Asparagine and glutamine metabolism in chicks

The potential of glutamine supplementation in reduced-crude protein diets for chicken-meat production

This review explores the potential of including glutamine, a so-called non-essential amino acid, in the formulation of reduced-crude protein (CP) diets for broiler chickens. There is a precedent for benefits when including glycine and serine in reduced-CP diets. Fundamentally this is due to decreases in non-essential amino acid concentrations in reduced-CP diets - an unavoidable consequence of reducing CP without amino acid supplementation. The situation for glutamine is complicated because analysed dietary concentrations are very rarely provided as standard assays do not differentiate between glutamine and glutamate and are reported on a combined basis as glutamic acid. The dietary requirement for glutamic acid is approximately 36.3 g/kg but it is increasingly unlikely that this requirement will be met as dietary CP levels are progressively reduced. Glutamine is an abundant and versatile amino acid and constitutes 50.5 mg/g of whole-body chicken protein and is the dominant free amino acid in systemic plasma where it has been shown to provide 22.6% (139.9 of 620.3 μg/mL) of the total in birds offered 215 g/kg CP, wheat-based diets. In addition to dietary intakes, glutamine biosynthesis is derived mainly from the condensation of glutamate and ammonia (NH3) catalysed by glutamine synthetase, a reaction that is pivotal to NH3 detoxification. Glutamate and NH3 are converted to glutamine by phosphate-dependent glutaminase in the reciprocal reaction; thus, glutamine and glutamate are interchangeable amino acids. However, the rate of glutamine biosynthesis may not be adequate in rapidly growing broiler chickens and exogenous and endogenous glutamine levels are probably insufficient in birds offered reduced-CP diets. The many functional roles of glutamine, including NH3 detoxification and maintenance of acid-base homeostasis, then become relevant. Twenty feeding studies were identified where dietary glutamine supplementation, usually 10 g/kg, was evaluated in birds kept under thermoneutral conditions. On balance, the outcomes were positive, but the average dietary CP was 213 g/kg across the twenty feeding studies, which indicates that CP and, in turn, glutamine concentrations would have been adequate. This suggests that glutamine inclusions in reduced-CP diets hold potential and consideration is given to how this may be best confirmed.

The influence of substituting dietary peptide-bound with free amino acids on nitrogen metabolism and acid-base balance of broiler chickens depends on asparagine and glutamine supply

Reducing dietary crude protein (CP) concentration while maintaining adequate amino acid (AA) supply by free AA inclusion can contribute to attenuate the negative environmental effects of animal farming. This study investigated upper limits of dietary free AA inclusions without undesirable effects including the dependence on asparagine (Asn) and glutamine (Gln) supply. Ten broilers were allocated to sixty-three metabolism units each and offered nine experimental diets from day (d) 7-21 (n 7). One diet (167 g CP/kg) contained 80 g soya protein isolate (SPI)/kg. In the other diets, 25, 50, 75 and 100 % of the digestible AA from SPI were substituted with free AA. Digestible Asn+aspartic acid (Asp) and Gln+glutamic acid (Glu) were substituted with Asp/Glu or 50/50 mixes of Asp/Asn and Glu/Gln, respectively. Total excreta were collected from d 11-14 and from d 18-21. Growth and nitrogen accretion were unaffected by 25 and 50 % substitution without and with free Asn/Gln, respectively, but decreased at higher substitution (P ≤ 0·024). Circulating concentrations of Asp, Glu and Gln were unaffected by treatment, while Asn decreased at substitution higher than 50 % when Asn/Gln were not provided (P ≤ 0·005). Blood gas analysis on d 21 indicated a compensated metabolic acidosis at substitution higher than 50 and 75 % without and with free Asn/Gln, respectively (P ≤ 0·017). Results suggest that adding Asn/Gln increased an upper limit for proportion of dietary free AA from 10 to 19 % of dietary CP and enabled higher free AA inclusion without affecting the acid-base balance.

