Purine metabolism in murine virus-induced erythroleukemic cells during differentiation in vitro.
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Purine metabolism was studied in murine virus-induced erythroleukemia cells stimulated to differentiate in vitro in the presence of dimethylsulfoxide. The activities of the enzymes that catalyze the synthesis of the first intermediate of the de novo purine pathway, phosphoribosyl-1-amine, were decreased while the enzymes that catalyze the conversion of purine bases to purine ribonucleotides remained unchanged at the time the cells acquired the specialized function of hemoglobin synthesis. In addition, cytidine deaminase (cytidine aminohydrolase, EC 3.5.4.5) activity increased with erythropoietic maturation, as it does during murine erythropoiesis in vivo. Stimulation of cellular proliferation of stationary erythroleukemic cells resulted in a marked increase in the activities of purine biosynthetic enzymes. These data provide a convincing example of repression and derepression of the PRA synthesizing enzymes in mammalian cells in vitro, and further evidence that the regulatory mechanisms operative in the normal development of erythrocytes can be activated by exposure of erythroleukemic cells to dimethylsulfoxide. (+info)
Wheat bran affects the site of fermentation of resistant starch and luminal indexes related to colon cancer risk: a study in pigs.
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BACKGROUND: Recent studies suggest that resistant starch (effective in producing butyrate and lowering possibly toxic ammonia) is rapidly fermented in the proximal colon; the distal colon especially would, however, benefit from these properties of resistant starch. AIMS: To determine whether wheat bran (a rich source of insoluble non-starch polysaccharides), known to hasten gastrointestinal transit, could carry resistant starch through to the distal colon and thus shift its site of fermentation. METHODS: Twenty four pigs were fed four human type diets: a control diet, or control diet supplemented with resistant starch, wheat bran, or both. Intestinal contents and faeces were collected after two weeks. RESULTS: Without wheat bran, resistant starch was rapidly fermented in the caecum and proximal colon. Supplementation with wheat bran inhibited the caecal fermentation of resistant starch, resulting in an almost twofold increase (from 12.9 (2.5) to 20.5 (2.1) g/day, p<0.05) in resistant starch being fermented between the proximal colon and faeces. This resulted in higher butyrate (133%, p<0.05) and lower ammonia (81%, p<0.05) concentrations in the distal colonic regions. CONCLUSIONS: Wheat bran can shift the fermentation of resistant starch further distally, thereby improving the luminal conditions in the distal colonic regions where tumours most commonly occur. Therefore, the combined consumption of resistant starch and insoluble non-starch polysaccharides may contribute to the dietary modulation of colon cancer risk. (+info)
Upper airway reactivity and upper respiratory tract infection: effect of nebulized lidocaine.
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Patients presenting for elective anaesthesia and surgery may be suffering with, or recovering from, a recent upper respiratory tract infection. Airway reflexes are heightened and these individuals may be more likely to suffer airway complications on administration of general anaesthesia. We have examined the effect of nebulized lidocaine on upper airway reflexes in such subjects. Using dilute ammonia as a chemical stimulus to the upper airway, we measured upper airway reactivity in 15 volunteers (aged 22-43 yr) with symptoms of an upper respiratory tract infection for 4 days or less. The threshold concentration of ammonia producing a brief reduction in inspiratory flow was determined. Measurements were made before and after administration of a nebulized solution of 4% lidocaine 4 ml or saline. After a 2-h interval the procedure was repeated with the alternative solution. The order of administration was randomized. The observer was blind to the solution given. Ammonia threshold was found to increase in subjects after nebulized lidocaine, from a median value of 327 (range 76-878) ppm to 878 (251-1620) ppm (P = 0.0007, Wilcoxon); there was no significant change after nebulized saline. After a convalescence period of at least 4 weeks, with no return of symptoms in the preceding 2 weeks, ammonia threshold was reassessed. It was found to be increased in all 15 subjects. Comparison of the five different times of measurement showed a highly significant difference (P < 0.001, Friedman). Subsequent analysis showed significant differences (P < 0.05, Wilcoxon) between convalescent ammonia threshold and both baseline and post-saline nebulizer values. There was no significant difference between convalescent and post-lidocaine ammonia threshold. We conclude that in adult subjects, nebulized lidocaine attenuated the heightened airway reflex sensitivity associated with symptoms and signs of upper respiratory tract infection. (+info)
Molecular orbital studies of enzyme activity: I: Charge relay system and tetrahedral intermediate in acylation of serine proteinases.
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The charge relay ststem and its role in the acylation of serine proteinases is studied using the partial retention of diatomic differential overlap (PRDDO) technique to perform approximate ab initio molecular orbital calculations on a model of the enzyme-substrate complex. The aspartate in the charge relay system is seen to act as the ultimate proton acceptor during the charging of the serine nucleophile. A projection of the potential energy surface is obtained in a subspace corresponding to this charge transfer and to the coupled motions of active site residues and the substrate. These results together with extended basis set results for cruder models suggest that a concerted transfer of protons from Ser-195 to His-57 and from His-57 to Asp-102 occurs with an energy barrier of 20-25 kcal/mole (84-105 kJ/mole). The subsequent nucleophilic attack on the scissile peptide linkage by the charged serine is then seen to proceed energetically downhill to the tetrahedral intermediate. The formation of the tetrahedral intermediate from the Michaelis complex is calculated to be nearly thermoneutral. (+info)
Ammonia as an attractive component of host odour for the yellow fever mosquito, Aedes aegypti.
