Intestinal amino acid transport: an overview

Sustainable diets with plant-based proteins require considerations for prevention of proteolytic fermentation

The human diet requires a more plant-based approach due to the exhaustive effects animal-based foods have on the environment. However, plant-based proteins generally miss a few or have a lower variety in essential amino acids and are more difficult to digest. Subsequently they might be prone to fermentation by the microbiome in the proximal colon. Proteolytic fermentation can induce microbial-metabolites with beneficial and negative health effects. We review current insight into how balances in saccharolytic and proteolytic fermentation can be maintained when the diet consists predominantly of plant-based proteins. Some proteolytic fermentation metabolites may negatively impact balances in gut microbiota composition in the large intestine and influence immunity. However, proteolytic fermentation can potentially be prevented in the proximal colon toward more saccharolytic fermentation through the addition of non-digestible carbohydrates in the diet. Knowledge on this combination of plant-based proteins and non-digestible carbohydrates on colonic- and general health is limited. Current data suggest that transitioning toward a more plant-based protein diet should be accompanied with a consumption of increased quantities and more complex structures of carbohydrates or by application of technological strategies to enhances digestibility. This can reduce or prevent proteolytic fermentation which might consequently improve human health.

Intestinal Permeability, Inflammation and the Role of Nutrients

The interaction between host and external environment mainly occurs in the gastrointestinal tract, where the mucosal barrier has a critical role in many physiologic functions ranging from digestion, absorption, and metabolism. This barrier allows the passage and absorption of nutrients, but at the same time, it must regulate the contact between luminal antigens and the immune system, confining undesirable products to the lumen. Diet is an important regulator of the mucosal barrier, and the cross-talk among dietary factors, the immune system, and microbiota is crucial for the modulation of intestinal permeability and for the maintenance of gastrointestinal tract (GI) homeostasis. In the present review, we will discuss the role of a number of dietary nutrients that have been proposed as regulators of inflammation and epithelial barrier function. We will also consider the metabolic function of the microbiota, which is capable of elaborating the diverse nutrients and synthesizing products of great interest. Better knowledge of the influence of dietary nutrients on inflammation and barrier function can be important for the future development of new therapeutic approaches for patients with mucosal barrier dysfunction, a critical factor in the pathogenesis of many GI and non-GI diseases.

Regulation of Intestinal Epithelial Cells Properties and Functions by Amino Acids

Intestinal epithelial cells (IECs) line the surface of intestinal epithelium, where they play important roles in the digestion of food, absorption of nutrients, and protection of the human body from microbial infections, and others. Dysfunction of IECs can cause diseases. The development, maintenance, and functions of IECs are strongly influenced by external nutrition, such as amino acids. Amino acids play important roles in regulating the properties and functions of IECs. In this article, we briefly reviewed the current understanding of the roles of amino acids in the regulation of IECs' properties and functions in physiological state, including in IECs homeostasis (differentiation, proliferation, and renewal), in intestinal epithelial barrier structure and functions, and in immune responses. We also summarized some important findings on the effects of amino acids supplementation (e.g., glutamine and arginine) in restoring IECs' and intestine functions in some diseased states. These findings will further our understanding of the important roles of amino acids in the homeostasis of IECs and could potentially help identify novel targets and reagents for the therapeutic interventions of diseases associated with dysfunctional IECs.

Molecular cloning and characterization of CT120, a novel membrane-associated gene involved in amino acid transport and glutathione metabolism

Within the minimum LOH region on chromosome 17p13.3 deleted in hepatocellular carcinoma, a novel human plasma membrane-associated gene, named CT120, was isolated from a human kidney cDNA library using electronical cloning and RACE. The novel gene CT120 consists of 2145bp and encodes a protein with 257 amino acids. Database search revealed that homologs of CT120 exist in different organisms from plant to animal kingdoms, which suggests that CT120 is a highly conserved gene during biological evolution. Different expression patterns of CT120 were observed in many different human normal tissues and in various human tumor cell lines. Transcript of CT120 was not detectable in normal lung tissue, but was abundant in SPC-A-1 (human epithelial-like lung adenocarcinoma) cell line, suggesting that CT120 may be involved in lung cancer development. Subcellular localization analysis showed that CT120 is a novel membrane-associated protein. CT120 can interact with SLC3A2 (member 2 of solute carrier family 3) and GGTL3B (isoform of gamma-glutamyltranspeptidase-like 3) in eukaryotic cells by yeast two-hybrid screen and co-immunoprecipitation assay, which suggested that CT120 may assume very essential physiological functions involved in amino acid transport and glutathione metabolism.