Oxidation of amino acids, glucose, and fatty acids as metabolic fuels in enterocytes of post-hatching developing chickens

This study determined the oxidation of amino acids, glucose and fatty acid in enterocytes of developing chickens. Jejunal enterocytes were isolated from 0-, 7-, 21-, and 42-d-old broiler chickens, and incubated at 40°C for 30 min in Krebs-Henseleit bicarbonate buffer (pH 7.4) containing 5 mM D-glucose and one of the following: 0.5-5 mM L-[U-14C]glutamate, 0.5-5 mM L-[U-14C]glutamine, 0.5-5 mM L-[U-14C]aspartate, 0.5-5 mM L-[U-14C]alanine, 0.5-2 mM [U-14C]palmitate, D-[U-14C]glucose, 0.5-5 mM [U-14C]propionate, and 0.5-5 mM [1-14C]butyrate. 14CO2 produced from each 14C-labeled substrate was collected for determination of radioactivity. Among all the substrates studied, glutamate had the greatest rate of oxidation in enterocytes from 0- to 42-d-old chickens. Glutamate transaminases, rather than glutamate dehydrogenase, may be primarily responsible for initiating glutamate degradation. Rates of amino acid and fatty acid oxidation by cells increased (P < 0.05) with increasing their extracellular concentrations from 0.5 to 5 mM. Rates of glutamate and glucose oxidation in enterocytes decreased (P < 0.05) with increasing age, and rates of glutamine, aspartate, propionate, and butyrate oxidation were lower (P < 0.05) in 42-d-old chickens than in 0-d-old chickens. By contrast, oxidation of palmitate at 2 mM increased (P < 0.05) by 118% in cells from 42-d-old chickens, compared with 0-d-old chickens. Compared with glutamate, oxidation of glutamine, aspartate, alanine, propionate, butyrate, and palmitate was limited in cells from all age groups of chickens. Collectively, these results indicate that glutamate is the major metabolic fuel in enterocytes of 0- to 42-d-old chickens.

Amino Acid Nutrition and Metabolism in Chickens

Both poultry meat and eggs provide high-quality animal protein [containing sufficient amounts and proper ratios of amino acids (AAs)] for human consumption and, therefore, play an important role in the growth, development, and health of all individuals. Because there are growing concerns about the suboptimal efficiencies of poultry production and its impact on environmental sustainability, much attention has been paid to the formulation of low-protein diets and precision nutrition through the addition of low-cost crystalline AAs or alternative sources of animal-protein feedstuffs. This necessitates a better understanding of AA nutrition and metabolism in chickens. Although historic nutrition research has focused on nutritionally essential amino acids (EAAs) that are not synthesized or are inadequately synthesized in the body, increasing evidence shows that the traditionally classified nutritionally nonessential amino acids (NEAAs), such as glutamine and glutamate, have physiological and regulatory roles other than protein synthesis in chicken growth and egg production. In addition, like other avian species, chickens do not synthesize adequately glycine or proline (the most abundant AAs in the body but present in plant-source feedstuffs at low content) relative to their nutritional and physiological needs. Therefore, these two AAs must be sufficient in poultry diets. Animal proteins (including ruminant meat & bone meal and hydrolyzed feather meal) are abundant sources of both glycine and proline in chicken nutrition. Clearly, chickens (including broilers and laying hens) have dietary requirements for all proteinogenic AAs to achieve their maximum productivity and maintain optimum health particularly under adverse conditions such as heat stress and disease. This is a paradigm shift in poultry nutrition from the 70-year-old "ideal protein" concept that concerned only about EAAs to the focus of functional AAs that include both EAAs and NEAAs.

