Effect of ammonia concentration on activity of enzymes of ammonia assimilation and on synthesis of amino acids by mixed rumen bacteria in continuous culture

Functional Analysis of a Glutamine Biosynthesis Protein from a Psychrotrophic Bacterium, Cryobacterium soli GCJ02

A putative glutamine synthetase (GS) was detected in a psychrophilic bacterium, Cryobacterium soli GCJ02. For gaining greater insight into its functioning, the gene was cloned and expressed in a heterologous host, Escherichia coli. The monomer enzyme with a molecular weight of 53.03 kDa was expressed primarily in cytosolic compartment. The enzyme activity was detected using glutamate and ATP. The optimum conditions of its biosynthesis were observed to be 60 °C and pH value 7.5. Its thermostability was relatively high with a half-life of 50 min at 40 °C. GS activity was enhanced in the presence of metal ions such as Mg²⁺ and Mn²⁺, whereas Fe²⁺, Cu²⁺ and Ca²⁺ proved inhibitory. The consensus pattern [EXE]-D-KP-[XGXGXH] in the GS lies between residues 132 and 272. The catalytic active sites consisting of EAE and NGSGMH were verified by site-directed mutagenesis. Based on the analysis of the consensus pattern, the GS/glutamate synthase cycle of C. soli GCJ02 is expected to contribute to the GS synthesic activity.

1.1 Nitrogen Metabolism in the Rumen

The concept that the protein reaching the duodenum of a ruminant comprises of two major components, feed and microbial, has been accepted for many years but recently there has been considerable interest in attempts to define and quantify those processes which have an influence on the quantity and quality of this protein. The main reason for this is the desire to predict accurately the total flow of protein to the duodenum when a particular diet is fed. The ability to do this, coupled with a refinement of knowledge on the needs of the animal, are essential steps in improving the efficiency with which ruminants are fed. This review examines some of the factors which control the breakdown of dietary protein and the synthesis of microbial protein in the rumen. The lack of space has prevented discussion of many important topics, for example, the contribution of endogenous proteins to the total protein entering the duodenum. Many reviews have been published in this area (see Egan, 1980; Demeyer and Van Nevel, 1980; others are referred to in the text).

Characterization of an L-phosphinothricin resistant glutamine synthetase from Exiguobacterium sp. and its improvement

A glutamine synthetase (GS; 1341 bp) gene with potent L-phosphinothricin (PPT) resistance was isolated and characterized from a marine bacterium Exiguobacterium sp. Molecular docking analysis indicated that the substitution of residues Glu60 and Arg64 may lead to significant changes in binding pocket. To enhance the enzymatic property of GS, variants E60A and R64G were obtained by site-directed mutagenesis. The results revealed a noteworthy change in the thermostability and activity in comparison to the wild type (WT). WT exhibited optimum activity at 35 °C, while E60A and R64G exhibited optimum activity at 45 and 40 °C, respectively. The mutant R64G was 4.3 times more stable at 70 °C in comparison to WT, while E60A was 5.7 times more stable. Kinetic analysis revealed that the k _cat value of R64G mutant was 8.10-, 7.25- and 7.63-fold that of WT for ADP, glutamine and hydroxylamine, respectively. The kinetic inhibition (K _i, 4.91 ± 0.42 mM) of R64G was 2.02-fold that of WT (2.43 ± 0.14 mM) for L-phosphinothricin. The analysis of structure and function relationship showed that the binding pocket underwent dramatic changes when Arg site of 64 was substituted by Gly, thus promoting the rapid capture of substrates and leading to increase in activity and PPT-resistance of mutant R64G. The rearrangements of the residues at the molecular level formed new hydrogen bonds around the active site, which contributed to the increase of thermostability of enzymes. This study provides new insights into substrate binding mechanism of glutamine synthetase and the improved GS gene also has a potential for application in transgenic crops with L-phosphinothricin tolerance.

