A kinetic study of the interactions between amino acids and monosaccharides at the intestinal brush-border membrane

The Dynamic Conversion of Dietary Protein and Amino Acids into Chicken-Meat Protein

This review considers the conversion of dietary protein and amino acids into chicken-meat protein and seeks to identify strategies whereby this transition may be enhanced. Viable alternatives to soybean meal would be advantageous but the increasing availability of non-bound amino acids is providing the opportunity to develop reduced-crude protein (CP) diets, to promote the sustainability of the chicken-meat industry and is the focus of this review. Digestion of protein and intestinal uptakes of amino acids is critical to broiler growth performance. However, the transition of amino acids across enterocytes of the gut mucosa is complicated by their entry into either anabolic or catabolic pathways, which reduces their post-enteral availability. Both amino acids and glucose are catabolised in enterocytes to meet the energy needs of the gut. Therefore, starch and protein digestive dynamics and the possible manipulation of this 'catabolic ratio' assume importance. Finally, net deposition of protein in skeletal muscle is governed by the synchronised availability of amino acids and glucose at sites of protein deposition. There is a real need for more fundamental and applied research targeting areas where our knowledge is lacking relative to other animal species to enhance the conversion of dietary protein and amino acids into chicken-meat protein.

Rapid protein disappearance rates along the small intestine advantage poultry performance and influence the post-enteral availability of amino acids

A foundation diet, an intermediate blend and a summit diet were formulated with different levels of soyabean meal, casein and crystalline amino acids to compare 'slow' and 'rapid' protein diets. The diets were offered to male Ross 308 chicks from 7 to 28 d post-hatch and assessed parameters included growth performance, nutrient utilisation, apparent digestibility coefficients and disappearance rates of starch and protein (N) in four small intestinal segments. Digestibility coefficients and disappearance rates of sixteen amino acids in three small intestinal segments and amino acid concentrations in plasma from portal and systemic circulations from the foundation and summit diets were determined. The dietary transition significantly accelerated protein (N) disappearance rates in the distal jejunum and ileum. The transition from foundation to summit diets significantly increased starch digestibility coefficients in the ileum and disappearance rates in all four small intestinal segments. These starch responses were associated with significant enhancements in nutrient utilisation. The dietary transition linearly increased digestibility coefficients and disappearance rates of amino acids in the majority of cases. The summit diet increased plasma concentrations of five amino acids but decreased those of four amino acids relative to the foundation diet to significant extents. Plasma concentrations of free amino acids were higher in the portal than systemic circulations. Rapid protein disappearance rates advantaged poultry performance and influenced post-enteral availability of amino acids. If the underlying mechanisms are to be identified, further research into the impact of protein digestive dynamics on broiler performance is required but appears justified.

Factorial effects of salinity, dietary carbohydrate and moult cycle on digestive carbohydrases and hexokinases in Litopenaeus vannamei (Boone, 1931)

Litopenaeus vannamei were reared in close cycle over seven generations and tested for their capacity to digest starch and to metabolise glucose at different stages of the moulting cycle. After acclimation with 42.3% of carbohydrates (HCBH) or 2.3% carbohydrates (LCBH) diets and at high salinity (40 g kg(-1)) or low salinity (15 g kg(-1)), shrimp were sampled and hepatopancreas (HP) were stored. Total soluble protein in HP was affected by the interaction between salinity and moult stages (p<0.05). Specific activity of alpha-amylase ranged from 44 to 241 U mg protein(-1) and a significant interaction between salinity and moult stages was observed (p<0.05), resulting in highest values at stage C for low salinity (mean value 196.4 U mg protein(-1)), and at D0 in high salinity (mean value 175.7 U mg protein(-1)). Specific activity of alpha-glucosidase ranged between 0.09 and 0.63 U mg protein(-1), an interaction between dietary CBH and salinity was observed for the alpha-glucosidase (p<0.05) and highest mean value was found in low salinity-LCBH diet treatment (0.329 U mg protein(-1)). Hexokinase specific activity (range 9-113 mU mg protein(-1)) showed no significant differences when measured at 5 mM glucose (p>0.05). Total hexokinase specific activity (range 17-215 mU mg protein(-1)) showed a significant interaction between dietary CBH and salinity (p<0.05) with highest value (mean value 78.5 mU mg protein(-1)) found in HCBH-high salinity treatment, whereas in the other treatments the activity was not significantly different (mean value 35.93 mU mg protein(-1)). A synergistic effect of dietary CBH, salinity and moult stages over hexokinase IV-like specific activity was also observed (p<0.05). As result of this interaction, the highest value (135.5+/-81 mU mg protein(-1)) was observed in HCBH, high salinity at D0 moult stage. Digestive enzymes activity is enhanced in the presence of high starch diet (HCBH) and hexokinase can be induced at certain moulting stages under the influence of blood glucose level. Perspectives are opened to add more carbohydrates in a growing diet, exemplifying the potential approach for less-polluting feed.