Effects of supplemented nonessential amino acids and nonprotein nitrogen on growth and nitrogen excretion characteristics of broiler chickens fed diets with very low crude protein concentrations

Reducing dietary CP for broiler chickens below a certain threshold results in decreased growth, even when the supply of essential amino acids and glycine equivalent (Gly_equi) is adequate, probably because other nonessential amino acids (neAA) are growth-limiting. Nonprotein nitrogen (NPN) might be used for the synthesis of neAA. Therefore, the effects of specific neAA and ammonium chloride (NH₄Cl) supplementation on the growth and N-excretion characteristics of broiler chickens were investigated. Nine male Ross 308 broiler chickens were kept in each of 81 metabolism units from day 7 to 21 and received 1 of 9 diets in 9 replicates in a one-factorial arrangement of treatments. Two diets with different neAA concentrations, except for Gly_equi, were mixed resulting in CP levels of 180 (CP180) and 160 (CP160) g/kg. In six other diets, CP160 was supplemented with either l-Ala, l-Pro, l-Asp, a mix of l-Asp and l-Asn·H₂O, l-Glu, or a mix of l-Glu and l-Gln to achieve concentrations of the respective neAA as formulated in CP180. In a further diet, NH₄Cl was added to CP160 to achieve the CP concentration of CP180. The ADG and gain:feed ratio (G:F) from day 7 to 21 were highest at CP180. Reduced neAA concentrations in CP160 decreased ADG and G:F. Supplementation of Asp+Asn, Glu, and Glu+Gln to CP160 increased ADG and G:F, but not to the level found for CP180. Compared with CP160, addition of Asp increased G:F but not ADG. Supplementation of Asp+Asn caused higher ADG and G:F than supplementation of Asp alone. The N-utilization efficiency was highest at CP160 and at CP160 supplemented with Ala, Pro, and Glu. Lower N-utilization efficiency was found at CP180 than at CP160, without and with supplemented neAA. The treatment containing NH₄Cl presented the lowest ADG, G:F, and N-utilization efficiency. These results showed that individual supplementation of Asp+Asn, Glu, and Glu+Gln partly compensates for the growth-reducing effects of very low CP diets. Supplementation of NH₄Cl as NPN source is not suitable for broiler chickens.

Interactive Effects of Glycine Equivalent, Cysteine, and Choline on Growth Performance, Nitrogen Excretion Characteristics, and Plasma Metabolites of Broiler Chickens Using Neural Networks Optimized with Genetic Algorithms

Simple Summary

The negative effects of nitrogen emissions caused by animal husbandry on the environment can be reduced by lowering the crude protein content in the diets of broiler chickens. The nonessential amino acids glycine and serine, investigated together as glycine equivalent, can limit the growth of broiler chickens fed diets that are low in crude protein. The response of broiler chickens to dietary glycine equivalent is not constant and is affected by endogenous precursors of glycine equivalent and metabolic processes that dissipate glycine equivalent. Choline can be converted to glycine, and glycine equivalent is required to form cysteine from methionine. The present study investigated interactive effects among dietary glycine equivalent, cysteine, and choline in broiler chickens. The results showed that the gain:feed ratio increased with dietary glycine equivalent supplementation. The extent of interactive effects among glycine equivalent, cysteine, and choline on the gain:feed ratio was hardly pronounced. Very high nitrogen-utilization efficiency with low variation among treatments was found. The findings indicate that small differences in nitrogen-utilization efficiency caused low glycine equivalent dissipation for nitrogen excretion, likely resulting in small interactive effects among dietary glycine equivalent, cysteine, and choline. These results contribute to further dietary crude protein reduction in feed for broiler chickens.