Effects of sucrose amendment on ammonia assimilation during sewage sludge composting

The aim of this study was to evaluate the laboratory-scale composting of sewage sludge and pumice mixtures that were amended with sucrose. The variation in temperature, pH, NH4(+)-N, ammonia emission, bacterial community, ammonia assimilating bacteria (AAB) populations and enzymatic activity related to ammonia assimilation were detected. The addition of sucrose increased the AAB population by 2.5-3.5 times, reduced ammonia emission by 24.7-31.1% compared with the control treatment, and promoted the growth of Bacillus and Wautersiella. The activities of glutamate dehydrogenase (GDH), glutamate synthase (GS) and glutamine synthetase (GOGAT), were enhanced by the addition of sucrose. GDH made a substantial contribution to ammonia assimilation when the ammonia concentration was high (⩾1.5g/kg) in the thermophilic phase. The GS/GOGAT cycle played an important role at low ammonia concentrations (⩽1.1g/kg) in the cooling phase. These results suggested that adding sucrose to sludge compost could promote ammonia assimilation and reduce ammonia emission.

Effect of carbohydrate sources and levels of cotton seed meal in concentrate on feed intake, nutrient digestibility, rumen fermentation and microbial protein synthesis in young dairy bulls

The objective of this study was to investigate the effect of levels of cottonseed meal with various carbohydrate sources in concentrate on feed intake, nutrient digestibility, rumen fermentation and microbial protein synthesis in dairy bulls. Four, 6 months old dairy bulls were randomly assigned to receive four dietary treatments according to a 2×2 factorial arrangement in a 4×4 Latin square design. Factor A was carbohydrate source; cassava chip (CC) and cassava chip+rice bran in the ratio of 3:1 (CR3:1), and factor B was cotton seed meal levels in the concentrate; 109 g CP/kg (LCM) and 328 g CP/kg (HCM) at similar overall CP levels (490 g CP/kg). Bulls received urea-lime treated rice straw ad libitum and were supplemented with 10 g of concentrate/kg BW. It was found that carbohydrate source and level of cotton seed meal did not have significant effects on ruminal pH, ammonia nitrogen concentration, microbial protein synthesis or feed intake. Animals which received CC showed significantly higher BUN concentration, ruminal propionic acid and butyric acid proportions, while dry matter, organic matter digestibility, populations of total viable bacteria and proteolytic bacteria were lower than those in the CR3:1 treatment. The concentration of total volatile fatty acids was higher in HCM than LCM treatments, while the concentration of butyric acid was higher in LCM than HCM treatments. The population of proteolytic bacteria with the LCM treatments was higher than the HCM treatments; however other bacteria groups were similar among the different levels of cotton seed meal. Bulls which received LCM had higher protein digestibility than those receiving HCM. Therefore, using high levels of cassava chip and cotton seed meal might positively impact on energy and nitrogen balance for the microbial population in the rumen of the young dairy bull.

Effect of carbohydrate source and cottonseed meal level in the concentrate on feed intake, nutrient digestibility, rumen fermentation and microbial protein synthesis in swamp buffaloes

The objective of this study was to investigate the effect of carbohydrate source and cottonseed meal level in the concentrate on feed intake, nutrient digestibility, rumen fermentation and microbial protein synthesis in swamp buffaloes. Four, 4-yr old rumen fistulated swamp buffaloes were randomly assigned to receive four dietary treatments according to a 2×2 factorial arrangement in a 4×4 Latin square design. Factor A was carbohydrate source; cassava chip (CC) and CC+rice bran at a ratio 3:1 (CR3:1), and factor B was level of cottonseed meal (CM); 109 g CP/kg (LCM) and 328 g CP/kg (HCM) in isonitrogenous diets (490 g CP/kg). Buffaloes received urea-treated rice straw ad libitum and supplemented with 5 g concentrate/kg BW. It was found that carbohydrate source did not affect feed intake, nutrient intake, digested nutrients, nutrient digestibility, ammonia nitrogen concentration, fungi and bacterial populations, or microbial protein synthesis (p>0.05). Ruminal pH at 6 h after feeding and the population of protozoa at 4 h after feeding were higher when buffalo were fed with CC than in the CR3:1 treatment (p<0.05). Buffalo fed with HCM had a lower roughage intake, nutrient intake, population of total viable and cellulolytic bacteria and microbial nitrogen supply than the LCM fed group (p<0.05). However, nutrient digestibility, ruminal pH, ammonia concentration, population of protozoa and fungi, and efficiency of microbial protein synthesis were not affected by cottonseed meal levels (p>0.05). Based on this experiment, concentrate with a low level of cottonseed meal could be fed with cassava chips as an energy source in swamp buffalo receiving rice straw.

