Regulation and roles of phosphoenolpyruvate carboxykinase in plants

Phosphoenolpyruvate carboxykinase (PCK) is probably ubiquitous in flowering plants, but is confined to certain cells or tissues. It is regulated by phosphorylation, which renders it less active by altering both its substrate affinities and its sensitivity to regulation by adenylates. In the leaves of some C4 plants, such as Panicum maximum, dephosphorylation increases its activity in the light. In other tissues such regulation probably avoids futile cycling between phosphoenolpyruvate and oxaloacetate. Although PCK generally acts as a decarboxylase in plants, its affinity for CO2 measured at physiological concentrations of metal ions is high and would allow it to be freely reversible in vivo. While its function in gluconeogenesis in seeds postgermination and in leaves of C4 and crassulacean acid metabolism plants is clearly established, the possible functions of PCK in other plant cells are discussed, drawing parallels with those in animals, including its integrated function in cataplerosis, nitrogen metabolism, pH regulation, and gluconeogenesis.     

Carboxyhemoglobin formation following smoke inhalation injury in sheep is interrelated with pulmonary shunt fraction

Carboxyhemoglobin (COHb) formation is triggered by the inducible isoform of heme oxygenase (HO-1) catalyzing carbon monoxide (CO) production through breakdown of heme molecules, exposure to CO or both. In the setting of CO poisoning, COHb is regarded as a reliable marker characterizing both severity of injury and efficacy of treatment strategies. This study was designed as a prospective laboratory experiment to elucidate potential interdependencies between COHb generation, oxygenation, and pulmonary shunt fraction (Qs/Qt) in an ovine model of smoke inhalation injury. Chronically instrumented ewes (n=15) were repeatedly subjected to cotton smoke (4 x 12 breaths) according to an established protocol. This approach resulted in a progressive increase in COHb formation that was interrelated with the degree of Qs/Qt (P<0.001) and inversely correlated with both arterial and mixed venous HbO(2) saturation (r=-0.96 and -0.93). Although the arteriovenous COHb gradient successively decreased over time, COHb determined in venous blood underestimated the arterial content.     

A computer-aided quantum chemical study of the N<Stack><Subscript>15</Subscript><Superscript>−</Superscript></Stack> cluster

Ab initio (RHF, MP2) and Density Functional Theory (DFT) methods have been used to examine six isomers of the N15m cluster with the 6-31+G* basis set. Different from the known odd-numbered anionic N7m, N9m, and N11m clusters, in which the open-chain structures are the most stable species, the most stable N15m isomer is structure 1 (C1), which may be considered as a complex between the fragments cyclic N5m (D5h) and staggered N10 (D2d). The decomposition pathways of structure 2 (CS), containing two aromatic N5 rings connected by a N5 chain, and the open-chain structure 3 (C2v) were studied at the B3LYP/6-31+G* level of theory. Relative energies were refined at the level of B3LYP/6-311+G(3df,2p)//B3LYP/6-31+G*+ZPE (B3LYP/6-31+G*). The barriers for N2 and N5m (D5h) fission reactions for structure 2 are predicted to be 18.2 and 14.2 kcal x mol(-1), respectively. The corresponding N2+N3m fission barrier for structure 3 is predicted to be 11.2 kcal x mol(-1). Supplementary material is available for this article if you access the article at http://dx.doi.org/10.1007/s00894-003-0118-0. A link in the frame on the left on that page takes you directly to the supplementary material. Figure Structure 1 of the N15m cluster, showing bond distances in A and bond angles in degrees     

Growth and net photosynthetic rates of <Emphasis Type="Italic">Hosta</Emphasis> ‘blue vision’ during acclimatization in bright,natural light with CO<Subscript>2</Subscript> enrichment

