Morphological pattern of development affects the contribution of nitrogen reserves to regrowth of defoliated white clover (Trifolium repens L.)

The contribution of nitrogen reserves to regrowth following defoliation was studied in white clover plants (Trifolium repens cv. Huia). This was found to be closely linked to the morphological pattern of development of the aerial parts during the same period. Low temperature (6 degrees C) and short day exposure (8 h photoperiod) were used to induce dwarf development, i.e. to increase branching rate and to enhance new sites of leaf production during a period of regrowth. Treated plants exhibited a large reduction in leaf area and a large increase in leaf pool size for the first 10 d of a subsequent regrowth under standard culture conditions (16 h daylight; 22/18 degrees C day/night). The contribution of nitrogen from storage compounds in organs remaining after defoliation (sources) to regrowing tissues (sinks) was assessed by 15N pulse-chase labelling during regrowth following shoot removal. The mobilization of nitrogen reserves from storage tissues of regrowing clover was closely linked to the pattern of differentiation of the newly developed organs. It appeared that regrowth was supported less by endogenous N for the first 10 d after defoliation in treated plants, compared with control plants grown continuously in standard conditions. It is assumed that dwarf plants exhibit a lower dependence upon the mobilization of soluble proteins previously accumulated in roots and uncut stolons. The relationship between leaf development rate and N-uptake recovery following defoliation is discussed.

Factors that influence assimilation rates and fractionation of nitrogen and carbon stable isotopes in avian blood and feathers

By switching great skuas Catharacta skua from one isotopically distinct diet to another, we measured diet-tissue discrimination factors and tested the assumption that dietary nitrogen and carbon isotope signatures are incorporated into blood and feathers at similar rates. We also examined the effects of metabolic rate and looked for evidence of isotopic routing. We found that blood delta(15)N and delta(13)C signatures altered after the diet switch at similar rates (14.4 d and 15.7 d, respectively). Qualitative analyses imply that the same was true with feathers. Mass balance calculations suggest that only a small amount of lipid is likely to be involved in the synthesis of blood and feathers. Differences in diet-tissue discrimination factors before and after the diet switch may mean that toward the end of the experiment, some of the nutrients for blood synthesis had been coming from stores. Repeated measures mixed models provided evidence that increases in metabolic rate might accelerate fractional turnover rates in blood. There is a need for more laboratory-based experimental isotope studies in order to address further questions that this study has raised.

Exploration of inorganic C and N assimilation by soil microbes with time-of-flight secondary ion mass spectrometry

Stable C and N isotopes have long been used to examine properties of various C and N cycling processes in soils. Unfortunately, relatively large sample sizes are needed for accurate gas phase isotope ratio mass spectrometric analysis. This limitation has prevented researchers from addressing C and N cycling issues on microbially meaningful scales. Here we explored the use of time-of-flight secondary ion mass spectrometry (TOF-SIMS) to detect 13C and 15N assimilation by individual bacterial cells and to quantify N isotope ratios in bacterial samples and individual fungal hyphae. This was accomplished by measuring the relative abundances of mass 26 (12C14N-) and mass 27 (13C14N- and 12C15N-) ions sputtered with a Ga+ probe from cells adhered to an Si contact slide. TOF-SIMS was successfully used to locate and quantify the relative 15N contents of individual hyphae that grew onto Si contact slides in intimate contact with a model organomineral porous matrix composed of kaolin, straw fragments, and freshly deposited manure that was supplemented with 15NO3-. We observed that the 15N content of fungal hyphae grown on the slides was significantly lower in regions where the hyphae were influenced by N-rich manure than in regions influenced by N-deficient straw. This effect occurred over distances of tens to hundreds of microns. Our data illustrate that TOF-SIMS has the potential to locate N-assimilating microorganisms in soil and to quantify the 15N content of cells that have assimilated 15N-labeled mineral N and shows promise as a tool with which to explore the factors controlling microsite heterogeneities in soil.

