Nucleotide intracellular concentrations

It was known that the intracellular concentrations of the reduced and oxidized forms of the pyridine nucleotides vary in different cell types and under different cell culture conditions.(17) Harrison and Chance applied the NAD(P)H fluorescence technique and found that culture fluorescence can be related to the metabolic state of the cells. 18,19) Since then, more than a hundred papers on NAD(P)H fluorometry have been published. However, they are primarily divided into three major categories ... 

Enzyme Synthesis Also Contributes to Regulation of Deoxyribonucleotides during the Cell Cycle Intracellular Concentrations of Nucleotides Vary According to the Physiological State of the Cell... 

Low activities of orotidine phosphate decarboxylase and (usually) orotate phosphoribosyltransferase are associated with a genetic disease in children that is characterized by abnormal growth, megaloblastic anemia, and the excretion of large amounts of orotate. When affected children are fed a pyrimidine nucleoside, usually uridine, the anemia decreases and the excretion of orotate diminishes. A likely explanation for the improvement is that the ingested uridine is phosphorylated to UMP, which is then converted to other pyrimidine nucleotides so that nucleic acid and protein synthesis can resume. In addition, the increased intracellular concentrations of pyrimidine nucleotides inhibit carbamoyl phosphate synthase, the first enzyme in the. naibwav of aro-tate synthesis. 

The equilibria in these phosphoribosyltransferase reactions favor nucleotide synthesis, and since the inorganic pyrophosphate released is rapidly hydrolyzed by inorganic pyrophosphatase, the coupling of these reactions makes the synthesis of nucleotide irreversible. However, the efficiency of salvage is heavily dependent on the intracellular concentration of PRPP. 

Dopamine activates adenylate cyclase and phospholipase C (PLC) via a D, receptor and inhibits through a D2 receptor, thereby regulating the production of intracellular second messengers, cAMP, Ca2+, and 1,2-diacylglycerol. D, and D2 receptors are decreased in the striatum of patients with dementia. There is considerable evidence to suggest that intracellular levels of cAMP have a protective role for dopaminergic neurons. Intracellular concentrations of cyclic nucleotides are regulated by cyclic nucleotide phosphodiesterases and CaMPDE, one of the most intensely studied and best-characterized phosphodiesterases. 

The basal concentration of cyclic AMP in various tissues is of the order of 0.1 to 1 nmol/g of tissue (wet weight) or about 0.5 to 5 pmol/mg of protein [6]. Assuming an even distribution throughout the intracellular water, the concentration would thus be in the range of 0.25 to 2.5 /xM. This is much less than the concentration of other adenine nucleotides. The concentration of ATP is usually on the order of 1 to 5 ju.ml/g of tissue (wet weight) or about 5 mM. The concentration of 5 -AMP is usually about one tenth of this, with the concentration of ADP most often falling somewhere in between. These nucleotides, therefore, exist in cells in concentrations which are 1000 to 10000 times greater than that of cyclic AMP [6],... 

Cidofovir (Figure 24.4) is an antiviral cytidine nucleotide analog with inhibitory activity against HCMV and other herpes viruses. Cidofovir is first converted to an active diphosphate form by cellular enzymes. Antiviral effects of cidofovir are due to inhibition of viral DNA polymerase by the diphosphate metabolite (Neyts and De Clercq, 1994 Plosker and Noble, 1999 Scholar and Pratt, 2000). The diphosphate probably interacts with DNA polymerase either as an alternate substrate (incorporation at the 3 end or within the interior of the DNA chain) or as a competitive inhibitor (with respect to the normal substrate dCTP). Cidofovir inhibits HCMV DNA synthesis at intracellular concentrations 1000-fold lower than are required to inhibit cellular DNA synthesis (Neyts and De Clercq, 1994). For HSV-1 and HSV-2 corresponding concentrations are at least 50-fold lower. 

PRPP synthetase requires inorganic phosphate as an allosteric activator. Its activity depends on intracellular concentrations of several end products of pathways in which PRPP is substrate. These end products are purine and pyrimidine nucleotides (Figure 27-12). 

Inhibition by AMP and GMP, is competitive with respect to PRPP. The human placental enzyme exists in a small form (M.W. 133,000) and a large form (M.W. 270,000). The small form is catalytically active. Ribonucleotides convert the active form to the large form, whereas PRPP does the opposite. The regulatory actions of PRPP synthetase and amidophosphoribosyltransferase are coordinated. When there is a decrease in the intracellular concentration of adenine ribonucleotides, PRPP synthetase is activated this results in increased synthesis of PRPP, which in turn converts the inactive form of amidophosphoribosyltransferase to the active form and increases production of purine nucleotides. 

Inosine is the only potential precursor of hypoxanthine, and it could be formed in some cells from inosinate by phosphatases. However, such a blind cycle is teleologically unsatisfactory, and furthermore, the nucleotide phosphatases are membrane bound (S13) and inhibited by the normal intracellular concentrations of ATP (M14). Thus, it must be assumed that inosine is formed from adenosine. 

It is apparent from the above considerations that an accelerated rate of purine biosynthesis de novo can occur, therefore, as a result of either elevated levels of PRPP in the cell or decreased levels of purine nucleotides especially the purine nucleoside monophosphates and ADP. Indeed, it seems likely that in many, if not all, clinical conditions to be described, alterations in the levels of both PRPP and purine nucleotides contribute to the accelerated rate of purine biosynthesis. Despite this prediction, the data currently available would suggest that the intracellular concentration of PRPP ordinarily plays a more... 

Other factors, however, may also be important. Edwards and co workers (15) have shown that the decreased reutilization of hypoanthine in patients with HPRT deficiency results in the loss of a normal major source of intracellular nucleotide. This drain of hypo-xanthine from the cell tends to decrease the consumption of PRPP and reduce the intracellular concentration of IMP. The elevated concentration of PRPP in turn presumably allows the cell to compensate for the loss of IMP by virtue of the accelerated rate of IMP synthesis. 

Adenylosuccinate synthetase enzymatic activity is responsive to the intracellular concentration of purine nucleotide monophosphates, but AMP-specific inhibition is probably not important. 

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