Abstract

Responses of broiler chickens to dietary glycine equivalent (Gly_equi) are affected by dietary cysteine and choline. Hence, this study investigated interactive effects among dietary Gly_equi, cysteine, and choline on the growth of broiler chickens. Male Ross 308 broiler chickens were maintained in 105 metabolism units (10 birds/unit) from days 7 to 22. Excreta were collected in 12-h intervals from days 18 to 21. Blood was sampled on day 22 (1 bird/unit). Five levels each of Gly_equi (9–21 g/kg), cysteine (2–5 g/kg), and choline (0.5–1.7 g/kg) were tested under 15 diets in 7 replicates each following a fractional central composite design. Another diet was provided to five metabolism units (15 birds/unit) to measure prececal amino acid digestibility. Data were evaluated using neural networks. The gain:feed ratio (G:F) increased with digestible Gly_equi intake. Differences between low and high digestible cysteine intake were low. Effects of choline intake on G:F were low. Nitrogen-utilization efficiency (NUE) was high (≥77%), with low variation among treatments. Plasma metabolites varied among treatments and indicated that metabolism of Gly_equi, cysteine, and choline was influenced. These findings showed that interactive effects of dietary Gly_equi, cysteine, and choline on growth were small, possibly because NUE was barely influenced.

Maturation-dependent neointima formation in fowl aorta

Fowl show spontaneous elevation of blood pressure (BP) and neointimal plaque formation in the abdominal aorta at young ages. Maturation/age-dependent modulation of vascular lesions and a causal relationship between elevated BP and neointima formation, however, have not been clarified. We therefore intended to characterize, first, maturation/age-dependent neointimal plaque formation and vascular lesions and, second, their relationship to BP elevation. The BP measured in conscious domestic fowl, Gallus gallus, White Leghorn breed, DeKalb strain, via an indwelling catheter inserted into the ischiadic artery, increased with maturation in males; and at plateau level, BP (mmHg) was significantly (P<0.01) higher in males (194.0+/-4.6, n=11) than in females (169.3+/-3.1, n=10). Neointimal plaques consisting of neointimal cells and abundant extracellular matrix appeared initially in the distal segment of the abdominal aorta (lesion-prone area) of chicks as early as 6 weeks old. The area (size) of neointimal plaques right above the ischiadic bifurcation increased with maturation, whereas the plaque area became smaller with some degenerative changes in adult birds. In some birds, diffuse subendothelial hyperplasia and more extensive plaque formation at the branching points of the aorta were observed. The plaque area appears to be larger in birds, particularly in males that have higher BP (r=0.68). The width of aortic smooth muscle (SM) layers, measured in regions with no plaque, increased with age, whereas the number of cells per unit of area decreased, suggesting that hypertrophy of vascular SM occurs in response to exposure of the vascular wall to high BP. The number of cells was significantly (P<0.01) higher in the plaque than in underlying aortic SM layers or in layers with no plaque formation. Both neointimal plaques and underlying SM layers are immunohistochemically positive for alpha SM actin, suggesting that neointimal cells are modulated SM cells, whereas the staining with SM myosin heavy chain antibody is low in neointimal plaques. Furthermore, plasma arginine levels dropped in accordance with the time of neointimal plaque formation, whereas plasma cholesterol levels showed an age-dependent increase. The results suggest that spontaneous development of neointimal plaques may be a consequence of exposure to high BP and associated local hemodynamic changes.

Distribution of phosphate-activated glutaminase isozymes in the chicken: absence from liver but presence of high activity in pectoralis muscle

The distribution of glutaminase expression in a uricotelic species, the chicken, has been examined using cDNA probes to the rat isozymes. The results suggest that chickens do not possess a glutaminase isozyme equivalent to the liver-type isozyme of mammalian liver. Measurements of enzymic activity also showed very low glutaminase activity in chicken liver. Extra-hepatic tissues in the chicken do express a glutaminase isozyme mRNA which is detected by rat kidney-type glutaminase cDNA. The abundance of this mRNA was highest in kidney and breast muscle and relatively abundant in brain, spleen and adipose tissue. Chicken small intestine expressed relatively low levels of the mRNA. The high level of glutaminase mRNA in chicken pectoralis muscle was accompanied by high glutaminase enzymic activity. In contrast, in mixed leg muscle glutaminase mRNA was barely detectable by Northern blot and glutaminase activity was relatively low. Starvation for 48 h resulted in a slight decrease in the activity of glutaminase in pectoralis muscle, but a large decrease in the relative abundance of the mRNA. The results suggest that in the chicken, hepatic glutamine hydrolysis is not quantitatively important, but skeletal muscle may be a major site of glutamine catabolism.