Small bowel adaptation is dependent on site of massive enterectomy

BACKGROUND

Changes in amino acid transport after massive enterectomy occur in a nutrient-dependent fashion and may affect long-term outcome. Epidermal growth factor (EGF) can enhance nutrient transport and a defective epidermal growth factor receptor (EGF-R) has been noted to attenuate adaptation. Most animal studies, however, have examined only a single site of resection. This does not mimic the clinical situation where disease dictates the site of resection leading to proximal, middle, or distal enterectomies. We hypothesize that the site of massive enterectomy will alter nutrient transport and EGF-R levels in the residual gut.

MATERIALS AND METHODS

New Zealand White rabbits were randomized to control, midgut division, or 70% resection (proximal, midgut, or distal). After 1 week, sodium-dependent transport of glucose, glutamine, alanine, and leucine into brush border membrane vesicles was quantitated. EGF-R protein was determined by Western blot analysis.

RESULTS

At baseline, amino acid transport was greater in ileum than jejunum. Surgery alone elevated glutamine and leucine jejunal transport by 130 and 97%, respectively, over controls (P < 0.05). Midgut resection increased jejunal glutamine transport 61% over control (P < 0.05). In contrast, distal resection increased jejunal alanine transport by 150% over controls with no change in glutamine (P < 0.05). After midgut resection, EGF-R was significantly greater (124%) in ileum then in jejunum in whole mucosa homogenates. Proximal resection significantly lowered ileal EGF-R compared to that seen in midgut resected residual ileum.

CONCLUSIONS

Site of massive resection is important in determining postoperative changes in nutrient transport and EGF-R.

Histidine and histamine metabolism in rat enterocytes

To study the metabolic fate of L-histidine and histamine in rat isolated enterocytes, enterocytes were incubated in the presence of 0.1 mM L-[U-14C] histidine. At the rate of 11.1 +/- 2.7 pmol/10(6) cells/30 min, the amino acid was incorporated into cellular proteins. 80 microM cycloheximide, i.e. a protein synthesis inhibitor, inhibited this incorporation by 70 +/- 17%. L-histidine was used for cellular protein synthesis which depended on time and concentration. 0.1 mM L-[U-14C] histidine was little oxidized by intestinal cells, i.e. 0.12 +/- 0.06 pmol/10(6) cells/30 min, and was not converted into histamine. When 10 mM histamine was added to the incubation medium, it completely inhibited the incorporation of 0.1 mM [1,4-14C] putrescine into isolated enterocytes. In enterocyte homogenates, this corresponded to inhibition by histamine of putrescine incorporation as catalyzed by transglutaminase activity. Since histamine incorporation into TCA-precipitable material derived from enterocyte homogenates depended on time and concentration, we concluded that exogenous, but not de novo-formed histamine was able to compete with putrescine incorporation into enterocytes as catalyzed by transglutaminase activity.

Human growth hormone induces system B transport in short bowel syndrome

BACKGROUND

After massive enterectomy, remnant intestine undergoes compensatory adaptation. A combination of human growth hormone (hGH) and a glutamine-enriched modified diet induces further adaptation in patients with short bowel syndrome (SBS) on long-term total parenteral nutrition. The specific actions of each component, however, are not well-defined.

METHODS

New Zealand White rabbits were randomized to control, sham operation, or SBS (70% midjejunoileal resection) groups and treated with either hGH or saline. Sodium-dependent uptake of glucose, glutamine, alanine, leucine, and arginine into brush border membrane vesicles was quantitated. Serum insulin-like growth factor-I (IGF-I) levels were determined by immunoradiometric assay. Mucosal mRNA expression of IGF-I and IGF binding protein 4 (IGFBP-4) was evaluated by northern blot analysis using rat cDNA probes.

RESULTS

Glutamine and leucine transports were 33 and 39% greater, respectively, in the hGH-treated versus saline-treated SBS group (P < 0.05), supporting induction of system B amino acid transport. This upregulation was due, in part, to an 88% increase in glutamine carrier capacity (Vmax) with no change in carrier affinity (Km). Both hGH treatment and SBS increased serum IGF-I levels without direct correlation with increased nutrient transport. IGFBP-4 mRNA expression in small bowel mucosa of saline-treated SBS animals was significantly greater than saline-treated unoperated control values. Mucosal IGFBP-4 mRNA was not significantly altered from control in the other study groups. IGF-I mRNA expression was not detected in mucosa, but weak hybridization was noted in rabbit liver.

CONCLUSIONS

Human growth hormone accelerates early adaptation in SBS by upregulation of system B carrier capacity. Serum IGF-I levels and mucosal IGF-I and IGFBP-4 mRNA expression did not directly correlate with this enhanced nutrient transport, suggesting that hGH might exert its adaptive effects by mechanisms that are independent from the IGF system in this model.