An isotope-edited FT-IR study of a symporter, the lactose permease

The lactose permease of Escherichia coli transports protons and lactose across the plasma membrane and uses a transmembrane ion gradient as the energy source to drive the uphill accumulation of lactose. In this report, the effect of the electrochemical gradient on the permease has been studied. Bacteriorhodopsin was co-reconstituted with the lactose permease to provide a light-triggered electrochemical gradient. Reaction-induced Fourier transform infrared spectra were acquired, and bacteriorhodopsin contributions were subtracted. In previous work, positive bands in the 1765-1730 cm(-1) region of the reaction-induced FT-IR spectrum were attributed to the perturbation of carboxylic acid residues in the permease [Patzlaff, J. S., Brooker, R. J., and Barry, B. A. (2000) J. Biol. Chem. 275, 28695-28700]. In this study, we have globally labeled the permease with (13)C or (15)N. Isotopic labeling demonstrates that features in the reaction-induced FT-IR spectrum arise from permease carboxylic acid, amide I, and amide II vibrational modes. In addition, isotope labeling leads to a tentative assignment of spectral features to lysine, arginine, histidine, glutamine, and/or asparagine in the permease. These results indicate that the electrochemical gradient causes changes in the environment or protonation state of carboxylic acid residues in the permease and suggest an interaction between these carboxylic acid side chains and nitrogen-containing amino acid side chains. Evidence for a change in secondary structure, corresponding to an interconversion of secondary structural elements, a change in the hydrogen-bonding strength, or coupling of peptide vibrational modes, is also presented. These experiments demonstrate the usefulness of reaction-induced spectroscopy in the study of transmembrane transport.

Stable isotope signature of philopatry and dispersal in a migratory songbird

Stable isotope analysis is widely promoted as a practical method for tracing the geographic origins of migratory birds. However, the extent to which geospatial patterns of isotope ratios in avian tissues are influenced by age-specific, altitudinal, and temporal factors remains largely unexplored. We measured carbon ((13)C/(12)C) and nitrogen ((15)N/(14)N) isotope ratios in feathers of black-throated blue warblers (Dendroica caerulescens) breeding along a relatively steep altitudinal gradient in the Appalachian Mountains to evaluate the effects of altitude and year on the isotopic signatures of yearling (first breeding season) and older males (>2 years). Breeding males (n = 302) collected during 7 consecutive years exhibited significant age-specific and altitudinal effects in delta(13)C values and age-specific and temporal effects in delta(15)N values. The delta(13)C values of older males increased with altitude at the rate of approximately 1.3 per thousand per 1,000 m, suggesting a high degree of year-to-year philopatry to narrow altitudinal zones, if not to breeding territories. In contrast, absence of altitudinal patterns in yearlings most likely reflects natal dispersal. Carbon isotope variation (delta(13)C = -26.07 to -20.86 per thousand) observed along a single altitudinal transect (755 m) nearly brackets the range of delta(13)C values recorded in feathers across the North American breeding range of the warbler from Georgia to New Brunswick (11 degrees of latitude) and from New Brunswick to Michigan (22 degrees of longitude). These data indicate that age-specific and altitudinal effects must be considered when using delta(13)C values to delineate the geographic origin of avian species with large altitudinal and latitudinal ranges.

Production of N(2) through anaerobic ammonium oxidation coupled to nitrate reduction in marine sediments

In the global nitrogen cycle, bacterial denitrification is recognized as the only quantitatively important process that converts fixed nitrogen to atmospheric nitrogen gas, N(2), thereby influencing many aspects of ecosystem function and global biogeochemistry. However, we have found that a process novel to the marine nitrogen cycle, anaerobic oxidation of ammonium coupled to nitrate reduction, contributes substantially to N(2) production in marine sediments. Incubations with (15)N-labeled nitrate or ammonium demonstrated that during this process, N(2) is formed through one-to-one pairing of nitrogen from nitrate and ammonium, which clearly separates the process from denitrification. Nitrite, which accumulated transiently, was likely the oxidant for ammonium, and the process is thus similar to the anammox process known from wastewater bioreactors. Anaerobic ammonium oxidation accounted for 24 and 67% of the total N(2) production at two typical continental shelf sites, whereas it was detectable but insignificant relative to denitrification in a eutrophic coastal bay. However, rates of anaerobic ammonium oxidation were higher in the coastal sediment than at the deepest site and the variability in the relative contribution to N(2) production between sites was related to large differences in rates of denitrification. Thus, the relative importance of anaerobic ammonium oxidation and denitrification in N(2) production appears to be regulated by the availability of their reduced substrates. By shunting nitrogen directly from ammonium to N(2), anaerobic ammonium oxidation promotes the removal of fixed nitrogen in the oceans. The process can explain ammonium deficiencies in anoxic waters and sediments, and it may contribute significantly to oceanic nitrogen budgets.