Purification, characterization, and expression of multiple glutamine synthetases from Prevotella ruminicola 23

The Prevotella ruminicola 23 genome encodes three different glutamine synthetase (GS) enzymes: glutamine synthetase I (GSI) (ORF02151), GSIII-1 (ORF01459), and GSIII-2 (ORF02034). GSI, GSIII-1, and GSIII-2 have each been heterologously expressed in and purified from Escherichia coli. The subunit molecular mass of GSI was 56 kDa, while GSIII-1 and GSIII-2 were both 83 kDa. Optimal conditions for γ-glutamyl transferase activity were found to be 35°C at pH 5.6 with 0.25 mM Mn(2+) ions (GSI) or 37°C at pH 6.0 (GSIII-1 and GSIII-2) with 0.50 to 1.00 mM Mn(2+) ions. GSIII biosynthetic activity was found to be optimal at 50 to 60°C and pH 6.8 to 7.0 with 10 mM Mn(2+) ions, while GSI displayed no GS biosynthetic activity. Kinetic analysis revealed K(m) values for glutamate and ammonium as well as for hydrolysis of ATP to be 8.58, 0.48, and 1.91 mM, respectively, for GSIII-1 and 1.72, 0.43, and 2.65 mM, respectively, for GSIII-2. A quantitative reverse transcriptase PCR assay (qRT-PCR) revealed GSIII-2 to be significantly induced by high concentrations of ammonia, and this corresponded with increases in measured GS activity. Collectively, these results show that both GSIII enzymes in P. ruminicola 23 are functional and indicate that GSIII-2, flanked by GOGAT (gltB and gltD genes), plays an important role in the acquisition and metabolism of ammonia, particularly under nonlimiting ammonia growth conditions.

Interactions between carbon and nitrogen metabolism in Fibrobacter succinogenes S85: a 1H and 13C nuclear magnetic resonance and enzymatic study

The effect of the presence of ammonia on [1-13C]glucose metabolism in the rumen fibrolytic bacterium Fibrobacter succinogenes S85 was studied by 13C and 1H nuclear magnetic resonance (NMR). Ammonia halved the level of glycogen storage and increased the rate of glucose conversion into acetate and succinate 2.2-fold and 1.4-fold, respectively, reducing the succinate-to-acetate ratio. The 13C enrichment of succinate and acetate was precisely quantified by 13C-filtered spin-echo difference 1H-NMR spectroscopy. The presence of ammonia did not modify the 13C enrichment of succinate C-2 (without ammonia, 20.8%, and with ammonia, 21.6%), indicating that the isotopic dilution of metabolites due to utilization of endogenous glycogen was not affected. In contrast, the presence of ammonia markedly decreased the 13C enrichment of acetate C-2 (from 40 to 31%), reflecting enhanced reversal of the succinate synthesis pathway. The reversal of glycolysis was unaffected by the presence of ammonia as shown by 13C-NMR analysis. Study of cell extracts showed that the main pathways of ammonia assimilation in F. succinogenes were glutamate dehydrogenase and alanine dehydrogenase. Glutamine synthetase activity was not detected. Glutamate dehydrogenase was active with both NAD and NADP as cofactors and was not repressed under ammonia limitation in the culture. Glutamate-pyruvate and glutamate-oxaloacetate transaminase activities were evidenced by spectrophotometry and 1H NMR. When cells were incubated in vivo with [1-13C]glucose, only 13C-labeled aspartate, glutamate, alanine, and valine were detected. Their labelings were consistent with the proposed amino acid synthesis pathway and with the reversal of the succinate synthesis pathway.