Responses of laying hens to diets containing up to 2% DL-methionine or equimolar (2.25%) 2-hydroxy-4-(methylthio)butanoic acid

Diets supplemented with up to .6% DL-Met (DLM) or .68% 2-hydroxy-4-(methylthio)butanoic acid (HMB, Alimet) acidify the urine and reduce the incidence of urolithiasis in pullets and laying hens. Excessive acidification potentially may reduce eggshell quality and bone mineralization by interfering with Ca metabolism and may severely challenge the liver and kidneys, which are the primary organs responsible for attenuating metabolic acidosis. To evaluate these possibilities, 30-wk-old Single Comb White Leghorn hens in full production (five hens per replicate, six replicates per diet treatment) were fed for 30 d a 15.7% CP corn and soybean meal-based control layer ration alone or supplemented with DLM (.5, 1, 1.5, or 2%) or equimolar HMB (.56, 1.13, 1.69, or 2.25%). None of the diets caused mortality or gross hepatic or renal damage. Hens fed diets supplemented with the highest levels of DLM and HMB exhibited significant reductions in feed intake, hen-day egg production, and liver mass and had lower plasma concentrations of alanine amino-transferase and isocitrate dehydrogenase when compared with hens fed the control diet. Kidney mass was not significantly affected by high levels of DLM or HMB, but plasma uric acid was significantly higher in hens fed 2% DLM compared with hens fed the control diet. The highest levels of DLM and HMB did not significantly alter total plasma Ca or inorganic phosphate concentrations, nor were percentage eggshell or femur mineralization (femur ash mass:defatted bone mass, femur ash mass:bone volume) significantly reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of dietary crude protein, electrolyte balance, and photoperiod on growth of broiler chickens

Three experiments were conducted with broiler chicks to study the effects of dietary electrolyte balance, dietary crude protein level, and photoperiod on performance. Increasing the electrolyte balance (Na + K -Cl) from about 200 meq/kg to 350 meq/kg or more improved gain and feed consumption of chicks fed high protein (28.6%) diets but depressed gain and feed consumption of chicks fed low protein (14.3%) diets. Neither sex nor photoperiod affected to nature or magnitude of the crude protein X electrolyte balance interaction, although birds housed under 16L:8D had significantly higher concentrations of plasma ammonia than did those housed under 23L:1D. Chicks fed diets containing a high vs. a low electrolyte balance had significantly higher concentrations of plasma uric acid and significantly lower kidney asparaginase activities. Electrolyte effects on kidney asparaginase activity were not affected by diet crude protein content. Results indicate that the electrolyte balance that provides for optimum growth is dependent upon dietary crude protein concentration. Further, diet electrolyte effects on metabolic acid-base homeostasis are not related to their effects on growth.

Time course of urinary excretion of intraportal ammonia as uric acid and ammonia in chickens fed low- or high-protein diet

15N-ammonia was intraportally infused for 6 hr into chickens fed 5% or 20% protein diet to examine the time course of urinary excretion of intraportal ammonia and dietary effects on it. Urinary ammonia increased linearly for the first hour to the same extent in both dietary groups and thereafter further in the low-protein group. Urinary uric acid derived from the intraportal ammonia adaptively increased and reached a steady state level within 1.5 hr. This level was four times higher in the high-protein group. The infused ammonia was excreted into urine as both ammonia and uric acid, in relatively high proportions in the chickens fed the low-protein diet but was almost all excreted as uric acid in those fed the high-protein diet.