Stimulation of non-sodium-dependent water, electrolyte, and glucose transport in rat small intestine by gum arabic

In experimental models of gastroenterological disease, the soluble fiber gum arabic (GA) acts as a proabsorptive adjuvant. This study investigated which specific transport pathway(s) are affected by GA. Rat jejunum was perfused under anesthesia with a standardized oral rehydration solution (ORS) containing D-glucose, with or without GA (2.5 g/liter). In some preparations either phloridizin, a competitive inhibitor of Na+-coupled D-glucose transport, or phloretin, an inhibitor of basolateral glucose transport, were added to the ORS, with or without GA. Diffusion and paracellular transport changes due to GA were evaluated with L-glucose and [14C]polyethlyene glycol 4000 (PEG). GA partially reversed water, Na+, and D-glucose absorption inhibition induced by phloridzin and normalized water and Na+ absorption in the presence of phloretin. GA also increased absorption of water, Na+, and PEG from an L-glucose ORS. The data suggest that GA does not act via Na+ dependent mechanism(s), but stimulates transcellular and/or transjunctional transport pathways; therefore GA may be useful to increase absorption of solutes transported by diffusion.

Nicarbazin stimulates intestinal transport of L-leucine in rabbit

Nicarbazin is an anticoccidial drug used mainly in birds, but also in rabbits. Besides the anticoccidial activity, nicarbazin has shown several other effects such as inhibition of growth and feed efficiency in poultries, and stimulation of sugar intestinal absorption in rabbits. The present work has been performed in order to check whether nicarbazin also affects L-leucine intestinal absorption. The results obtained show that nicarbazin decreases L-leucine accumulation in the jejunal tissue, and increases mucosal to serosal transepithelial fluxes of this amino acid in a dose-dependent way, without modifying its diffusion across the intestinal epithelium. The drug stimulates the amino acid uptake in brush-border and basolateral membrane vesicles, thus suggesting that nicarbazin increases the absorption of L-leucine mediated by carriers.

Modification of leucine transport across bovine pigment epithelium by metabolic stress

The transport of leucine in the apical-to-basal (retina to choroid) direction across the isolated bovine retinal pigment epithelium is mediated predominantly by the L amino transport system at low carrier (10 microns) concentrations. There is no evidence of an active or facilitated transport system operating in the opposite direction. The identification of the L system is based on the lack of sodium dependence, specific inhibition of leucine transport by 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH), and the demonstration of trans-stimulation. Lysine, glutamate, and 2-methylaminoisobutyric acid (MeAIB) did not provide any competitive inhibition. Ouabain and iodoacetate were also ineffective in modifying leucine transport. The transport mediated by the L system was markedly temperature sensitive, whereas no temperature dependence was apparent in the transport of leucine in the basal-to-apical direction (choroid to retina). When treated with dinitrophenol (DNP), the transport of leucine in the apical-to-basal direction was greatly enhanced, but no effect was observed on the leucine movement in the opposite direction. Azide and rotenone had an effect similar to DNP, as did reducing the partial pressure of O2 to less than 40 Torr. The enhancement of transport appeared to be mediated by the activation of an ancillary system, since it was susceptible to different classes of metabolic and competitive inhibitors as well as the observed ionic dependency. After DNP treatment, the transport of leucine was inhibited by lysine and BCH, revealed a sodium dependence, and could be inhibited by iodoacetate. The characteristics of the enhanced transport appear to be similar to those of the recently described G system(s) of amino acid transport.

Inhibition of the intestinal transport of uracil by hexoses and amino acids

Various hexoses and amino acids were tested as potential inhibitors of the active mucosal to serosal transport of uracil across the everted rat jejunum. Uracil transport displayed Michaelis-Menten type kinetics with a Vmax of 10.4 +/- 0.2 mumol X g-1 X h-1 and an apparent Km of 0.047 +/- 0.002 mM (means +/- S.D.). Scilliroside, an inhibitor of the basolateral (Na+ + K+)-ATPase, dose-dependently inhibited the transport of uracil consistent with the Na+ dependency of uracil transport. Thymine was a full competitive inhibitor (Ki = 0.021 +/- 0.002 mM) of uracil transport. All actively transported substances tested including L-phenylalanine, L-leucine, D-galactose, D-glucose, and 3-O-methylglucose inhibited the transport of uracil. In contrast, L-glucose and fructose, substances which are not actively transported, were without effect on uracil transport. Further studies with D-galactose indicated that it acts as a partial noncompetitive inhibitor (Ki = 6.0 +/- 1.4 mM) of uracil transport. This Ki is in good agreement with the apparent Kt (5.8 +/- 1.1 mM) for D-galactose transport. Phlorizin (0.1 mM), an inhibitor of galactose transport, blocked the inhibitory effect of galactose on uracil transport. In the ileum D-galactose had no effect on uracil transport but thymine caused the same degree of inhibition as in the jejunum. The results demonstrate that heterologous inhibition is a more general phenomenon than had previously been realized.

