Pentose phosphate pathway in rat colonic epithelium

The colonic cells of the large intestine are one of the most proliferative tissues of the animal body. The pentose pathway has an essential role in cell division and growth being the only pathway forming ribose 5-P necessary for all nucleotide and nucleic acid sunthesis. The pentose pathway may also provide reducing potential as NADPH for biosynthesis and C-3- C-8 glycolyl compounds. The maximum catalytic capacities of the reactions of the non-oxidative pentose pathway for the conversion of ribose 5-P to hexose and triose phosphates by the proximal and distal colon under feeding and starvation regimes are among the highest in the animal body. The qualitative presence of the oxidative pentose pathway was assessed by measurement of the C-1/C-6 ratio value of 1.67-1.82. Enzymes of the F-type and L-type pentose pathways are present in colonocytes and their maximum catalytic activities in colonocyte cytosol are reported. The contribution of the F-type pentose cycle to the total glucose metabolism of colonocytes, measured by the specific yield method, is negligibly low (approximately 1.5%). Colonic epithelial cells use glucose at a high rate (7.1 +/- 0.33 mumol min-1g-1 dry wt) and 79% of the glucose is converted to lactate. Arabinose 5-P has an intermediary role in the formation of keto pentose, sedoheptulose and hexose phosphates from ribose 5-P by colonocyte cytosol. The intermediary and reaction products of [1-13C] ribose 5-P dissimilation by colonocytes is investigated by 13C NMR spectroscopy. The 13C positional isotope distributions show labelling of C-1 and C-3 of hexose 6-phosphates consistent with either the theoretical predictions of the F-type pentose pathway or of the activities of exchange reactions catalysed by transketolase and/or transaldolase. Measurements of exchange reactions showed that the C-1/C-3 labelling of these compounds is mostly, if not wholly, attributable to exchange catalysis by these group transferring enzymes. The results suggest that the F-type PC has little role in the glucose metabolism of colonocytes and pentose phosphate formation may thus occur by a contribution (approx 20% of the total glucose metabolism) by the alternate L-type pathway.     

Pentose phosphate pathway mutants of yeast

Summary A glucose-negative mutant of Saccharomyces cerevisiae lacking 6-phosphogluconate dehydrogenase, the second enzyme of the pentose phosphate pathway, has been obtained by inositol starvation. Suppression of this mutant for growth on glucose takes place by the loss of glucose 6-phosphate dehydrogenase. A lesion in the latter enzyme alone leaves growth paractically unaffected. The mutations define the respective structural genes.     

Pentose phosphate pathway and nucleic acid metabolism in red and white muscles of fish

The activities of the pentose phosphate cycle enzymes, the content of nucleic acids and the activities of acid and alkaline RNAses in the heart and red and white muscles of cartilaginous and teleost fish were determined. It was found that the rates of the dehydrogenase and transferase reactions of the pentose phosphate pathway and the nucleic acid metabolism in the red muscles and heart are much higher than those in the white muscles of the species under study.     

Pentose phosphate pathway and nucleic acid synthesis in human funicular tissue. In vitro studies]

Umbilical cord slices were incubated with either 1- or 6-14C-glucose, the radioactivities of which were measured in CO2 evolved. The ratio, 1 CO2/6 CO2 was low, being comprised between 1 and 2. Furthermore, this incubation of cord slices with tritiated uridine or thymidine resulted in very low incorporations, especially for the latter. Therefore, both the pentose phosphate pathway and the synthesis of nucleic acid have a low activity in the cord tissue: these might be signs of senescence in this otherwise fetal organ.     

Pentose phosphate pathway flux analysis for glycopeptide antibiotic vancomycin production during glucose-limited cultivation of Amycolatopsis orientalis

In vivo pentose phosphate pathway (PPP) enzymes such as glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), and transaldolase (TAL) activities as well as ATP- and ADP-level variations of Amycolatopsis orientalis were investigated with respect to glucose concentration and incubation period. G6PDH, 6PGDH, and TAL activities of A. orientalis reached maximum levels at 48 hr for all glucose concentrations used, after which the levels began to decline. G6PDH, 6PGDH, and TAL activities showed positive correlation with the glucose concentration up to 15 g/L, while further increases had an opposite effect. Intracellular ATP level showed a positive correlation with glucose concentrations, while ADP level increased up to 15 g/L. ATP concentration of A. orientalis increased rapidly at 48 hr of incubation, as was the case also for G6PDH, 6PGDH, and TAL activities, although the incubation period corresponding to maximum values of ADP shifted to 60 hr. Production of the glycopeptide antibiotic vancomycin increased with the increases in glucose concentrations up to 15 g/L, by showing coherence in the rates of oxidative and nonoxidative parts of the PPP.     