Ecology, metabolism, and genetics of ruminal selenomonads

Selenomonas ruminantium is one of the more prominent and functionally diverse bacteria present in the rumen and can survive under a wide range of nutritional fluctuations. Selenomonas is not a degrader of complex polysaccharides associated with dietary plant cell wall components, but is important in the utilization of soluble carbohydrates released from initial hydrolysis of these polymers by other ruminal bacteria. Selenomonads have multiple carbon flow routes for carbohydrate catabolism and ATP generation, and subspecies differ in their ability to use lactate. Some soluble carbohydrates (glucose, sucrose) appear to be transported via the phosphoenolpyruvate phosphotransferase system, while arabinose and xylose are transported by proton symport. High cell yields and the presence of electron transport components in Selenomonas strains has been documented repeatedly and this may partially account for the energy partitioning observed between energy consumed for growth and maintenance functions. Most strains can utilize ammonia, protein, and/or amino acids as a nitrogen source. Some strains can hydrolyze urea and/or reduce nitrate and use the ammonia for the biosynthesis of amino acids. Experimental evidence suggests that ammonia assimilatory enzymes in some strains may possess unique properties with respect to other presumably similar bacteria. Little is known about the genetics of ruminal selenomonads. Plasmid DNA has been isolated from some strains, but it is unknown what physiological functions may be encoded on these extrachromosomal elements. Due to the predominance of S. ruminantium in the rumen, it is an ideal candidate for genetic manipulation. Once the genetics of this bacterium are better understood, it may be possible to amplify its role in the rumen.

Alanine as an end product during fermentation of monosaccharides by Clostridium strain P2

The thermophilic Clostridium P2 was isolated from a semi-continuously fed reactor with high ammonium concentration. This bacterium formed substantial amounts of L-alanine as a major fermentation product from glucose, fructose and mannose. Low amounts of acetate, butyrate, carbon dioxide and hydrogen were also formed. A high partial pressure of hydrogen inhibited the degradation of the monosaccharides, whereas hydrogen removal, in the form of methanogenesis was found to be stimulatory. However, the amount of alanine produced per mole of hexose degraded did not change. Hexose degradation and alanine production were favoured by high ammonium concentrations. Nuclear magnetic resonance spectroscopy studies provided strong evidence that an active Embden-Meyerhof-Parnas pathway existed and that alanine was produced via an amination of pyruvate.

Chemical factors involved in ruminal fiber digestion

In the United States, cattle are commonly fed diets containing cereal grains. The presence of starch and sugars reduces fiber digestion, which may in turn depress intake. In this paper, chemical constraints that may be responsible for the decrease in fiber digestion are explored. A major factor appears to be rumen pH. Moderate depression in pH, to approximately 6.0, results in a small decrease in fiber digestion, but numbers of fibrolytic organisms are usually not affected. Further decreases to 5.5 or 5.0 result in depressed growth rates and decreased fibrolytic microbes, and fiber digestion may be completely inhibited. Proliferation of organisms on readily fermentable carbohydrates may increase the need for total nitrogen as both ammonia and amino acids. The value of amino acids to cellulolytic organisms appears to be primarily as sources of isobutyric, isovaleric, and 2-methylbutyric acids. This reinforces the need to establish dietary requirements for nonprotein nitrogen, degradable protein, and isoacids. Other factors affecting fiber digestion, such as inhibition of cellulytic enzymes and plant concentrations of lignins and phenyl propanoids, are also discussed.