A revision of current data and views on membrane hydrolysis and transport in the mammalian small intestine based on a comparison of techniques of chronic and acute experiments: experimental re-investigation and critical review

Literature data and the results of our investigations using both generally accepted and original perfusion techniques of the isolated loop of the rat small intestine in in vivo experiments are reviewed. Significant differences in the functioning of the small intestine under conditions of acute and chronic experiments are revealed. It has been established that in chronic experiments as compared to acute ones: (a) the absorption of glucose, galactose, fructose and glycine is 2-5 times higher; (b) Kt as well as Jmax values of the transport of these nutrients differ considerably; (c) Na+-independent mechanism of glucose and glycine transport predominates; (d) higher rates of membrane hydrolysis and more effective interactions between enzyme and transport systems of the enterocyte brush border membranes are observed; (e) functional characteristics of the small intestine affected by various experimental factors are more stable. The conclusion is made that it is necessary to revise current views of the scale and regularities of digestive-transport processes in the small intestine under physiological conditions. The importance of the suggested approaches for general and comparative physiology and biochemistry is discussed.

Effectors of amino acid transport processes in animal cell membranes

Various effectors, which act upon ion gradients, protein synthesis, membrane components or cellular functional groups, have been employed to provide insights into the nature of amino acid-membrane transport processes in animal cells. Such effectors, for example, include ions, hormones, metabolites and various organic reagents and their judicious use has allowed the following list of conclusions. Sodium ion has been found to stimulate amino acid transport in a wide variety of cell systems, although depending on the tissue and/or substrate, this ion may have no effect on such transport, or even inhibit it. Amino acid transport can be stimulated in some cell systems by other ions such as K+, Li+, H+ or Cl-. Both H+ and K+ have been found to be inhibitory in other systems. Amino acid transport is dependent in many cell systems upon an inwardly directed Na+ gradient and is stimulated by a membrane potential (negative cell interior). In some cell systems an inwardly directed Cl- and H+ gradient or an outwardly directed K+ gradient can energize transport. Structurally dissimilar effectors such as ouabain, Clostridium enterotoxin, aspirin and amiloride inhibit amino acid transport presumably through dissipation of the Na+ gradient. Inhibition by certain sugars or metabolic intermediates of the tricarboxylic acid cycle may compete with the substrate for the energy of the Na+ gradient or interact with the substrate at the carrier level either allosterically or at a common site. Stimulation of transport by other sugars or intermediates may result from their catabolism to furnish energy for transport. Insulin and glucagon stimulate transport of amino acids in a variety of cell systems by a mechanism which involves protein synthesis. Microtubules may be involved in the regulation of transport by insulin or glucagon. Some reports also suggest that insulin has a direct effect on membranes. In addition, a number of growth hormones and factors have stimulatory effects on amino acid transport which are also mediated by protein synthesis. Steroid hormones have been noted to enhance or diminish transport of amino acids depending on the nature of the hormone. These agents appear to function at the level of protein synthesis. While stimulation may involve increased carrier synthesis, inhibition probably involves synthesis of a labile protein which either decreases the rate of synthesis or increases the rate of degradation of a component of the transport system.(ABSTRACT TRUNCATED AT 400 WORDS)

The mechanism of Na+-L-lactate cotransport by brush border membrane vesicles from horse kidney: analysis of rapid equilibrium kinetics in absence of membrane potential

Membrane transport of lactate was studied using vesicles prepared from horse kidney brush border. It is shown that the carrier-mediated transport of L-lactate is Na dependent and the D-lactate Na dependence seems weaker than the L stereoisomer. Augmented transport rate is observed following imposition of an artificial chemical Na+ gradient of electrical potential difference. The effect of Na+ chemical gradient on the L-lactate uptake was analyzed using membrane vesicles incubated with 50 mM KCl and valinomycin in order to short circuit any contribution of transmembrane electrical potential to the transport. Kinetics results and principally the absence of linearity between l/v (lactate) versus l/Na+ show that the L-lactate transport mechanism fit the properties of an ordered process with two Na+ ions cotransported with one L-lactate anion. The L-lactate and sodium affinities (Km) determined under Na+ chemical gradient were 1.05 and 48 mM for L-lactate and Na, respectively. The sodium activation was shown to be highly cooperative with a Hill number of 2 although no "sigmoidal" activation effect was observed.