Effects of a stay-green mutation on plant nitrogen relations in Lolium perenne during N starvation and after defoliation

The stay-green mutation of the nuclear gene sid results in inhibition of chlorophyll degradation during leaf senescence in grasses, reducing N remobilization from senescing leaves. Effects on growth of Lolium perenne L. were investigated during N starvation (over 18 d) and after severe defoliation, when leaf growth depends on the remobilization of internal N. Rates of dry mater production, partitioning between shoots and roots, and re-partitioning of N from shoots to roots were very similar in stay-green and normal plants under N starvation. Km and Vmax for net uptake of NH4+ were also similar for both genotypes, and Vmax increased with the duration of N deprivation. The mutation had little effect on recovery of leaf growth following severe defoliation, but stay-green plants recommenced NO3- and K+ uptake 1 d later than normal plants. Import of remobilized N into new leaves was generally similar in both lines. However, stay-green plants remobilized less N from stubble compared with normal plants. It was concluded that the sid locus stay-green mutation has no significant adverse effect on the growth of L perenne during N starvation, or recovery from severe defoliation when plants are grown under an optimal regime of NO3- supply both before and after defoliation. The absence of any effect on leaf dry matter production implies that the difference in foliar N availability attributable to this mutation has little bearing on productivity, at least in the short to medium term.

Isotopic double-labeling of two honeybee odorant-binding proteins secreted by the methylotrophic yeast Pichia pastoris

Odorant-binding proteins (OBPs) are soluble, low-molecular-weight proteins secreted in the sensillum lymph surrounding the dendrites of olfactory sensilla from a wide range of insect species. These proteins play a role in the solubilization, transport and/or deactivation of pheromones and odorants. In order to study the relationships between the molecular structure in solution and their ligand-binding properties, we have (13)C/(15)N-double-labeled two divergent honeybee OBPs, called ASP1 and ASP2, in sufficient quantities to permit a full determination of the structure and dynamics using heteronuclear NMR spectroscopy. The recombinant labeled proteins produced by the methylotrophic yeast Pichia pastoris have been secreted into a buffered minimal medium using native insect signal peptide. Mass spectrometry and Edman sequencing showed a native-like processing with a labeling efficiency of secreted proteins greater than 98%. After dialysis, the recombinant proteins were purified to homogeneity by one-step reversed-phase liquid chromatography. The final yield after 4-day shake-flask liquid culture was approximately 60 and 100 mg/L for ASP1 and ASP2, respectively. The inexpensive overproduction of labeled recombinant ASP1 and ASP2 should allow NMR studies of the structures and ligand-binding analysis in order to understand the relationships between structure and biological function of these proteins.

Cost-effective and uniform (13)C- and (15)N-labeling of the 24-kDa N-terminal domain of the Escherichia coli gyrase B by overexpression in the photoautotrophic cyanobacterium Anabaena sp. PCC 7120