Glutamine synthetase activity in the ruminal bacterium Succinivibrio dextrinosolvens

Succinivibrio dextrinosolvens C18 was found to possess glutamine synthetase (GS), urease, glutamate dehydrogenase, and several other nitrogen assimilation enzymes. When grown in continuous culture under ammonia limitation, both GS and urease activities were high and glutamate dehydrogenase activity was low, but the opposite activity pattern was observed for growth in the presence of ample ammonia. The addition of high-level (15 mM) ammonium chloride to ammonia-limited cultures resulted in a rapid loss of GS activity as measured by either the gamma-glutamyl transferase or forward assay method with cells or extracts. No similar activity losses occurred for urease, glutamate dehydrogenase, or pyruvate kinase. The GS activity loss was not prevented by the addition of chloramphenicol and rifampin. The GS activity could be recovered by washing or incubating cells in buffer or by the addition of snake venom phosphodiesterase to cell extracts. Manganese inhibited the GS activity (forward assay) of untreated cells but stimulated the GS activity in ammonia-treated cells. Alanine, glycine, and possibly serine were inhibitory to GS activity. Optimal pH values for GS activity were 7.3 and 7.4 for the forward and gamma-glutamyl transferase assays, respectively. The glutamate dehydrogenase activity was NADPH linked and optimal in the presence of KCl. The data are consistent with an adenylylation-deadenylylation control mechanism for GS activity in S. dextrinosolvens, and the GS pathway is a major route for ammonia assimilation under low environmental ammonia levels. The rapid regulation of the ATP-requiring GS activity may be of ecological importance to this strictly anaerobic ruminal bacterium.

Nitrogen metabolism in the rumen

Nitrogen metabolism is reviewed with emphasis on methods for quantitating various nitrogen-transactions in the rumen of animals on a variety of diets. Ammonia kinetics, microbial cell synthesis, the inputs of endogenous nitrogen, degradation of dietary protein, and availability to the animal of dietary bypass protein are discussed. The efficiency of microbial protein from the rumen is discussed in relation to the ratio of protein to energy in the nutrients available to meet the requirements of the animal. The ratio is determined largely by the maintenance requirements of microbes and the breakdown of microbial materials, which result in the recycling of microbial nitrogen in the rumen. Emphasis is placed on the role of rumen protozoa in decreasing the ratio of protein to energy in absorbed nutrients in ruminants on diets that are marginally deficient in protein. Recent studies of the dynamics of protozoa in the rumen and their contribution to microbial protein outflow are summarized.

Incorporation of nitrogen into rumen bacterial fractions of steers given protein- and urea-containing diets. Ammonia assimilation into intracellular bacterial amino acids

Experiments were carried out in vivo to investigate the pathways of ammonia incorporation into rumen bacteria, bacterial fractions and free amino acids within the bacteria. Steers were alternately given two isoenergetic, isonitrogenous diets containing the nitrogen mainly as either urea or decorticated groundnut meal (DCGM). At the end of each period on a given diet, a solution of 15NH4Cl was infused into the rumen and samples of rumen contents were removed at 2, 10, 20 and 90 min and 5, 10 and 24 h afterwards. Concentrations of ammonia and its 15N enrichment were determined and samples of mixed rumen bacteria were prepared. Bacteria were disrupted ultrasonically and separated into bacterial protein, cell wall and protein-free cell supernatant fractions. Amino acids were separated after hydrolysis and their 15N contents determined. A rumen fluid circulation pump was developed so that representative samples could be taken at very short time intervals after the introduction of the 15N label. Rumen pH changes, rumen fluid dilution rates and patterns of rumen ammonia concentrations were consistent with normal rumen metabolism. Net bacterial synthesis (as calculated from the net outflow of bacteria from the rumen) was significantly (P less than 0.05) greater with the DCGM diet (12.4 g bacterial N/d) than with the urea diet (9.24 g bacterial N/d). With both diets the 15N label rapidly left the rumen ammonia pool and entered the rumen bacteria. Analysis of the bacterial fractions indicated that the label appeared rapidly in the protein-free cell supernatant fraction and more slowly in the bacterial protein and cell wall fractions. With the DCGM diet bacteria apparently utilized intracellular label less efficiently than with the urea diet. The proportion of N in the protein-free cell supernatant was higher with the DCGM diet, suggesting increased levels of intracellular amino acids and peptides, following extracellular protein degradation. Levels of enrichment of the amino acids alanine and glutamate in the protein-free cell supernatant fraction suggested that the enzymes alanine dehydrogenase (EC 1.4.1.1) and glutamate dehydrogenase (EC 1.4.1.2 and 1.4.1.4) may be the major enzymes for assimilating ammonia when concentrations of soluble carbohydrate and rumen ammonia are high in the rumen. The high levels of intracellular alanine are discussed with reference to published work on the excretion of alanine by rumen bacteria.

