Adenine nucleotide synthesis from

Seasonal variations in the metabolic fate of adenine nucleotides prelabelled with [8—1-4C] adenine were examined in leaf disks prepared at 1-month intervals, over the course of 1 year, from the shoots of tea plants (Camellia sinensis L. cv. Yabukita) which were growing under natural field conditions by Fujimori et al.33 Incorporation of radioactivity into nucleic acids and catabolites of purine nucleotides was found throughout the experimental period, but incorporation into theobromine and caffeine was found only in the young leaves harvested from April to June. Methy-lation of xanthosine, 7-methylxanthine, and theobromine was catalyzed by gel-filtered leaf extracts from young shoots (April to June), but the reactions could not be detected in extracts from leaves in which no synthesis of caffeine was observed in vivo. By contrast, the activity of 5-phosphoribosyl-1-pyrophosphate synthetase was still found in leaves harvested in July and August. 

The chemi-osmotic theory of oxidative phosphorylation has been reviewed,74 a model for mitochondrial oxidative phosphorylation in which a membrane potential or proton gradient might transmit energy from an oxidation step to ATP synthesis has been proposed,76 and adenine nucleotide transport in mitochondria has been reviewed.76... 

The amount of nutrient degradation and ATP synthesis have to be continually adjusted to the body s changing energy requirements. The need to coordinate the production and consumption of ATP is already evident from the fact that the total amounts of coenzymes in the organism are low. The human body forms about 65 kg ATP per day, but only contains 3-4 g of adenine nucleotides (AMP, ADP, and ATP). Each ADP molecule therefore has to be phosphorylated to ATP and dephosphory-lated again many thousand times a day. 

In a series of papers, we have proposed the torsional mechanism of energy transduction and ATP synthesis, the only unified and detailed molecular mechanism of ATP synthesis to date [16-20,56] which addresses the issues of ion translocation in Fq [16, 20, 56], ionmotive torque generation in Fq [16, 20, 56], torque transmission from Fq to Fj [17,18], energy storage in the enzyme [17], conformational changes in Fj [18], and the catalytic cycle of ATP synthesis [18, 19]. We have also studied the thermodynamic and kinetic aspects of ATP synthesis [19,20,41,42,56]. A kinetic scheme has been developed and mathematically analyzed to obtain a kinetic model relating the rate of ATP synthesis to pHjn and pH m in the Fq portion and the adenine nucleotide concentrations in the Fj portion of ATP synthase. Analysis of these kinetic models reveals a wealth of mechanistic details such as the absence of cooperativity in the Fj portion of ATP synthase, order of substrate binding and product release events, and kinetic inequivalence of ApH and Aip. 

Additional information <1-7, 11, 14, 15, 17-19, 21, 30> (<7> cell-free synthesis in mRNA-dependent rabbit reticulocyte lysate system [40] <2,4,5> high activities in tissues where turnover of energy from adenine nucleotides is great, e. g. muscle [3] <1-6,11,14,15> tissue distribution [3,46] <2,5> rabbit and human carry a minimum of 2 sets of isozymes within an individual one set in muscle, erythrocytes, brain and another in liver, kidney and spleen [3]) [3, 40, 46]... 

Nucleotides play important roles in all major aspects of metabolism. ATP, an adenine nucleotide, is the major substance used by all organisms for the transfer of chemical energy from energy-yielding reactions to energy-requiring reactions such as biosynthesis. Other nucleotides are activated intermediates in the synthesis of carbohydrates, lipids, proteins, and nucleic acids. Adenine nucleotides are components of many major coenzymes, such as NAD+, NADP+, FAD, and CoA. (See chapter 10 for structures of these coenzymes.)... 

While the PRTases salvage nucleobases within cells, nucleosides such as adenosine and uridine are present in the blood at much higher concentrations ( 1 pM) than the equivalent nucleobases, adenine and uracil. Indeed, the brain synthesizes pyrimidine nucleotides (UTP and CTP) via salvage synthesis from uridine produced by the liver and released into the circulation. Human cells may contain at least three types of nonspecific nucleoside transporters, and nucleosides are internalized more rapidly than nucleobases. 

The control of the respiration process and ATP synthesis shifts as the metabolic state of the mitochondria changes. In an isolated mitochondrion, control over the respiration process in state 4 is mainly due to the proton leak through the mitochondrial inner membrane. This type of control decreases from state 4 to state 3, while the control by the adenine nucleotide and the dicarboxylate carriers, cytochrome oxidase, increases. ATP utilizing reactions and transport activities also increase. Therefore, in state 3, most of the control is due to respiratory chain and substrate transport. 

According to the methanochondrion concept ATP is synthesized by A/iH across internal membranes and ATP is transported from the organelle into the cytoplasm via an ATP/ADP translocator (for a cartoon see ref [162]). Thus, the observed uncoupler insensitivity of ATP synthesis might be explained on the assumption that the internal membranes are not accessible to these compounds. However, upon reinvestigation of the electron microscopic data and the adenine nucleotide transport this explanation could be ruled out the internal membranes which had been described in the literature for Methanobacterium thermoautotrophicum were found to be artefacts of fixation [163] (see also ref [164]). The adenine nucleotide transport could be explained by a tight and... 

A second special case is that where the ATP synthesis has come to a stop, because the maximal level of phosphorylation of the adenine nucleotides has been reached (State 4 condition). Assuming that no contaminating ATP hydrolyzing enzymes are present, this is the condition where the ATP-driven proton pump has come to a true equilibrium. From the relation Jp = 0, we derive easily ... 