Determination of pentose phosphate and Embden-Meyerhof pathway activities in bovine embryos

Quantitative determination was made of the activity of pentose phosphate pathway (PPP) and Embden-Meyerhof pathway (EMP) in individual bovine embryos from the six-cell to the hatched blastocyst stage. Embryos were collected from superovulated cross-bred heifers and classified into good and poor categories. A single embryo in 1 ul of medium was mixed with 2 ul of medium containing 3 to 30 nCi radiolabeled glucose previously placed on a detached lid of the 1.5-ml polypropylene microcentrifugé vial. The lid was then fitted to its vial which had been loaded in advance with 1.5 ml of 0.1 N NaOH. Vials were then incubated at 37 degrees C for 3 h. At the end of the incubation period, a 1.5-ml NaOH trap was inverted and placed into a 20-ml scintillation vial containing 10 ml of aqueous counting solution and counted in a liquid scintillation spectrophotometer. The PPP activity in good-quality embryos was greatest at the six-cell stage and decreased with increasing embryo development. The EMP activity showed the reverse trend. Poor- quality embryos had a lower glucose metabolism and higher PPP activity. Similar measurements were made on embryos following 24 h of culture, and total glucose metabolism and percentage of PPP activity were increased. In conclusion, these data suggest that in good quality bovine embryos total glucose utilization is low until 16-cell stage, with PPP being the predominant pathway. Total glucose utilization increases significantly at the morula stage; EMP activity increases with increasing embryo development; and PPP activity increases significantly in poor quality embryos and in embryos 24 h in culture.     

Pentose cycle pathway in normal and tumoral islet cells

Relative to protein content, the activity of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase and the rate of glucose metabolism by the pentose cycle pathway in tumoral insulin-producing cells were similar to or higher than those found in normal rat islets. Hence, the decreased secretory response of tumoral cells to glucose is apparently not attributable to any major anomaly in glucose handling by the hexose monophosphate pathway.     

N-terminal chromogranin-derived peptides as dilators of bovine coronary resistance arteries

N-terminal peptides of chromogranin A and B (CGA and CGB) were compared for dilator responses in isolated bovine coronary arteries (bCoA), measuring diameter changes as a function of pressure. bCoA developed and maintained myogenic tone (MYT) at approximately 20% from 50 to 150 mm Hg. In contrast to CGB(1-40), CGA(1-40) and CGA(1-76) (VS-I) both displayed significant intrinsic vasodilator effects. CGA(1-40) reduced myogenic reactivity from 70 to 150 mm Hg (p<0.05, n=6). At 75 mm Hg, CGA(1-40) showed a concentration-dependent dilatation at 0.1 nM-10 microM. The dilator effect of CGA(1-40) persisted at moderately elevated [K(+)](e) (8.4-16 mM). However, this effect was diminished by pertussis toxin (PTX) and abolished by antagonists to several subtypes of K(+) channels (tetraethylammonium, Ba(2+) and glibenclamide). These results demonstrate that the N-terminal domain of CGA has dilator effect in the myogenically active bCoA. We propose that CGA(1-40) and the naturally occurring vasostatin I are regulatory peptides of relevance for the coronary microcirculation and that a G(alphai) sub-unit and K(+) channel activation may be involved in the signal pathway.     

Pentose phosphate metabolism during dormancy breakage in Corylus avellana L.

Commercially obtained fruits of Corylus avellana exhibit the characteristic loss of dormancy of this seed following chilling under moist conditions. The activities of cytosolic and organellar enzymes of pentose phosphate pathway in cotyledonary tissue were assayed throughout stratification and over a similar period in damp vermiculite at 20° C. Glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconic acid dehydrogenase (6PGDH) were both found in cytosolic extracts in all treatments; only 6PGDH was present in the organellar fraction.The enzyme activities monitored in seeds at 20° C remained relatively constant over the course of the investigation except in the case of cytosolic 6PGDH where it is suggested an inhibitor of the enzyme accumulated. This inhibitor was removed by the partial purification procedure. Increases in the activities of the enzymes occurred during stratification, the major increase coinciding exactly with dormancy breakage but prior to the initiation of germination. The marked increase in G6PDH and 6PGDH concurrent with the change in germination potential of the chilled seed may have considerable biochemical significance in breaking down the dormant state.Abbreviations G6P glucose-6-phosphate - G6PDH glucose-6 phosphate dehydrogenase - NADP nicotinamide adenine dinucleotide phosphate - 6 PGDH 6-phosphogluconic acid dehydrogenase - PPP pentose phosphate pathway     

Hypoxia promotes relaxation of bovine coronary arteries through lowering cytosolic NADPH