Microbial ecology and activities in the rumen: part 1

This review describes the progress which has been made during the last 10 to 15 years in the field of rumen microbiology. It is basically an account of new discoveries in the bacteriology, protozoology, biochemistry, and ecology of the rumen microbial population. As such it covers a wide range of subjects including the isolation and properties of methanogenic bacteria, the role of rumen phycomycete fungi, anaerobic energy conservation, and general metabolic aspects of rumen microorganisms. It also attempts, however, to describe and develop new concepts in rumen microbiology. These consist principally of interactions of the microbemicrobe, microbe-food and microbe-host types, and represent the main areas of recent advance in our understanding of the rumen ecosystem. The development of experimental techniques such as chemostat culture and scanning electron microscopy are shown to have been instrumental in progress in these areas. The paper is concluded with an assessment of our present knowledge of the rumen fermentation, based on the degree of success of experiments with gnotobiotic ruminants inoculated with defined flora and in mathematical modeling of the fermentation. The efficacy of chemical manipulation of the fermentation in ruminant is also discussed in this light.

Distribution and changes in urease (EC 3.5.1.5) activity in Rumen Simulation Technique (Rusitec)

1. The Rumen Simulation Technique (Rusitec) was used in a series of long-term experiments to study the distribution and changes of urease (EC 3.5.1.5) activity in a heterogeneous fermentation system. 2. It was shown that in Rusitec the high urease activity from the inoculum decreased to low values, that the rate of decrease was consistent with simple dilution of ureolytic micro-organisms and that the urease activity could be restored to original values by infusion of urea into the reaction vessels. The magnitude of this urease activity was a direct function of the amounts of urea infused. Single daily additions of the same or greater amounts of urea in food or as solid failed to increase the urease activity significantly. 3. In general, urease activity increased 2-6 h after feeding and the increases were greater with roughage diets. 4. The ureolytic activity per unit volume was always higher in compartment 2(space occupied by micro-organisms that are loosely associated with the solid) than in compartment 1 (strained rumen contents) or compartment 3 (space occupied by microbial population that cannot be washed out of the solid matrix). 5. The distribution of urease activity between the compartments was different from the distribution of certain other enzymes (e.g. protease and alkaline phosphatase (EC 3.1.3.1)). 6. Apart from the boundary region, the concentrations of urease, ammonia and volatile fatty acids in compartment 2 were constant, while the concentrations of protein, DNA and another enzyme (alkaline phosphatase) increased with the depth of the compartment. Specific urease activity (per unit weight of protein or DNA) was much higher in compartment 1 than in compartment 2 and it decreased markedly with depth of compartment. 7. The concentrations of ammonia were always much higher in the solid matrix (compartments 2 and 3) than in the free suspension of micro-organisms (compartment 1). There was a linear relation between these two quantities. 8. The results are discussed in relation to published work on the entry and metabolism of urea in the rumen.

Effect of fermentation and formalin preservation on the protein component of bovine colostrum

Colostrum was inoculated with Streptococcus lactis or yogurt culture or preserved with .1% (vol/vol) formalin in two separate experiments. All preparations then were stored at ambient temperature for 24 days. With increasing storage time, a larger proportion of the total colostrum nitrogen was not precipitated in 10% (wt/vol) trichloroacetic acid. By day 24, this represented 30 to 35% for the fermented samples and 10 to 15% for the formalin preserved samples. The majority of this nonprecipitable nitrogen was amino acids and small peptides. Most of the nitrogen in colostrum even after 24 days of storage should be nutritionally useful to the calf because even the nonprecipitable portion is amino acids and peptides.