Structural studies of biomolecules using nuclear magnetic resonance (NMR) rely on the availability of samples enriched in (13)C and (15)N isotopes. While (13)C/(15)N-labeled proteins are generally obtained by overexpression in transformed Escherichia coli cells cultured in the presence of an expensive mixture of labeled precursors, those of the photoautotrophic cyanobacterium Anabaena sp. PCC 7120 can be uniformly labeled by growing them in medium containing Na(15)NO(3) and NaH(13)CO(3) as the sole nitrogen and carbon sources. We report here a novel vector-host system suitable for the efficient preparation of uniformly (13)C/(15)N-labeled proteins in Anabaena sp. PCC 7120. The 24-kDa N-terminal domain of the E. coli gyrase B subunit, used as a test protein, was cloned into the pRL25C shuttle vector under the control of the tac promoter. The transformed Anabaena cells were grown in the presence of the labeled mineral salts and culture conditions were optimized to obtain over 90% of (13)C and (15)N enrichment in the constitutively expressed 24-kDa polypeptide. The yield of purified 24-kDa protein after dual isotope labeling under anaerobic conditions was similar to that obtained with E. coli cells bearing a comparable expression vector and cultured in parallel in a commercially available labeling medium. Furthermore, as probed by NMR spectroscopy and mass spectrometry, the 24-kDa N-terminal domain expressed in Anabaena was identical to the E. coli sample, demonstrating that it was of sufficient quality for 3D-structure determination. Because the Anabaena system was far more advantageous taking into consideration the expense for the labels that were necessary, these results indicate that Anabaena sp. PCC 7120 is an economic alternative for the (13)C/(15)N-labeling of soluble recombinant proteins destined for structural studies.

Foliar nitrogen concentrations and natural abundance of (15)N suggest nitrogen allocation patterns of Douglas-fir and mycorrhizal fungi during development in elevated carbon dioxide concentration and temperature

Pseudotsuga menziesii (Mirb.) Franco (Douglas-fir) seedlings were grown in a 2 x 2 factorial design in enclosed mesocosms at ambient temperature or 3.5 degrees C above ambient, and at ambient CO2 concentration ([CO2]) or 179 ppm above ambient. Two additional mesocosms were maintained as open controls. We measured the extent of mycorrhizal infection, foliar nitrogen (N) concentrations on both a weight basis (%N) and area basis (Narea), and foliar delta15N signatures (15N/14N ratios) from summer 1993 through summer 1997. Mycorrhizal fungi had colonized nearly all root tips across all treatments by spring 1994. Elevated [CO2] lowered foliar %N but did not affect N(area), whereas elevated temperature increased both foliar %N and Narea. Foliar delta15N was initially -1 per thousand and dropped by the final harvest to between -4 and -5 per thousand in the enclosed mesocosms, probably because of transfer of isotopically depleted N from mycorrhizal fungi. Based on the similarity in foliar delta15N among treatments, we conclude that mycorrhizal fungi had similar N allocation patterns across CO2 and temperature treatments. We combined isotopic and Narea data for 1993-94 to calculate fluxes of N for second- and third-year needles. Yearly N influxes were higher in second-year needles than in third-year needles (about 160 and 50% of initial leaf N, respectively), indicating greater sink strength in the younger needles. Influxes of N in second-year needles increased in response to elevated temperature, suggesting increased N supply from soil relative to plant N demands. In the elevated temperature treatments, N effluxes from third-year needles were higher in seedlings in elevated [CO2] than in ambient [CO2], probably because of increased N allocation below ground. We conclude that N allocation patterns shifted in response to the elevated temperature and [CO2] treatments in the seedlings but not in their fungal symbionts.

Effect of protein restriction on (15)N transfer from dietary [(15)N]alanine and [(15)N]Spirulina platensis into urea

Six normal men consumed a mixed test meal while adapted to high (1.5 g. kg(-1) x day(-1)) and low (0.3 g. kg(-1) x day(-1)) protein intakes. They completed this protocol twice: when the test meals included 3 mg/kg of [(15)N]alanine ([(15)N]Ala) and when they included 30 mg/kg of intrinsically labeled [(15)N]Spirulina platensis ([(15)N]SPI). Six subjects with insulin-dependent diabetes mellitus (IDDM) receiving conventional insulin therapy consumed the test meal with added [(15)N]Ala while adapted to their customary high-protein diet. Protein restriction increased serum alanine, glycine, glutamine, and methionine concentrations and reduced those of leucine. Whether the previous diet was high or low in protein, there was a similar increase in serum alanine, methionine, and branched-chain amino acid concentrations after the test meal and a similar pattern of (15)N enrichment in serum amino acids for a given tracer. When [(15)N]Ala was included in the test meal, (15)N appeared rapidly in serum alanine and glutamine, to a minor degree in leucine and isoleucine, and not at all in other circulating amino acids. With [(15)N]SPI, there was a slow appearance of the label in all serum amino acids analyzed. Despite the different serum amino acid labeling, protein restriction reduced the postmeal transfer of dietary (15)N in [(15)N]Ala or [(15)N]SPI into [(15)N]urea by similar amounts (38 and 43%, respectively, not significant). The response of the subjects with IDDM was similar to that of the normal subjects. Information about adaptive reductions in dietary amino acid catabolism obtained by adding [(15)N]Ala to a test meal appears to be equivalent to that obtained using an intrinsically labeled protein tracer.