Summary Hosta ‘Blue Vision’, a shade-adapted perennial, was successfully acclimatized in high, natural light conditions in the research Acclimatron^TM at Clemson University, Clemson, SC during the summer of 2000. The supplemental CO₂ levels achieved during acclimatization were 710±113, 2396±121, and 5641±119 μmol mol⁻¹, approximately 2×, 6×, and 15× ambient CO₂. Plants were maintained in H₂O-saturated atmospheres and protected from temperature increases associated with high light intensity. In the 5 wk following ex vitro transfer, plantlet roots grew at the 2× CO₂ level, but shoot biomass was unaffected. Results for the 6× and 15× CO₂ levels were comparable and provided the best plantlet growth. The “doubling time’ that is characteristic of exponential growth was 10.8 and 9.8 d for root and shoot dry weights, respectively. There was no indication of light saturation of net photosynthetic rate (NPR) over the photosynthetic photon flux density (PPFD) range of 100–1200 μmolm⁻²s⁻¹ experienced during this study. An interaction between CO₂ and light intensity levels was detected for NPR of Hosta ‘Blue Vision’ with CO₂ saturation occurring at approximately 2800 μmol mol⁻¹. regardless of light level. Furthermore, at the optimal CO₂ level, NPR increased quadratically as light intensity increased, and NPR was greatest at the maximum light intensity (PPFD: 1200 μmol m⁻²s⁻¹).     

Do mammals,birds, reptiles and fish have similar nitrogen conserving systems?

Comparative physiological studies are a powerful tool for revealing common animal adaptations. Amino acid catabolism produces ammonia which is detoxified through the synthesis of urea (mammals, some fish), uric acid (birds), or urea and uric acid (reptiles). In mammalian herbivores and omnivores, urea nitrogen is salvaged by a series of steps involving urea transfer into the intestine, microbial mediated urea hydrolysis with synthesis of amino acids utilizing the liberated ammonia and transfer of the amino acids back to the host. A similar series of steps occur in omnivorous/granivorous and herbivorous birds, although in this case urine, containing uric acid, is refluxed directly into the intestine where microbes degrade the uric acid and utilize the liberated ammonia for amino acid synthesis. These amino acids are transferred back to the host. In reptiles and ureotelic fish not all of these steps have been experimentally confirmed. Reptiles like birds, reflux urine into the intestine where it is exposed to the microflora. However, the capacity of these microbes to breakdown the uric acid and urea and utilize ammonia for amino acid synthesis has not been documented. Ureotelic fish transfer urea into the intestine where urease (presumably of bacterial origin) hydrolyzes the urea. However, the amino acid synthesizing capacity of the intestinal microflora has not been studied. The series of steps, as outlined, would define the prevailing nitrogen conservation system for herbivores and omnivores at least. However, it would appear that some animals, in particular the fruit-eating bat and perhaps the fruit-eating bird, may have evolved alternative, as yet uncharacterized, adaptations to a very limited nitrogen intake.     

The in vivo nitrogen isotope discrimination among organic plant compounds

The bulk delta 15 N-value of plant (leaf) biomass is determined by that of the inorganic primary nitrogen sources NO(3)(-), NH(4)(+) and N(2), and by isotope discriminations on their uptake or assimilation. NH(4)(+) from these is transferred into "organic N" mainly by the glutamine synthetase reaction. The involved kinetic nitrogen isotope effect does not become manifest, because the turnover is quantitative. From the product glutamine any further conversion proceeds in a "closed system", where kinetic isotope effects become only efficient in connection with metabolic branching. The central and most important corresponding process is the GOGAT-reaction, involved in the de novo nitrogen binding and in recycling processes like the phenylpropanoid biosynthesis and photorespiration. The reaction yields relatively 15N-depleted glutamate and remaining glutamine, source of 15N-enriched amide-N in heteroaromatic compounds. Glutamate provides nitrogen for all amino acids and some other compounds with different 15N-abundances. An isotope equilibration is not connected to transamination; the relative delta 15 N-value of individual amino acids is determined by their metabolic tasks. Relative to the bulk delta 15 N-value of the plant cell, proteins are generally 15N-enriched, secondary products like chlorophyll, lipids, amino sugars and alkaloids are depleted in 15N. Global delta 15 N-values and 15N-patterns of compounds with several N-atoms can be calculated from those of their precursors and isotope discriminations in their biosyntheses.     