Not all the transporters discussed above are present in aU types of mitochondria the set of activities present in mitochondria depends on the functional needs of the cells from which the mitochondria are isolated. The adenine nucleotide and phosphate transporters are present in all mitochondria thus far studied. This reflects the fact that the major function of mitochondria is the synthesis of ATP. Even in the rare instances (e.g., brown fat mitochondria [55] and mitochondria in anaerobically growing yeast [56]) where the major function is not ATP synthesis, mitochondria normally have active adenine nucleotide transport. The pyruvate transporter also appears to be ubiquitous. The carnitine transporter has been studied in liver [57], heart [35] and sperm [58] and is probably present in all mitochondria which use long-chain fatty acids. 

Alcoholism affects about 10% of the drinking population and alcohol (ethanol) abuse has been implicated in at least 20% of admissions to general hospitals. This chronic disease exhibits high mortality due to a wide variety of factors. Ethanol produces effects in virtually every organ system. The biochemical effects of ethanol are due to increased production of NADH that decreases the [NAD ]/[NADH] ratio in the cytoplasm of liver cells at least tenfold from the normal value of about 1000. Increased production of lactate and inhibition of gluconeo-genesis (Chapter 15) result. The hyperuricemia associated with ethanol consumption has been attributed to accelerated turnover of adenine nucleotides and their catabolism to uric acid (Chapter 27). Alcohol increases hepatic fatty acid and triacylglycerol synthesis and mobilization of fat from adipose tissue, which can lead to fatty liver, hepatitis, and cirrhosis. These effects are complicated by a deficiency of B vitamins and protein. 

Immune system dysfunction in ADA deficiency has also been ascribed to the inhibition of pyrimidine nucleotide synthesis by adenosine, known as pyrimidine starvation. This may arise from inhibition of conversion of orotic acid to orotidine 5 -monophosphate or from inhibition of PRPP synthesis by excessive synthesis of adenine nucleotides. 

TSH also increased pyrimidine nucleotide synthesis in the bovine thyroid (H2, L6). Hall and Tubman postulated that increases in nucleotide synthesis may be mediated by increased ribose generated from the hexose monophosphate shunt. Incorporation of labeled formate and adenine into nuclear, cytoplasmic, or whole RNA was not inhibited by puromycin at a time when protein synthesis was inhibited as measured... 

DNA and RNA each consist of only four different nucleotides. Recall from Chapter 2 that all nucleotides consist of an organic base linked to a five-carbon sugar that has a phosphate group attached to carbon 5. In RNA, the sugar is rlbose in DNA, deoxyribose (see Figure 2-14). The nucleotides used in synthesis of DNA and RNA contain five different bases. The bases adenine (A) and guanine (G) are purines, which con-... 

At very low values of EC, when AMP is elevated it is deaminated via AMP deaminase to inosine monophosphate (IMP). This further displaces the adenylate kinase reaction in the direction of ATP synthesis. The IMP is dephosphorylated by nucleotide phosphatase, and the inosine is phosphorylyzed via purine nucleotide phosphorylase, releasing hypoxanthine and ribose 1-phosphate. The latter is metabolized via the pentose phosphate pathway, and most of the carbon atoms enter glycolysis. Because this course of events depletes the overall adenine nucleotide pool, and hence the scope for ATP production in the longer term, it represents a metabolic last ditch stand by the cell to extract energy even from the energy currency itself ... 

Secondly, the transport inhibitor must be able to pass the cell membrane. The inability of benzene-1,2,3-tricarboxylate to inhibit gluconeogenesis from lactate in perfused pigeon liver, a tissue in which mitochondrial efflux of phosphoenolpyruvate is obligatory for glucose synthesis, is presumably due to lack of penetration through the plasma membrane [16], This observation is interesting since it shows that the ability of an inhibitor to penetrate the cell membrane may vary from tissue to tissue benzene-1,2,3-tricarboxylate does inhibit lipogenesis in hepatocytes of neonatal chicks, as discussed above. Another example is the apparent relative impermeability of the plasma membrane of isolated foetal rat hepatocytes, as compared with that from adult rats, for atractyloside, the inhibitor of the adenine nucleotide translocator [17]. 

Figure 3. Oxidative phosphorylation (OXPHOS). It is composed by electron transport chain (ETC) and ATP synthase. In ETC, oxidation of reducing equivalents (NADH and FADH2) allow the electron transport from complex I and II to complex III by ubiquinone (violet circle) and from complex III to complex IV by cytochrome -c (pink circle). This process finishes in reduction of molecular oxygen, occurring mitochondrial respiration. Mitochondrial potential membrane (A nn) generated by ETC is used for ATP synthesis by ATP synthase. Adenine nucleotides are transported by voltage-dependent anion channel (VDAC) and adenine nucleotide translocator (ANT).

The fatty infiltration of the liver which accompanies the ingestion of orotic acid does not seem to be accompanied by serious pathological disturbances [293] and is readily reversible, unlike the development of fatty liver induced by a choline deficient diet. Supplementation of the orotic acid diet with adenine essentially modifies the effect of orotic acid [294]. Since PRPP is required for both the synthesis of purines and the metabolism of orotic acid, the decrease in the pool of adenine nucleotides is caused [295,296] by an inhibition of purine synthesis de novo due to extensive depletion of PRPP during the conversion of orotic acid to UMP. After the disappearance of orotic acid from the liver of animals previously fed a diet containing orotic acid, stimulation of the synthesis of adenine nucleotides occurred. 

FIGURE 11.18 A -Acyl iminodiacetic acids library. (From Pei, Y. et al., Design and combinatorial synthesis of N-acyl iminodiacetic acids as bongkrekic acid analogs for the inhibition of adenine nucleotide translocase,... 

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