Total and individual amino acids in strained ruminal liquor from cows fed graded amounts of urea

Total and individual amino acids were determined in samples of strained ruminal liquor from an experiment in which cows were fed a low protein, high concentrate diet supplemented with graded amounts of urea to elicit a range of ruminal ammonia concentrations. Total amino acids plateaued beyond 2% crude protein from urea and 6.5 mM ruminal ammonia. There was a negative quadratic relationship between concentrations of total amino acids and ammonia with total amino acids maximized at 16.2 mM ammonia. Individual amino acids followed trends similar to that of total amino acids, increasing with dietary urea and ruminal ammonia before plateauing or declining slightly. Contribution to the animal's amino acid requirements from passage of free amino acids in strained ruminal liquor were estimated for three essential amino acids. Based on mean concentration in strained ruminal liquor, liquid turnover rates, and requirements for gain, 7, 16, and 12% of the requirements for methonine, lysine, and threonine may be contributed from passage of ruminal free amino acids.

Degradation of maize protein in rumen contents. Influence of ammonia concentration

1. The influence of ammonia concentration on the distribution of nitrogen derived from opaque-2 maize uniformly-labelled with 15N has been investigated during short-term in vitro incubation of bovine rumen contents. 2. Less 15N derived from maize was found in the non-protein-N (NPN) fraction during incubation without added NH3 than with added NH3, due entirely to differences in the amount of N derived from maize in the NH3 fraction. 3. From calculations based on the transfer of N derived from maize to the NPN pool and to a bacterial fraction, it was concluded that degradation of maize protein was not influenced by NH3 concentration within the examined limits. 4. The decrease in relative amount of N derived from maize in the NH3 fraction at low concentrations of NH3, together with evidence for an increased fractional turnover rate of NH3-N suggests that a deficient supply of NH3 is compensated for by increased catabolism of nitrogenous compounds derived from the rumen micro-organisms.

Ammonia saturation constants for predominant species of rumen bacteria

Ammonia saturation constants were determined for representative pure cultures of predominant, anaerobic, fermentative rumen bacteria. Based on growth experiments with ammonia limited continuous cultures, average estimates for ammonia saturation constants of Bacteroides amylophillus and Bacteroides ruminicola were 10.5 and 23.5 microM ammonia-nitrogen, respectively. With ammonia-limited linear-growth cultures, the estimates for the ammonia saturation constants of B. amylophilus, B. ruminicola, and Selenomonas ruminantium were, respectively, 6, 33.5, and 18 microM ammonia nitrogen. By a third method, which involved estimation of ammonia concentration in the medium when the growth rate of ammonia-limited batch cultures reached half maximal, the ammonia saturation constant was determined for the species mentioned as well as Megaspaera elsdenii and Ruminococcus flavefaciens. Except for M. elsdenii, saturation constants of the other bacteria were less than 50 microM ammonia nitrogen. An organism with a saturation constant for ammonia of 50 microM growing in a medium containing 1 mM ammonia should achieve 95% of its maximum specific growth rate. Many of the predominant species of rumen bacteria are efficient scavengers of ammonia.

Ammonia assimilation and glutamate formation in the anaerobe Selenomonas ruminantium

Selenomonas ruminantium was found to possess two pathways for NH4+ assimilation that resulted in net glutamate synthesis. One pathway fixed NH4+ through the action of an NADPH-linked glutamate dehydrogenase (GDH). Maximal GDH activity required KCl (about 0.48 M), but a variety of monovalent salts could replace KCl. Complete substrate saturation of the enzyme by NH4+ did not occur, and apparent Km values of 6.7 and 23 mM were estimated. Also, an NADH-linked GDH activity was observed but was not stimulated by KCl. Cells grown in media containing non-growth-rate-limiting concentrations of NH4+ had the highest levels of GDH activity. The second pathway fixed NH4+ into the amide of glutamine by an ATP-dependent glutamine synthetase (GS). The GS did not display gamma-glutamyl transferase activity, and no evidence for an adenylylation/deadenylylation control mechanism was detected. GS activity was highest in cells grown under nitrogen limitation. Net glutamate synthesis from glutamine was effected by glutamate synthase activity (GOGAT). The GOGAT activity was reductant dependent, and maximal activity occurred with dithionite-reduced methyl viologen as the source of electrons, although NADPH or NADH could partially replace this artificial donor system. Flavin adenine dinucleotide, flavin mononucleotide, or ferredoxin could not replace methyl viologen. GOGAT activity was maximal in cells grown with NH4+ as sole nitrogen source and decreased in media containing Casamino Acids.

