Hypoxanthine formation

Slide 3 (Fig.l) shows that adenosine from the medium is rapidly converted to inosine,probably on the cell membrane, since no adenosine is detectable inside the cell. Inosine is rapidly converted to IMP in the normal cell,but in mutant cells it is converted to hypoxanthine. Effective AMP formation from adenosine in mutant cells is lower than in normal cells, indicating a shift towards hypoxanthine formation. 

Allopurinol is an analog of hypoxanthine and is converted to alloxanthine by XOD. Both allopurinol and hypoxanthine inhibit XOD (Fig. 1). Alloxanthine is a noncompetitive inhibitor of XOD as is allopurinol at high concentrations. At low concentrations, allopurinol is a competitive inhibitor of XOD. As a result of XOD inhibition, the formation of the poorly soluble... 

The molyhdopterin cofactor, as found in different enzymes, may be present either as the nucleoside monophosphate or in the dinucleotide form. In some cases the molybdenum atom binds one single cofactor molecule, while in others, two pterin cofactors coordinate the metal. Molyhdopterin cytosine dinucleotide (MCD) is found in AORs from sulfate reducers, and molyhdopterin adenine dinucleotide and molyb-dopterin hypoxanthine dinucleotide were reported for other enzymes (205). The first structural evidence for binding of the dithiolene group of the pterin tricyclic system to molybdenum was shown for the AOR from Pyrococcus furiosus and D. gigas (199). In the latter, one molyb-dopterin cytosine dinucleotide (MCD) is used for molybdenum ligation. Two molecules of MGD are present in the formate dehydrogenase and nitrate reductase. 

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.

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. 

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. 

The first surprise was that these molecules are much longer than seems necessary for the formation of adapters. In addition, 10-20% of their bases are modified greatly from their original form.171 Another surprise was that the anticodons are not all made up of "standard" bases. Thus, hypoxanthine (whose nucleoside is inosine) occurs in some anticodons. Conventional "cloverleaf" representations of tRNA, which display their secondary structures, are shown in Figs. 5-30 and 29-7. However, the molecules usually have an L shape rather than a cloverleaf form (Figs. 5-31 and 29-6),172 and the L form is essential for functioning in protein synthesis as indicated by X-ray and other data.173 Three-dimensional structures, now determined for several different tRNAs,174 175 are all very similar. Structures in solution are also thought to be... 

Adenine aminohydrolase of A. vinelandii does not catalyze the back incorporation of products, hypoxanthine or chloride, into 6-chloropurine during the course of hydrolysis when examined over a wide range of pH in contrast to the back incorporation of oxygen-18 into hypoxanthine catalyzed by adenosine aminohydrolase (80) (see Section III). These results are consistent with a direct displacement of the 6 substituent by water rather than the intermediate formation of purinyl enzyme or chloroenzyme during catalysis. 

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). 

In mammals specific enzymes for converting purine bases to nucleotides are present in many organs, and in heart muscle this may be the main source of purine nucleotides. The most important of these enzymes is hypoxanthine-guanine phosphoribosyltransferase, which catalyzes the formation of IMP from hypoxanthine and GMP from guanine ... 

Some phosphoric acid derivatives of 2-desoxy-D-ribose have been obtained by enzymic methods of preparation. A reaction analogous to the phosphorolysis of glycogen to D-glucose 1-phosphate241 has been effected with either hypoxanthine- or guanine-D-riboside, both of which could be split by enzymic phosphorolysis with the formation of D-ribose 1-phosphate.242 The successful conclusion of these experiments prompted similar investigations with desoxyribonucleosides. 

Manson and Lampen243 reported that they obtained the phosphorolysis and arsenolysis of hypoxanthine desoxyriboside by enzyme preparations from calf-thymus gland and rat liver. An acid-stable phosphate ester was isolated as a product of phosphorolysis. Results to be outlined suggested that this ester was 2-desoxy-D-ribose 5-phosphate and evidence was obtained for its formation by a mutase type reaction from 2-desoxy-D-ribose 1-phosphate. This evidence was extended and reinforced when Manson and Lampen244 obtained indications for the formation of desoxy-D-ribose 1-phosphate during the phosphorolysis of thymidine. Consequently the conversions outlined may be depicted as shown. 

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