Purine synthesis, assay

This enzyme catalyses the reaction which is the first step in purine synthesis. It may be assayed radiochemically [595]. 

Fig, 1. - Effect of fructose on isolated rat hepatocyte.s content of ATP, PP-rib-P and Pi, on the rate of de novo purine synthesis (( C) formate incorporation) and on PP-rib-P availability for metabolic reaction (( C) adenine incorporation). The time of incubation indicates the time of incubation with 28mM glucose or fructose before the assay of the specified parameter. The measurement of the rate of incorporation of ( C) formate and of ( C) adenine required additional incubation of 30 min, following the incubation period indicated in the figure. Thus, for these two measurements, 0 time of incubation values refer to values obtained during the 30 min assay incubation period alone (without preincubation). 0 - incubation with glucose o - incubation with fructose. 

If pterinoids and riboflavin do compete for a common purine-like precursor, then there shoud be a certain reciprocal sparing between pterinoids and riboflavin—complicated by the feedback represented by the folic pterinoids being essential for purine synthesis. A stematic study of folic-riboflavin-Be-purine relations in the classical strain of Streptococcus faecalis ( K , ATCC 8043) has not been made. According to Niven and Sherman (1944) this strain does not need riboflavin nevertheless riboflavin is mcluded in all the basal media recommended for vitamin assays. 

The chemistry, metabolism, and clinical importance of folic acid have been the subject of many excellent reviews (A7, Gil, H14, H20, Rl). Folic acid deficiency leads to a macrocytic anemia and leucopenia. These symptoms are due to inadequate synthesis of nucleic acid. The synthesis of purine bases and of thymine, required for nucleic acid synthesis, is impaired in folic acid deficiency. Detection of folic acid activity in biologic fluids and tissues is of the utmost importance it distinguishes between the various anemias, e.g., those due to vitamin Bi2 or folic acid deficiency. Because morphology of the abnormal red cell does not help in diagnosing vitamin deficiency, one must rely on assay methods for differential diagnosis. Treatment of pernicious anemia with folic acid has led to subacute combined degeneration of the spinal cord despite... 

In the enzyme catalysis of the first committed step in the de novo synthesis of purines, an amino group from L-glutamine is transferred to 5-phosphoribosyl-l-pyrophosphate to form glutamate and 5-phosphoribosyl-1-amine. The assay includes glycinamide ribonucleotide synthetase, which converts 5-phosphoribosyl-l-amine to glycinamide ribonucleotide, which is the reaction product quantitated. 

Purlnerglc receptors - Receptors for the purines may be divided into P-1 and P-2 subtypes. P-1 receptors are adenosine-sensitive and cyclase-linked, while P-2 receptors are ATP-sensltlve, affect prostaglandin synthesis, and have no effect on cyclic AMP production. Two subtypes of the P-1 receptor exist he A-1 or R1 and A-2 or Ra, which, respectively, inhibit or activate adenylate cyclase. Both A-1 and A-2 receptors are sensitive to blockade by xanthines such as caffeine and theophylline. To date, ligand binding assays have only been described for the A-l and P-2 receptors. Binding studies have led to the description of further subtypes which show species... 

The positions that remain unmodified at the end of the in vitro selection procedure (e.g., the purine residnes in a selection carried out with 2 -fluoro-pyrimidine triphosphates) can be modified post-SELEX for further optimization of the aptamers. A systematic stndy of the 64 variants of the six-membered apical loop of an anti-TAR aptamer led to the identification of locked nucleic acid/2 -0-methyl chimeras fully resistant to nncleases that displayed anti-HIV-1 properties in a cell cnlture assay (Di Primo et al., 2007). Identification of the few residues that cannot be modified in an RNA aptamer can be carried ont by chemical interference, a method nsed to identify chemical variants of the aptamer originally selected. Snch an approach led to the synthesis of a modified anti-HIV-1 reverse transcriptase in which all bnt two of the positions of the RNA aptamer were snbstitnted by 2 -0-methyl residnes (Green et al., 1995). This was also the case for the aptamer nsed for age-related macnla degeneration in hnman beings (Ruckman et al., 1998). 

A. Nucleotides, Acyl Nucleotides, and Purines. B. Synthesis and Isolation of RN.A., Aminoacyl-sRNA, and DNA. C. Nearest Neighbor Seepiences. D. UDPG and Related Compounds. E. Assay of Pyridine Nucleotides. F. Folic Acid and Derivatives. 

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