Facilitating effect of amino acids on fructose and sorbitol absorption in children

The facilitating effect of glucose on free fructose absorption has been suggested to be due to a sucrase-related transport mechanism. In contrast, the conditions influencing the absorption of sorbitol have hardly been investigated. As amino acids promote transcellular water flow, we investigated their effects on the absorption of fructose and sorbitol. We studied 15 healthy children using breath hydrogen tests following the ingestion of fructose and sorbitol, alone and in combination with glucose or amino acids. Similarly, the effect of acarbose pretreatment on sucrose and fructose-glucose absorption was investigated. The inhibition of sucrase isomaltase by acarbose impedes the absorption of sucrose but not of the fructose-glucose mixture. Fructose absorption is enhanced by glucose and by the amino acids L-alanine, L-glutamine, L-phenylalanine, and L-proline. Similarly, the absorption of sorbitol is facilitated by glucose and L-alanine. These results are not in concordance with a sucrase-related fructose-transport system and suggest another mechanism for glucose-induced enhancement of fructose (and sorbitol) absorption. We hypothesize that the absorption of fructose and sorbitol may be stimulated by the increased water flux induced by active absorption of glucose as well as amino acids.

Role of membrane transport in interorgan amino acid flow between muscle and small intestine

In the fasting state, amino acids are released from the periphery to be used in splanchnic tissues. To understand the mechanism of such interorgan substrate exchange at the tissue level, we have determined the relationships between inward and outward amino acid transport and intracellular amino acid kinetics in the small intestine and skeletal muscle of postabsorptive anesthetized dogs. In the gut, amino acids appearing intracellularly (from inward transport, protein degradation, and absorption from the lumen) were used for protein synthesis more efficiently (P < .05) than in muscle (phenylalanine, 55% +/- 5% v 13% +/- 3%; lysine, 70% +/- 7% v 28% +/- 3%). In contrast, in muscle, amino acids appearing intracellularly (from inward transport and protein degradation) were preferentially (P < .05) released into the bloodstream, as opposed to being incorporated into protein (phenylalanine, 87% +/- 4%; lysine, 72% +/- 3%). Inward transport accounted for a greater (P < .05) proportion of total intracellular amino acid appearance in the gut than in muscle (leucine, 63% +/- 3% v 37 +/- 3%; valine, 75% +/- 5% v 53% +/- 3%; phenylalanine, 66% +/- 1% v 50% +/- 4%; lysine, 52% +/- 2% v 31% +/- 2%). We conclude that differences in transmembrane amino acid transport kinetics in both the inward and outward directions contribute to the net flow of amino acids from the muscle to the gut in the fasting state.

Intestinal arginine metabolism during development. Evidence for de novo synthesis of L-arginine in newborn pig enterocytes

The capacity for L-arginine metabolism was studied in villus enterocytes isolated from pigs at birth, after 2-8 days suckling and after weaning. Immediately after birth, enterocytes were able to convert 1 mM L-citrulline, 2 mM L-glutamine or 1 mM L-ornithine to L-arginine. In 2-8-day-old animals, the net production of L-arginine from L-citrulline (2.00 +/- 0.45 nmol x 10(6) cells-1 x 30 min-1), or from L-ornithine (0.29 +/- 0.06 nmol x 10(6) cells-1 x 30 min-1) was similar to the values obtained at birth. Furthermore, 40% of L-arginine synthetized de novo from L-citrulline were released into the incubation medium. In 2-8-day-old animals, the production of L-arginine from L-glutamine represented only 5% of the production at birth (the latter being 0.73 +/- 0.15 nmol x 10(6) cells-1 x 30 min-1). In enterocytes isolated from post-weaned pigs, no significant production of L-arginine from either L-glutamine or L-ornithine was detected. In contrast, although the L-arginine production from L-citrulline was very low in post-weaned animals, it was significantly enhanced in the presence of L-glutamine, representing 23% of the production measured in suckling animals. The capacity of enterocytes to cleave L-arginine to L-ornithine and urea was very limited at birth, but was increased more than threefold in 2-day-old animals. This was concomitant with a marked increase in arginase activity. In post-weaned animals, the flux through arginase in intact enterocytes, and the arginase activity were both threefold higher than in 2-8-day-old animals. It is concluded that enterocytes isolated from neonatal pigs exhibit the capacity for a net production of L-arginine since the metabolism of this amino acid is oriented to anabolism rather than catabolism. The results are discussed in relation to L-arginine metabolism in the neonatal liver.

Diamines interfere with the transport of L-ornithine in Ehrlich-cell plasma-membrane vesicles

1. L-Ornithine transport by plasma-membrane vesicles isolated from Ehrlich cells is Na(+)-independent and shows a saturable and a diffusional component. 2. Putrescine, histamine, 5-hydroxytryptamine and 2,3-diaminopropane at 55 microM concentration significantly inhibit 0.5 mM-L-ornithine transport at least for the first 10 min of incubation. 3. There is a trans-stimulatory effect of putrescine on L-ornithine transport.