The de novo and salvage pathways for the synthesis of pyrimidine residues of RNA predominate in different locations within the mouse doudenal epithelium

Dietary nucleotides can prevent glucocorticoid-induced telomere attrition in a fast-growing wild vertebrate

Telomeres are chromosome protectors that shorten during eukaryotic cell replication and in stressful conditions. Developing individuals are susceptible to telomere erosion when their growth is fast and resources are limited. This is critical because the rate of telomere attrition in early life is linked to health and life span of adults. The metabolic telomere attrition hypothesis (MeTA) suggests that telomere dynamics can respond to biochemical signals conveying information about the organism's energetic state. Among these signals are glucocorticoids, hormones that promote catabolic processes, potentially impairing costly telomere maintenance, and nucleotides, which activate anabolic pathways through the cellular enzyme target of rapamycin (TOR), thus preventing telomere attrition. During the energetically demanding growth phase, the regulation of telomeres in response to two contrasting signals - one promoting telomere maintenance and the other attrition - provides an ideal experimental setting to test the MeTA. We studied nestlings of a rapidly developing free-living passerine, the great tit (Parus major), that either received glucocorticoids (Cort-chicks), nucleotides (Nuc-chicks) or a combination of both (NucCort-chicks), comparing these with controls (Cnt-chicks). As expected, Cort-chicks showed telomere attrition, while NucCort- and Nuc-chicks did not. NucCort-chicks was the only group showing increased expression of a proxy for TOR activation (the gene TELO2), of mitochondrial enzymes linked to ATP production (cytochrome oxidase and ATP-synthase) and a higher efficiency in aerobically producing ATP. NucCort-chicks had also a higher expression of telomere maintenance genes (shelterin protein TERF2 and telomerase TERT) and of enzymatic antioxidant genes (glutathione peroxidase and superoxide dismutase). The findings show that nucleotide availability is crucial for preventing telomere erosion during fast growth in stressful environments.

Biochemical and structural analysis of the Klebsiella pneumoniae cytidine deaminase CDA

The emergence of drug-resistant strains of Klebsiella pneumoniae, has exacerbated the treatment and control of the disease caused by this bacterium. Cytidine deaminases (CDA) are zinc-dependent enzymes involved in the pyrimidine salvage pathway and catalyze the formation of uridine and deoxyuridine from cytidine and deoxycytidine, respectively. To illustrate the structural basis of CDA for a deeper knowledge of the molecular mechanisms underlying the salvage pathway, we reported here the biochemical and structural analysis of CDA from pathogenic K. pneumonia. KpCDA showed deaminase activity against cytidine as well as its analog cytarabine. The deaminase activity of KpCDA on cytarabine was 1.8 times higher than that on cytidine. KpCDA is composed of an N-terminal catalytic domain and a C-terminal noncatalytic domain. Zinc, which is involved in the activity of the catalytic domain, is coordinated by His102, Cys129, and Cys132, and two 1,4-dioxane molecules were present at the active sites. KpCDA exists as a dimer and shows distinct dimeric interface compared with other CDAs. Our results provide the structural features of KpCDA, and KpCDA might be a potential antibacterial target for the disease caused by K. pneumoniae.

The role of nucleotides in the immune and gastrointestinal systems: potential clinical applications

Nucleotides are low molecular weight biological molecules key to biochemical processes. Sources include de novo synthesis, recovery via salvage mechanisms, and dietary intakes. Although endogenous production serves as the main nucleotide source, evidence suggests that exogenous sources are essential to immune competence, intestinal development, and recovery. Dietary nucleotides serve a marked role in rapidly proliferating cells where they are necessary for optimal function. Accordingly, dietary nucleotides are deemed conditionally essential in the presence of various physiological stresses, including growth and development, recovery from injury, infection, and certain disease states. Clinical studies that evaluated nutrition formulations of nucleotides in combination with other specific nutrient substances demonstrated improved clinical outcomes in patients characterized as critically ill, injured, immune suppressed, or with chronic gastrointestinal conditions. However, conclusions regarding specific benefits of nucleotides are limited. Scientific substantiation of nucleotide supplementation in infant formula has been reported to improve the maturation and development of the intestinal tract as well as immune function. All medical nutrition products except for one immune-modulating formulation are devoid of nucleotides. In an effort to build on this, the current review presents the data to support potential clinical applications for nucleotides in enteral nutrition that may contribute to improved outcomes in physiologically stressed patients.

Exogenous nucleosides modulate proliferation of rat intestinal epithelial IEC-6 cells

Exogenous nucleotides are considered semiessential nutritional components that play an important role in intestinal development, maintenance, and recovery from tissue damage. Nucleosides (NS) are the best-absorbed chemical form of nucleotides in the intestinal epithelium. The aim of this work was to clarify, at the cellular level, the effects described in vivo. Under conditions of high intracellular availability of NS, we studied the effects of 2 NS mixtures on the NS uptake and intracellular distribution and on the proliferation, morphology, viability, and cell-cycle phase distribution of rat intestinal epithelial cell line 6. Purine and pyrimidine NS showed a similar uptake profile, but the intracellular incorporation of guanosine was greater than that of uridine, without differences in intracellular distribution. Proliferation assays demonstrated that IEC-6 cell proliferation is increased by a mixture containing thymidine but decreased by one containing uridine. In fact, the antiproliferative effect started at 75 micromol/L, which indicated that it may not be correct to consider concentrations of uridine >75 micromol/L as physiological. Interestingly, these effects were not related to increased cell necrosis or apoptosis or to changed cell morphology but rather to a reduced S-phase and increased G0/G1 phase of the cell cycle. In summary, our results suggest that NS molecules are well-absorbed by rat intestinal epithelial cell line 6 cells, whose proliferation can be promoted or inhibited (according to the NS mixtures used) by a mechanism that is not dependent on the toxicity of the mixtures.

