co-injected with a solution containing creatinine (Crt), uric acid (UrcA). tyrosine (Tyr), hypoxanthine (Hyp), uridine (Urd), inosine (Ino), guanosine (Guo), hippuric acid (HipA), tryptophan (Trp). theobromine (Thb), and caffeine (Cat). Conditions same as in Fig. 10. Reprinted with permission from Hartwick et at. (H7). Copyright by Elsevier Scientific Publishing Company, Amsterdam.

Free radical alkylation procedures have proved a useful route to alkylpurines which are not readily available by other methods. Thus 6-substituted purines including adenine and hypoxanthine may be converted into 8-methyl derivatives with t-butyl hydroperoxide in the presence of iron(II) ions and acid (74T2677), although small amounts of 2-methyl and 2,8-dimethyl derivatives were formed simultaneously. Adenosine and guanosine similarly furnished the corresponding 8-methyl derivatives with diacetyl peroxide (as a source of methyl radicals) and iron(II) ions (76T337). 

Stereochemical Configuration. Deamination by means of nitrous acid transforms adenosine into inosine (ribosyl-hypoxanthine), and guanosine into xanthosine (ribosyl-xanthine). These four purine nucleosides are hydrolyzed rapidly, and with equal ease, by dilute mineral acids. They are also all hydrolyzed by the same enzyme, so it is reasonable to presume that they are similarly constituted as regards the stereochemical disposition at the sugar-base link. Whether this is of the a- or i8-configuration is still unknown. 

Catabolism of the nucleotides (Figure 24-3, B) begins with removal of their ribose-linked phosphate, a process catalyzed by purine 5 -nucleotidase. Removal of the ribose moiety of inosine and guanosine by the action of purine-nucleoside phosphorylase forms hypoxanthine and guanine, both of which are converted to xanthme. Xanthine is converted to uric acid through the action of xanthine oxidase. 

---