Efficient 13C/15N double labeling of the avirulence protein AVR4 in a methanol-utilizing strain (Mut+) of Pichia pastoris

Cost effective 13C/15N-isotope labeling of the avirulence protein AVR4 (10 kDa) of the fungal tomato pathogen Cladosporium fulvum was achieved with the methylotrophic yeast Pichia pastoris in a fermentor. The 13C/15N-labeled AVR4 protein accumulated to 30 mg/L within 48 h in an initial fermentation volume of only 300 mL, while prolonged optimized overexpressions yielded 126 mg/L. These protein yields were 24-fold higher in a fermentor than in flask cultures. In order to achieve these protein expression levels, we used the methanol-utilizing strain (Mut+) of Pichia pastoris which has a high growth rate while growing on methanol as the only carbon source. In contrast, the methanol-sensitive strain (MutS) could intrinsically yield comparable protein expression levels, but at the expense of additional carbon sources. Although both strains are generally used for heterologous protein expression, we show that the costs for 13C-isotope labeling can be substantially reduced using the Mut+ strain compared to the MutS strain, as no 13C3-glycerol is required during the methanol-induction phase. Finally, nitrogen limitations were precluded for 15N-labeling by an optimal supply of 10 g/L (15NH4)2SO4 every 24 h.

Relationship of maternal protein turnover and lean body mass during pregnancy and birth length

Epidemiological evidence shows that small size at birth is associated with an increased risk of developing cardiovascular and metabolic disease in adult life. We have examined the relationships between size at birth and maternal body composition and protein turnover in normal pregnant women. A group of 27 multiparous Caucasian women with singleton pregnancies were studied at around 18 and 28 weeks' gestation. Body composition was determined by anthropometry, and whole-body protein turnover was estimated by using a single oral dose of [(15)N]glycine and the end-product method. The baby's weight and length were measured within 48 h of birth. Mothers with a greater lean body mass had higher rates of protein turnover at 18 weeks' gestation. This association was largely accounted for by differences in the mother's visceral, rather than muscle, mass. Mothers who had higher protein turnover at 18 weeks' gestation had babies that were longer at birth. After adjustment for the duration of gestation and the baby's sex, 26% of the variation in length at birth was accounted for by maternal protein synthesis at 18 weeks' gestation. Maternal protein intake was not associated with the baby's birth length. Thus the mother's ability to nourish her fetus is influenced by her body composition and her rate of protein turnover. Dietary intake does not adequately characterize this ability.

Does glutamine act as a substrate for transamination reactions in the liver of fed and fasted sheep?