Methods for measuring tissue RNA turnover

Measuring RNA turnover is important because of the significance of rRNA, tRNA and mRNA in tissue protein synthesis. Changes in turnover of each of these species precede important cellular events such as hormone or cytokine action or cell-division itself. Isotopic methods have relied on decay of pulse-labelled RNA or on incorporation of isotopically-labelled precursors. However, recycling of labels may lead to under or overestimation of synthesis rates respectively. The labelling of the intracellular precursor pool must be known if accurate RNA synthesis rates are to be calculated from the degree of incorporation. However, the intracellular nucleotide pools may be anatomically or metabolically compartmented (i.e. via de novo or salvage synthesis routes) and this complicates many study designs. The use of[methyl-14C]- or [methyl-3H]methionine as a means of labelling methylated nucleosides in RNA and protein simultaneously is described in addition to new stable isotopic techniques based on 13C-glycine as a de novo label. Urinary excretion of the numerous modified nucleosides in cellular RNA can be used to calculate whole-body turnover rates of each of the major RNA species. Examples of the effects of critical-illness and glutamine supplementation on RNA turnover are given. We conclude by suggesting that whole-body RNA turnover rates have been significantly underestimated and that this has implications for nutritional therapy, especially with regard to nucleotide supplementation.

Glutamine and nucleotide metabolism within enterocytes

Glutamine has an important role as a source of energy for enterocytes. However, it may also have a key role as a source of nitrogen for the synthesis of nucleotides. The relative contribution of de novo synthesis and salvage pathways seems to be affected by the position of enterocytes within the crypt-villus axis as well as the dietary intake of nucleic acids and glutamine. Nucleotides are especially important to enterocytes during intestinal development, maturation, and repair. Hence an understanding of nucleotide metabolism within enterocytes has important implications regarding both the composition and route of administration of nutrient solutions. Many important questions remain unanswered, in particular: Does glutamine stimulate intestinal de novo pyrimidine synthesis via the action of carbamoyl phosphate synthetase I? Can de novo purine synthesis maintain intestinal purine pools in the absence of dietary nucleic acids? And, what are the specific effects of parenterally administered nucleotides on the metabolism and well-being of enterocytes? A greater understanding of these issues will lead to a more rational approach toward the nutritional modulation of gut dysfunction.

Morphological changes in hepatocytes of rats deprived of dietary nucleotides

The aim of the present study was to investigate the influence of dietary nucleotides on liver morphology. Adult rats were fed for 21 d on a nucleotide-containing diet or the same diet free of nucleotides. Liver sections were examined by light and transmission electron microscopy, as well as for nucleic acid and protein contents. Morphometric analysis was performed for different variables. Deprivation of dietary nucleotides resulted in a reduction in hepatocyte nuclear and nucleolar areas as well as in nuclear chromatin condensation. In addition, the rough endoplasmic reticulum was reduced, as were ribosome association and abundance, whereas fat accumulated. These findings portray dietary nucleotides as required nutrients for the liver under normal physiological conditions and suggest that an inadequate supply of nucleotides for a certain period of time has transient negative effects on liver ultrastructure and function.

Pyrimidine nucleotide metabolism in rat hepatocytes: evidence for compartmentation of nucleotide pools

Pyrimidine nucleotide metabolism in rat hepatocytes was studied by measurement of the labelling kinetics of the various intermediates after double labelling with [14C]orotic acid and [3H]cytidine, the precursors for the de novo and the salvage pathways respectively. For the uridine nucleotides, differences were found for the 14C/3H ratios in the UDP-sugars, in UMP (of RNA) and in their precursor UTP, suggesting the existence of separated flows of the radioactive precursors through the de novo and the salvage pathways. Higher ratios in the UDP-sugars, which are synthesized in the cytoplasm, and a lower ratio in UMP (of RNA) relative to the 14C/3H ratio in UTP indicated that UTP derived from orotic acid is preferentially used for the cytoplasmic biosynthesis of the UDP-sugars. Uridine, derived from cytidine, is preferentially used for the nuclear-localized synthesis of RNA. In contrast to these findings, the 14C/3H ratios in the cytidine derivatives CMP-NeuAc and CMP (of RNA), and in the liponucleotides CDP-choline and CDP-ethanolamine, were all lower than that in the precursor CTP. This indicates a preferential utilization of the salvage-derived CTP for the synthesis of the liponucleotides as well as for RNA and CMP-NeuAc. Similar conclusions could be drawn from experiments in which the intracellular amounts of several uridine- and cytidine-nucleotide-containing derivatives were increased by preincubating the hepatocytes with unlabelled pyrimidine nucleotides or ethanolamine. Based on these data, we propose a refined model for the intracellular compartmentation of pyrimidine nucleotide biosynthesis in which three pools of UTP are distinguished: a pool of de novo-derived molecules and a pool of salvage-derived molecules, both of which are channelled to the site of utilization; in addition an 'overflow' pool exists, consisting of molecules having escaped from channelling. An overflow pool could also be distinguished for CTP, but no discrimination between de novo and salvage-derived molecules could be made.