Hypoxanthine, synthesis

After carbon dioxide, formate, and glycine were established as precursors of uric acid in vivo, it was shown that these compounds were utilized for hypoxanthine synthesis by pigeon liver homogenates (41, 4 )- It was demonstrated that, with no additions other than these substrates, hypoxanthine synthesis took place in a particulate-free extract of pigeon liver by the reaction of formate, CO2, and glycine in the approximate... 

Hypoxanthine, 7-ethyl-synthesis, 5, 584 Hypoxanthine, 1-methyl-deuterium-hydrogen exchange, 5, 527 synthesis, 5, 594 Hypoxanthine, 2-methyl-methylation, 5, 532 synthesis, 5, 587 Hypoxanthine, 3-methyl-free radical methylation, 5, 544 irradiation, 5, 543 synthesis, 5, 584 Hypoxanthine, 8-methyl-synthesis, 5, 584 Hypoxanthine, 9-methyl-methylation, 5, 532... 

Hypoxanthine, 2-trifluoromethyl-synthesis, 5, 587 Hypoxanthine, trimethylsilyl-glycosylation, 5, 536 Hypoxanthinium nitrate, 1,3,7-trimethyl-thiation, 5, 540 Hypsochromic shift, 1, 344... 

The free bases of the purines can be salvaged to spare de novo synthesis. The only hard thing is remembering what the names stand for. HGPRTase is hypoxanthine-guanine phosphoribosyltransferase, and it makes both IMP and GMP. A separate enzyme exists for the salvage of adenine. The salvage pathways are included in Fig. 19-1. 

For design of a simple manufacturing process, the thermostability of the NP enzymes is a very useful feature. Although heat treatment can be used as part of a purification protocol to isolate the enzymes from contaminating materials, the high temperature of operation itself excludes undesired enzyme-catalysed side reactions. For example, in the synthesis of 9-p-D-arabinofuranosyladenine from Ara-U and adenine, using a wet cell paste of Enterobacter aerogenes, adenine and Ara-U mainly underwent deamination at lower temperatures to form hypoxanthine and uracil respectively. At elevated temperature, deamination was completely eliminated and the rate of transarabinosylation increased. 

In many cells, the capacity for de novo synthesis to supply purines and pyrimidines is insufficient, and the salvage pathway is essential for adequate nucleotide synthesis. In patients with Lesch-Nyhan disease, an enzyme for purine salvage (hypoxanthine guanine phosphoribosyl pyrophosphate transferase, HPRT) is absent. People with this genetic deficiency have CNS deterioration, mental retardation, and spastic cerebral palsy associated with compulsive self-mutilation, Cells in the basal ganglia of the brain (fine motor control) normally have very high HPRT activity. These patients also all have hyperuricemia because purines cannot be salvaged. 

The major intermediates in the biosynthesis of nucleic acid components are the mononucleotides uridine monophosphate (UMP) in the pyrimidine series and inosine monophosphate (IMP, base hypoxanthine) in the purines. The synthetic pathways for pyrimidines and purines are fundamentally different. For the pyrimidines, the pyrimidine ring is first constructed and then linked to ribose 5 -phosphate to form a nucleotide. By contrast, synthesis of the purines starts directly from ribose 5 -phosphate. The ring is then built up step by step on this carrier molecule. 

The synthesis of purine nucleotides (1) starts from IMP. The base it contains, hypoxanthine, is converted in two steps each into adenine or guanine. The nucleoside monophosphates AMP and CMP that are formed are then phos-phorylated by nucleoside phosphate kinases to yield the diphosphates ADP and GDP, and these are finally phosphorylated into the triphosphates ATP and CTP. The nucleoside triphosphates serve as components for RNA, or function as coenzymes (see p. 106). Conversion of the ribonucleotides into deoxyribo-nucleotides occurs at the level of the diphosphates and is catalyzed by nucleoside diphosphate reductase (B). 

Successful fusion (2) is a rare event, but the frequency can be improved by adding polyethylene glycol (PEG). To obtain only successfully fused cells, incubation is required for an extended period in a primary culture with HAT medium (3), which contains hypoxan-thine, aminopterin, and thymidine. Amino-pterin, an analogue of dihydrofolic acid, competitively inhibits dihydrofolate reductase and thus inhibits the synthesis of dTMP (see p. 402). As dTMP is essential for DNA synthesis, myeloma cells cannot survive in the presence of aminopterin. Although spleen cells are able to circumvent the inhibitory effect of aminopterin by using hypoxanthine and thymidine, they have a limited lifespan and die. Only hybridomas survive culture in HAT medium, because they possess both the immortality of the myeloma cells and the spleen cells metabolic side pathway. 

Allopurinol, in contrast to the uricosuric drugs, reduces serum urate levels through a competitive inhibition of uric acid synthesis rather than by impairing renal urate reabsorption. This action is accomplished by inhibiting xanthine oxidase, the enzyme involved in the metabolism of hypoxanthine and xanthine to uric acid. After enzyme inhibition, the urinary and blood concentrations of uric acid are greatly reduced and there is a simultaneous increase in the excretion of the more soluble uric acid precursors, xanthine and hypoxanthine. 

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