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Behavioural responses of Aedes aegypti mosquitoes to ammonia were investigated in a modified Y-tube olfactometer. Ammonia was attractive in concentrations from 17 ppb to 17 ppm in air when presented together with lactic acid. Aqueous solutions of ammonia salts in concentrations comparable to those found in human sweat also increased the attractiveness of lactic acid. The role of lactic acid as an essential synergist for ammonia became further apparent by the fact that ammonia alone or in combination with carbon dioxide was not effective, even though the synergistic effect of carbon dioxide and lactic acid was corroborated. An extract from human skin residues, which attracts approximately 80% of the tested mosquitoes, contains both lactic acid and ammonia. The combination of these compounds, however, attracts no more than 45%, indicating that other components on human skin also play a role in host finding. Preparative liquid chromatography of the skin extract yielded three behaviourally active fractions which work together synergistically. Fraction III contains lactic acid as the effective principle; the compositions of the other two have not been clarified yet. The attractiveness of fraction I was augmented considerably when ammonia was added, whereas the effect of fraction II was not influenced by ammonia. These results suggests that ammonia is part of the effective principle of fraction II and contributes to the attractive effect of host odours. (+info)
The mudskipper, Periophthalmodon schlosseri, actively transports NH4+ against a concentration gradient.
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Periophthalmodon schlosseri can maintain ammonia excretion rates and low levels of ammonia in its tissues when exposed to 8 and 30 mM NH4Cl, but tissue ammonia levels rise when the fish is exposed to 100 mM NH4Cl in 50% seawater. Because the transepithelial potential is not high enough to maintain the NH4+ concentration gradient between blood and water, ammonia excretion under such a condition would appear to be active. Branchial Na+-K+-ATPase activity is very high and can be activated by physiological levels of NH4+ instead of K+. Ammonia excretion by the fish against a concentration gradient is inhibited by the addition of ouabain and amiloride to the external medium. It is concluded that Na+-K+-ATPase and an Na+/H+ exchanger may be involved in the active excretion of ammonia across the gills. This unique ability of P. schlosseri to actively excrete ammonia is related to the special structure of its gills and allows the fish to continue to excrete ammonia while air exposed or in its burrow. (+info)
In rat tIMCD, NH4+ uptake by Na+-K+-ATPase is critical to net acid secretion during chronic hypokalemia.
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The purpose of this study was to determine the magnitude of Na+ pump-mediated NH4+ uptake in the terminal inner medullary collecting duct (tIMCD) at K+ and NH4+ concentrations observed in vivo in the inner medullary interstitium of normal and in K+-restricted rats. Interstitial K+ and NH4+ concentrations in the terminal half of the inner medulla were taken to be 10 and 6 mM in K+-restricted rats, but 30 and 6 mM in K+-replete rats. In tubules from K+-restricted rats, when perfused at a K+ concentration of 10 mM, addition of ouabain to the bath reduced total bicarbonate flux (JtCO2) by 40% and increased intracellular pH (pHi), indicating significant NH4+ uptake by the Na+-K+-ATPase. In tubules from K+-restricted rats, JtCO2 was reduced with increased extracellular K+. At a K+ concentration of 30 mM, ouabain addition neither reduced JtCO2 nor increased pHi in tubules from rats of either treatment group. In conclusion, in the tIMCD from hypokalemic rats, 1) acute changes in extracellular K+ concentration modulate net acid secretion, and 2) Na+ pump-mediated NH4+ uptake should be an important pathway mediating transepithelial net acid secretion in vivo. (+info)
Turning on of activities in unfertilized sea urchin eggs: correlation with changes of the surface.
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Unfertilized sea urchin eggs exposed to low concentrations of ammonia enter into a number of activities which normally appear after fertilization. It is shown that the effects are attributable to ammonia, rather than to NH4+ ions of elevated pH. The same effects are obtained by exposure to isotonic urea and to glycerol at very low ionic strengths. All treatments which produce these changes (such as the turning on of chromosome replication and condensation in unfertilized eggs) also bring about changes of the outer cell surface which are visible in the scanning electron microscope. The most striking indicator is the elongation of the microvilli which cover the surface of the unfertilized egg. The changes of the surface are interpreted as the dissociation of a component from the outer surface layer. This component is not the "vitelline" sheet as defined morphologically or by the ability of the egg to form a fertilization membrane upon insemination. It is proposed further that this component is a peripheral component of the plasma membrane, whose removal modifies the membrane functionally and leads to the derepression of various processes within the egg. (+info)