The present study investigated the relative importance of glutamine as a transamination source in the ovine liver by examination of the labelling of amino acids (AA) in the hepatic free pool, mixed liver and plasma proteins of fed and fasted sheep, following infusion of isotopically-labelled glutamine. In a cross-over design four sheep were either fasted for 3 d or fed to maintenance and finally euthanased. At each intake, the sheep were infused for 6 h with [2-15N]glutamine (150 micromol/h) and samples of total plasma protein isolated. Following the terminal infusion, liver tissue total proteins were prepared and hydrolysed and 15N-enrichments in seventeen AA were determined by GC-combustion-isotope-ratio mass spectrometry. All AA were enriched (relative to natural abundance) except lysine and threonine, with the lowest enrichments in phenylalanine and histidine. There was no effect of the fed v. fasted state, except for leucine and isoleucine in liver protein Enrichments in liver protein were greater than in plasma protein except proline) and probably reflect the faster turnover rate of hepatic constitutive proteins compared with export proteins. Amination to methionine was greater than that to phenylalanine suggesting a mechanism for preferentially protecting the former. This factor could be important for ruminant production, as methionine is often considered to be the first limiting AA for animals offered certain silages and conserved forages. Enrichments in all AA (except for glutamine, alanine and aspartate) were less than that for glutamate and thus transaminations may have occurred with glutamine directly or via glutamate, following the action of hepatic glutaminase.

Uptake of glycine by non-mycorrhizal Lolium perenne

Plants of Lolium perenne were grown in sterile solution culture. 15N-labelled glycine (Gly) coupled with gas chromatograph mass spectrometry was used to prove that non-mycorrhizal plants of L. perenne are capable of acquiring N in the form of intact Gly. It was estimated that a minimum of 80% of Gly-N uptake, over a 3 h period, was as intact Gly, though possible processes resulting in deviation from this estimate are discussed. The relative incorporation of 15N derived from Gly uptake into serine (Ser) compared with other amino acids in the root amino acid pool suggested the enzyme serine:glyoxylate aminotransferase was at least partly responsible for the synthesis of Ser from Gly. Defoliation was shown to reduce Gly uptake by L. perenne. The addition of either 25 mol x m(-3) sucrose or 50 mol x m(-3) glucose to the uptake solution of defoliated plants increased Gly-N uptake compared with both defoliated plants without sugars and with undefoliated plants. Addition of a glucose analogue, 3-O-methyl-D-glucopyranose, that is absorbed but not metabolized by plants, did not affect Gly uptake by defoliated plants. Increasing pH from 3.5 to 9.2 caused a reduction in Gly uptake. Results of the effects of defoliation and pH are consistent with Gly uptake by L. perenne being by an energy-dependent proton symport. When either or Gly were supplied to plants at equimolar concentrations, uptake was five times greater than that of Gly at pH 6 and 13 times greater at pH 9.

Simulated and NMR-derived backbone dynamics of a protein with significant flexibility: a comparison of spectral densities for the betaARK1 PH domain

A 7.6 ns molecular dynamics trajectory of the betaARK1 PH domain in explicit water with appropriate ions was calculated at 300 K. Spectral densities at omega = 0, omega(N), and 0.87omega(H) and the model-free parameters were evaluated from the experimental as well as the simulated data, taking the anisotropic overall motion of the protein into account. Experimental and simulated spectral densities are in reasonable general agreement for NH bond vectors, where the corresponding motions have converged within the simulation time. A sufficient sampling of the motions for NH bonds within flexible parts of the protein requires a longer simulation time. The simulated spectral densities J(0) and J(omega(N)) are, on average, 4.5% and 16% lower than the experimental data; the corresponding numbers for the core residues are about 6%; the high-frequency spectral densities J(0.87omega(H)) are lower by, on average, 16% (21% for the core). The simulated order parameters, S(2), are also lower, although the overall disagreement between the simulation and experiment is less pronounced: 1% for all residues and 6% for the core. The observed systematic decrease of simulated spectral density and the order parameters compared to the experimental data can be partially attributed to the ultrafast librational motion of the NH bonds with respect to their peptide plane, which was analyzed in detail. This systematic difference is most pronounced for J(0.87omega(H)), which appears to be most sensitive to the slow, subnanosecond time scale of internal motion, whereas J(0) and J(omega(N)) are dominated by the overall rotational tumbling of the protein. Similar discrepancies are observed between the experimentally measured (15)N relaxation parameters (R(1), R(2), NOE) and their values calculated from the simulated spectral densities. The analysis of spectral densities provides additional information regarding the comparison of the simulated and experimental data, not available from the model-free analysis.