Microbial transaminase activities and their relationship with bovine rumen metabolites

Two each adult male crossbred cattle and murrah buffalo were fed a diet of alfalfa hay, chopped wheat straw, and concentrate mixture. Total rumen transaminase activity of cattle was higher than that of buffalo. Rumen protozoal fractions showed higher total transaminase activity than bacterial fractions in both ruminant species. Besides generally studied glutamate oxalacetate transaminase and glutamate pyruvate transaminase, a large number of other microbial transaminases also have been detected in the rumen of both the ruminant species. Bacterial fractions of rumen liquor were devoid of transaminases utilizing tryptophan, threonine, and lysine as their substrates. Ruminal ammonia and nonprotein nitrogen were correlated positively with microbial transaminases in both species. Transamination reactions may be important for assimilation of ruminal ammonia to cellular proteins.

The origin of nitrogen incorporated into compounds in the rumen bacteria of steers given protein- and urea-containing diets

1. Two young Friesian steers fitted with rumen cannulas were each given three different isonitrogenous and isoenergetic diets for successive periods of 2-3 weeks. The diets consisted mainly of straw and tapioca, with the nitrogen supplied mainly as decorticated groundnut meal(DCGM; diet A), in approximately equal amounts of DCGM and urea (diet B), or entirely as urea (diet C). 2. At the end of each period on a given diet, part of the dietary urea of a morning feed was replaced by a solution of [15N]urea which was infused into the rumen. Samples of rumen contents were removed just before giving the 15N dose and at 1,3,5,7 and 24 h afterwards, concentrations of ammonia and its 15N enrichment were determined and samples of mixed bacteria were prepared. Amino acids, ammonia derived mainly from amide groups, and hexosamines were prepared by ion-exchange chromatography of acid-hydrolysates of the bacteria and analysed for 15N. 3. Approximate estimates of net bacterial N synthesis were made from turnover data for rumen fluid and 15N enrichments in rumen fractions. From the determined efficiency of incorporation of urea-N into bacteria recovered at the duodenum, it was calculated that on diets A, B and C respectively 82%, 37% and 0% of the bacterial N was derived from dietary protein or other non-urea sources. 4. [15N]urea was converted rapidly to ammonia and the 15N then incorporated into bacterial amide-N; it appeared at a slower rate in total bacterial non-amide-N. Rates of incorporation into non-amide-N were highest for glutamic acid, aspartic acid and alanine, and generally lowest for proline (pro), histidine (his), phenylalanine(phe), arginine(arg), methionine(met) and galactosamine. A similar ranking was also generally observed for relative 15N abundances (15N atoms % excess in N component divided by 15N atoms % excess in total bacterial N) achieved after several hours. Relative 15N abundances in his, arg and pro increased with decreasing protein (DCGM) in the diet but those in the other protein amino acids, including the poorly labelled met, phe (and its derivative tyrosine) did not. 5. It was concluded that different extents of labelling of the amino acids (at least those present mainly in protein) indicated that different amounts of preformed units (amino acids or peptides) were used. When an adequate supply of such units was available (particularly on diet A) pro, arg, his, met and phe were derived in this way to a greater extent than the other amino acids, but whereas synthesis of pro, arg and his increased on the low-protein diet C, that of met and phe did not. Thus met and phe may be limiting for bacterial growth on diets low in protein and high in non-protein-N. 6. Differences in the extent of labelling of other bacterial N components may be due to different turnover rates.