Hypoxia relaxes endothelium-denuded bovine coronary arteries (BCA) through mechanisms that do not appear to involve reactive oxygen species, prostaglandins, or nitric oxide. Because of similarities in the relaxation of BCA to hypoxia (Po(2) = 8-10 Torr) and inhibitors of the pentose phosphate pathway (PPP) including 6-aminonicotinamide and epiandrosterone, we measured NADPH and NADP and found that hypoxia caused NADPH oxidation (decreased NADPH/NADP). The relaxation to hypoxia was similar to previously reported properties of relaxation to PPP inhibitors in that both responses were associated with glutathione oxidation and depressed intracellular calcium release and calcium influx-mediated contractile responses. Inhibitors of potassium channels had minimal effects on these relaxation responses. Relaxation to hypoxia and PPP inhibitors were attenuated by a thiol reductant (3 mM dithiothreitol) and by eliciting contraction with an activator of protein kinase C (phorbol 12,13-dibutyrate). In the presence of contraction to U-46619, relaxation to hypoxia and PPP inhibitors were attenuated by the sarco(endo)plasmic reticulum Ca(2+)-ATPase pump inhibitor 200 microM cyclopiazonic acid and by 10 mM pyruvate. Hypoxia decreased BCA levels of glucose-6-phosphate but not ATP. Pyruvate prevented the hypoxia-elicited decrease in glucose-6-phosphate and glutathione oxidation, and it increased NADPH levels under hypoxia to levels observed under normoxia. Thus hypoxia causes a metabolic stress on the PPP that promotes BCA relaxation through processes controlled by lowering the levels of cytosolic NADPH.     

Functional identification of epithelial and smooth muscle histamine-dependent relaxing mechanisms in the bovine trachea,but not in bronchi

Theoretically, the overall effect of histamine on respiratory smooth muscle is the result of a subtle balance of contraction and relaxation. The aim of the study was to identify histamine type 2 (H2) and 3 (H3) receptor-dependent relaxing mechanisms in the contractile elements of the bovine tracheobronchial tree. In bronchial preparations, histamine induced very weak contractions, which were not exacerbated with the H2-antagonist cimetidine. Moreover, precontracted bronchial rings never relaxed in response to cumulative doses of histamine or amthamine (H2-agonist). In intact tracheal preparations, histamine induced strong contractions that were exacerbated by cimetidine (E(max): +17.2+/-6.6%) but not by thioperamide (H3-antagonist). Precontracted tracheal bundles did not relax in response to cumulative doses of the H3-agonist R-alpha-methylhistamine. The tracheal contractile response was higher in denuded compared to intact preparations (11.0+/-1.2 vs. 6.0+/-1.7 g). Cimetidine effect was dramatically potentiated in denuded tracheal strips (+40.0+/-11.7%). It is concluded that the weak response of bovine bronchi to histamine is due to a relative scarcity of H1 receptors on bronchial smooth muscle rather than to H2- or H3-dependent relaxation. In the bovine trachea, the smooth muscle possesses relaxing H2 but no H3 receptors. The epithelium exercises a relaxation, which is independent from H2 and H3 receptors.     

Endothelium-derived 2-arachidonylglycerol: an intermediate in vasodilatory eicosanoid release in bovine coronary arteries

Acetylcholine stimulates the release of endothelium-derived arachidonic acid (AA) metabolites including prostacyclin and epoxyeicosatrienoic acids (EETs), which relax coronary arteries. However, mechanisms of endothelial cell (EC) AA activation remain undefined. We propose that 2-arachidonylglycerol (2-AG) plays an important role in this pathway. An AA metabolite isolated from bovine coronary ECs was identified as 2-AG by mass spectrometry. In ECs pretreated with the fatty acid amidohydrolase inhibitor diazomethylarachidonyl ketone (DAK; 20 micromol/l), methacholine (10 micromol/l)-stimulated 2-AG release was blocked by the phospholipase C inhibitor U-73122 (10 micromol/l) or the diacylglycerol lipase inhibitor RHC-80267 (40 micromol/l). In U-46619-preconstricted bovine coronary arterial rings, 2-AG relaxations averaging 100% at 10 micromol/l were inhibited by endothelium removal, by DAK, by the hydrolase inhibitor methyl arachidonylfluorophosphate (10 micromol/l), by the cyclooxygenase inhibitor indomethacin (10 micromol/l), but not by the CB1 cannabinoid receptor antagonist SR-141716 (1 micromol/l). The cytochrome P-450 inhibitor SKF-525a (10 micromol/l) and the 14,15-epoxyeicosa-5Z-enoic acid EET antagonist (14,15-EEZE; 10 micromol/l) further attenuated the indomethacin-resistant relaxations. The nonhydrolyzable 2-AG analogs noladin ether, 2-AG amide, and 14,15-EET glycerol amide did not induce relaxation. N-nitro-L-arginine-resistant relaxations to methacholine were also inhibited by U-73122, RHC-80267, and DAK. 14,15-EET glycerol ester increased opening of large-conductance K(+) channels 12-fold in cell-attached patches of isolated smooth muscle cells and induced relaxations averaging 95%. These results suggest that methacholine stimulates EC 2-AG production through phospholipase C and diacylglycerol lipase activation. 2-AG is further hydrolyzed to AA, which is metabolized to vasoactive eicosanoids. These studies reveal a role for 2-AG in EC AA release and the regulation of coronary tone.