Simultaneous measurement of denitrification and nitrogen fixation using isotope pairing with membrane inlet mass spectrometry analysis

A method for estimating denitrification and nitrogen fixation simultaneously in coastal sediments was developed. An isotope-pairing technique was applied to dissolved gas measurements with a membrane inlet mass spectrometer (MIMS). The relative fluxes of three N(2) gas species ((28)N(2), (29)N(2), and (30)N(2)) were monitored during incubation experiments after the addition of (15)NO(3)(-). Formulas were developed to estimate the production (denitrification) and consumption (N(2) fixation) of N(2) gas from the fluxes of the different isotopic forms of N(2). Proportions of the three isotopic forms produced from (15)NO(3)(-) and (14)NO(3)(-) agreed with expectations in a sediment slurry incubation experiment designed to optimize conditions for denitrification. Nitrogen fixation rates from an algal mat measured with intact sediment cores ranged from 32 to 390 microg-atoms of N m(-2) h(-1). They were enhanced by light and organic matter enrichment. In this environment of high nitrogen fixation, low N(2) production rates due to denitrification could be separated from high N(2) consumption rates due to nitrogen fixation. Denitrification and nitrogen fixation rates were estimated in April 2000 on sediments from a Texas sea grass bed (Laguna Madre). Denitrification rates (average, 20 microg-atoms of N m(-2) h(-1)) were lower than nitrogen fixation rates (average, 60 microg-atoms of N m(-2) h(-1)). The developed method benefits from simple and accurate dissolved-gas measurement by the MIMS system. By adding the N(2) isotope capability, it was possible to do isotope-pairing experiments with the MIMS system.

Millipedes and earthworms increase the decomposition rate of 15N-labelled winter rape litter in an arable field

Effects of millipedes and earthworms on the decomposition of 15N-labelled litter of winter oilseed rape were investigated in a microcosm field experiment over a period of 264 days on an oat field near Göttingen managed by integrated farming. A total of 32 microcosms were filled with defaunated soil. 15N-labelled rape litter was placed either on top of the soil or buried into the soil simulating mulching and ploughing, respectively. To the microcosms nine adult individuals of Blaniulus guttulatus (Diplopoda) and two of Aporrectodea caliginosa (Lumbricidae) were added separately or in combination. In general, the presence of the animals accelerated the decomposition rate of the litter material. The effects were most pronounced in the presence of Aporrectodea caliginosa. The total amount of nitrate, ammonium and the amount of 35N leached from the microcosms was increased in the presence of earthworms or of both earthworms and millipedes. Both species proved to be important members of the detritus food web of the agricultural system studied.

Rapid determination of protein folds using residual dipolar couplings

Over the next few years, various genome projects will sequence many new genes and yield many new gene products. Many of these products will have no known function and little, if any, sequence homology to existing proteins. There is reason to believe that a rapid determination of a protein fold, even at low resolution, can aid in the identification of function and expedite the determination of structure at higher resolution. Recently devised NMR methods of measuring residual dipolar couplings provide one route to the determination of a fold. They do this by allowing the alignment of previously identified secondary structural elements with respect to each other. When combined with constraints involving loops connecting elements or other short-range experimental distance information, a fold is produced. We illustrate this approach to protein fold determination on (15)N-labeled Eschericia coli acyl carrier protein using a limited set of (15)N-(1)H and (1)H-(1)H dipolar couplings. We also illustrate an approach using a more extended set of heteronuclear couplings on a related protein, (13)C, (15)N-labeled NodF protein from Rhizobium leguminosarum.

Compartmental modeling with nitrogen-15 to determine effects of degree of fat saturation on intraruminal N recycling