Photosynthetic energy conversion under extreme conditions--I: important role of lipids as structural modulators and energy sink under N-limited growth in Antarctic sea ice diatoms

The availability of dissolved nutrients such as nitrate under extreme low temperatures is a strong determinant in the development and growth of ice diatoms. Consequently we investigated regulation of photosynthesis in a mixed culture of three diatom species, which grew in chemostats at -1 degrees C, 15 micromol photons m(-2) s(-1) under N-limitation. When nitrogen is limiting, pigment-protein complexes are one of the most affected structures under low-light conditions. The loss of integral polar thylakoid components destabilized the bilayer structure of the membrane with consequences for lipid composition and the degree of fatty acid desaturation. N-Limitation caused a decrease in monogalactosydiacylglycerol (MGDG) and a simultaneous increase in bilayer forming digalactosyldiacylglycerol (DGDG). Their ratio MGDG:DGDG decreased from 3.4 +/- 0.1 to 1.1 +/- 0.4, while 20:5 n-3 fatty acids of chloroplast related phospholipid classes such as phosphatidylglycerol (PG) increased under N-limitation. These data reveal that lipids are important components, required to sustain membrane structure under a deficiency of integral membrane bound proteins and pigments. Nonetheless, energy conversion at photosystem II is still affected by N-limitation despite this structural regulation. Photosynthetic quantum yield (F(v)/F(m)) and electron transport rates decreased under N-limitation caused by an increasing amount of electron acceptors (second stable electron acceptor = Q(B)) which had slower reoxidation kinetics. The energy surplus under these conditions is stored in triacylglycerols, the main energy sink in Antarctic sea ice diatoms under N-limitation.     

Description of the cytochrome c oxidase subunit II gene in some genera of New World monkeys (primates,Platyrrhini)

Nucleotide sequence variation at the mitochondrial cytochrome c oxidase subunit II gene (COII) was analyzed in 27 New World monkey specimens, nine newly reported herein. The study involved comparisons among platyrrhines and also between platyrrhines and catarrhines. The analysis of the frequencies of transitions and transversions at each codon position showed transitional saturation at third codon position. Neighbor-Joining trees obtained from genetic distances estimated by means of Kimura's (1980) two-parameter model showed poor resolution of phylogenetic relationships among platyrrhine genera. Rates of nucleotide substitutions were largely homogeneous except in the genus Saimiri that showed low numbers of unique substitutions suggesting the maintenance of ancestral or plesiomorphic states of platyrrhine mt DNA nucleotide characters probably due to its large population sizes as compared to other platyrrhines.     

High rate of aerobic nitrification and denitrification by Nitrosomonas eutropha grown in a fermentor with complete biomass retention in the presence of gaseous NO2 or NO

A pure culture of the obligately lithoautotrophic ammonia-oxidizer Nitrosomonas eutropha was grown in a laboratory-scale bioreactor with complete biomass retention. The air supply was supplemented with nitrogen dioxide (NO₂; 25 or 50 ppm) or nitric oxide (NO; 25 or 50 ppm). Compared to cultures grown without these nitrogenous oxides, the addition of NO₂ or NO to the culture resulted in a significant increase of the nitrification rate, specific activity of ammonia oxidation, growth rate, and maximum cell densities. In contrast, the growth yield slightly decreased in the presence of NO or NO₂. Maximum cell densities of about 2 × 10¹⁰ cells ml^–1 and a maximum nitrification rate of about 221 mmol NH₄ ⁺ l^–1 day^–1 were obtained after 3 weeks in the presence of 50 ppm NO₂. Furthermore, in the stationary phase about 50% of the nitrite produced was aerobically denitrified to dinitrogen (N₂) and traces of nitrous oxide (N₂O). When cells were supplemented with NO, a high rate of aerobic denitrification occurred only during the first days of the exponential growth phase. Received: 12 May 1997 / Accepted: 10 November 1997     

The rnf gene products in Rhodobacter capsulatus play an essential role in nitrogen fixation during anaerobic DMSO-dependent growth in the dark

The rnf genes in Rhodobacter capsulatus are essential for nitrogen fixation in the light. Because R. capsulatus grows readily on N₂ in the dark by anaerobic respiration with dimethylsulfoxide, the diazotrophic capacities of various strains in the dark were examined. No rnf mutants tested grew diazotrophically, and a nonpolar fdxN-null mutant showed decreased diazotrophic growth in the dark, suggesting that the Rnf and FdxN proteins form the primary electron donor pathway to nitrogenase in the dark as well as in the light. Nonphotosynthetic mutants lacking the component of cyclic electron transport grew diazotrophically and the levels of Rnf proteins were similar to those of the wild-type. These results indicate that rnf gene products play an essential role in nitrogen fixation without any functional link to the cyclic electron transport system. Received: 19 August 1997 / Accepted: 20 January 1998