Two- and three-compartment models were developed to describe N kinetics within the rumen using three Holstein heifers and one nonlactating Holstein cow fitted with ruminal and duodenal cannulas. A 4 x 4 Latin square design included a control diet containing no supplemental fat and diets containing 4.85% of diet dry matter as partially hydrogenated tallow (iodine value = 13), tallow (iodine value = 51), or animal-vegetable fat (iodine value = 110). Effects of fat on intraruminal N recycling and relationships between intraruminal N recycling and ruminal protozoa concentration or the efficiency of microbial protein synthesis were determined. A pulse dose of 15(NH4)2SO4 was introduced into the ruminal NH3 N pool, and samples were taken over time from the ruminal NH3 N and nonammonia N pools. For the three-compartment model, precipitates of nonammonia N after trichloroacetic acid and ethanol extraction were defined as slowly turning over nonammonia N; rapidly turning over nonammonia N was determined by difference. Curves of 15N enrichment were fit to models with two (NH3 N and nonammonia N) or three (NH3 N, rapidly turning over nonammonia N, and slowly turning over nonammonia N) compartments using the software SAAM II. Because the three-compartment model did not remove a small systematic bias or improve the fit of the data, the two-compartment model was used to provide measurements of intraruminal N recycling. Intraruminal NH3 N recycling (45% for control) decreased linearly as fat unsaturation increased (50.2, 43.0, and 41.7% for partially hydrogenated tallow, tallow, and animal-vegetable fat, respectively). Intraruminal nitrogen recycling was not correlated with efficiency of microbial protein synthesis or ruminal protozoa counts.

Role of insulin and glucose oxidation in mediating the protein catabolism of burns and sepsis

We have investigated the responsiveness of protein kinetics to insulin and the role of glucose oxidation rate as a mediator of the protein catabolic response to burn injury and sepsis by assessing the response of leucine and urea kinetics to a 5-h hyperinsulinemic euglycemic clamp with and without the simultaneous administration of dichloroacetate (DCA) (to further increase glucose oxidation via stimulation of pyruvate dehydrogenase activity) in eight severely burned and eight septic patients. Leucine and urea kinetics were measured by the primed-constant infusions of [1(-13)C]leucine and [15N2]urea. Compared with controls, basal leucine kinetics (flux and oxidation) were significantly elevated (P less than 0.01) in both groups of patients. Hyperinsulinemia elicited significant (P less than 0.05) decreases in leucine kinetics in both groups of patients. Consistent with this observation, hyperinsulinemia caused urea production to decrease significantly (P less than 0.05) in both patient groups. The administration of DCA to patients during hyperinsulinemia elicited a significant increase in glucose oxidation rate compared with the clamp rate (P less than 0.05), and the percent of glucose uptake oxidized increased from 45.5 +/- 5.5 to 53.5 +/- 4.8%; yet the response of leucine and urea kinetics to the clamp plus DCA was not different from the response to the clamp alone. These results suggest that the maximal effectiveness of insulin to suppress protein breakdown is not impaired and that a deficit in glucose oxidation or energy supply is probably not playing a major role in mediating the protein catabolic response to severe burn injury and sepsis.

Assessment of alanine, urea, and glucose interrelationships in normal subjects and in patients with sepsis with stable isotopic tracers

The kinetic interactions among glucose, alanine, and urea metabolism were studied in both normal volunteers and in patients with sepsis by means of a primed, constant infusion of stable isotopes. In the normal volunteers, infusion of glucose at 4 mg/kg/min suppressed total glucose production, the rate of gluconeogenesis from alanine, and the production of urea, despite an increase in the rate of release and uptake of alanine. When the glucose infusion rate was increased to 8 mg/kg/min, the production of urea decreased further, even though gluconeogenesis from alanine was already suppressed by the first infusion. This additional N-sparing effect was explainable by an increase in glucose oxidation. In the patients with sepsis the basal rates of production of glucose and urea were elevated significantly. Glucose infusion (4 mg/kg/min) decreased hepatic glycogenolysis but not gluconeogenesis from alanine or urea production. At the glucose infusion rate of 8 mg/kg/min, glucose oxidation increased in the patients and urea production decreased. Thus in patients with sepsis a higher rate of glucose infusion is necessary to achieve nitrogen-sparing effects than is necessary in controls because of a lack of suppressibility of gluconeogenesis. Because of continued glucose production during glucose infusion, hyperglycemia commonly develops during glucose infusion in sepsis. However, this effect does not necessarily indicate a complete inability of the patient with sepsis to benefit nutritionally from infused glucose, as we observed no decrement in the ability to oxidize infused glucose.