Intestinal absorption of an arginine-containing peptide in cystinuria

Amino acid absorption and homeostasis in mice lacking the intestinal peptide transporter PEPT1

The intestinal peptide transporter PEPT1 mediates the uptake of di- and tripeptides derived from dietary protein breakdown into epithelial cells. Whereas the transporter appears to be essential to compensate for the reduced amino acid delivery in patients with mutations in amino acid transporter genes, such as in cystinuria or Hartnup disease, its physiological role in overall amino acid absorption is still not known. To assess the quantitative importance of PEPT1 in overall amino acid absorption and metabolism, PEPT1-deficient mice were studied by using brush border membrane vesicles, everted gut sacs, and Ussing chambers, as well as by transcriptome and proteome analysis of intestinal tissue samples. Neither gene expression nor proteome profiling nor functional analysis revealed evidence for any compensatory changes in the levels and/or function of transporters for free amino acids in the intestine. However, most plasma amino acid levels were increased in Pept1(-/-) compared with Pept1(+/+) animals, suggesting that amino acid handling is altered. Plasma appearance rates of (15)N-labeled amino acids determined after intragastric administration of a low dose of protein remained unchanged, whereas administration of a large protein load via gavage revealed marked differences in plasma appearance of selected amino acids. PEPT1 seems, therefore, important for overall amino acid absorption only after high dietary protein intake when amino acid transport processes are saturated and PEPT1 can provide additional absorption capacity. Since renal amino acid excretion remained unchanged, elevated basal concentrations of plasma amino acids in PEPT1-deficient animals seem to arise mainly from alterations in hepatic amino acid metabolism.

Molecular and Integrative Physiology of Intestinal Peptide Transport

Intestinal protein digestion generates a huge variety and quantity of short chain peptides that are absorbed into intestinal epithelial cells by the PEPT1 transporter in the apical membrane of enterocytes. PEPT1 operates as an electrogenic proton/peptide symporter with the ability to transport essentially every possible di- and tripeptide. Transport is enantio-selective and involves a variable proton-to-substrate stoichiometry for uptake of neutral and mono- or polyvalently charged peptides. Neither free amino acids nor peptides containing four or more amino acids are accepted as substrates. The structural similarity of a variety of drugs with the basic structure of di- or tripeptides explains the transport of aminocephalosporins and aminopenicillins, selected angiotensin-converting inhibitors, and amino acid-conjugated nucleoside-based antiviral agents by PEPT1. The high transport capacity of PEPT1 allows fast and efficient intestinal uptake of the drugs but also of amino acid nitrogen even in states of impaired mucosal functions. Transcriptional and post-transcriptional regulation of PEPT1 occurs in response to alterations in the nutritional status and in disease states, suggesting a prime role of this transporter in amino acid absorption.

Molecular biology of mammalian plasma membrane amino acid transporters

Molecular biology entered the field of mammalian amino acid transporters in 1990-1991 with the cloning of the first GABA and cationic amino acid transporters. Since then, cDNA have been isolated for more than 20 mammalian amino acid transporters. All of them belong to four protein families. Here we describe the tissue expression, transport characteristics, structure-function relationship, and the putative physiological roles of these transporters. Wherever possible, the ascription of these transporters to known amino acid transport systems is suggested. Significant contributions have been made to the molecular biology of amino acid transport in mammals in the last 3 years, such as the construction of knockouts for the CAT-1 cationic amino acid transporter and the EAAT2 and EAAT3 glutamate transporters, as well as a growing number of studies aimed to elucidate the structure-function relationship of the amino acid transporter. In addition, the first gene (rBAT) responsible for an inherited disease of amino acid transport (cystinuria) has been identified. Identifying the molecular structure of amino acid transport systems of high physiological relevance (e.g., system A, L, N, and x(c)- and of the genes responsible for other aminoacidurias as well as revealing the key molecular mechanisms of the amino acid transporters are the main challenges of the future in this field.

Cystinuria calls for heteromultimeric amino acid transporters

The proteins rBAT (related to bo,+ amino acid transporter) and 4F2hc (the heavy chain of the surface antigen 4F2) are homologous proteins that induce amino acid transport in Xenopus oocytes. The role of rBAT in amino acid transport is substantiated by the fact that mutations in the gene encoding it cause cystinuria, a heritable disease characterised by high concentrations of cystine in the urine. Structural and functional evidence supports the hypothesis that both rBAT and 4F2hc proteins form part of heterodimeric amino acid transporters. There is new evidence that the functional unit of system y+L amino acid transporter is a disulfide bridge-dependent complex of 4F2hc with a Xenopus oocyte plasma membrane protein.

Enteral nutrition: background, indications and management

Enteral nutrition is only part of the wider field of clinical nutrition in which great advances in both theory and practice have been made over the last decade. We have attempted to summarize what we consider to be the advances that have most relevance to the clinical practice of enteral nutrition. This chapter reviews our present understanding of the processes of digestion and absorption of protein, carbohydrate and fats, and examines how this theoretical understanding can be applied to patients in the clinical situation. A broad classification of the different enteral diets is undertaken, and the reasons for the development of particular diets are discussed. The clinical value of these diets is assessed. The wide variety of indications for enteral (as opposed to parenteral) nutrition are discussed and the specific benefits of enteral nutrition for the patient are highlighted. Techniques of administration of enteral nutrition are reviewed in detail, and the methods by which enteral nutrition should be monitored are outlined. Finally, complications of enteral nutrition are summarized and advice given on how to prevent or treat them.

Cystinuria

Cystinuria is an inherited metabolic disease resulting in renal stone formation. An incidence of 1 in 7000 makes it a relatively common genetic disease. The biochemical defect is a carrier protein in the epithelial cells of certain organs. This carrier protein is responsible for the transport of cystine and the dibasic amino acids. Cystine is a poorly soluble compound which precipitates in acid urine and results in renal calculi. Cystine stones account for 1 to 2% of all renal calculi. Homozygotes are detected by the high concentration of cystine in their urine. Treatment consists of sulfhydryl compounds that form more soluble compounds with cystine through sulfhydryl exchange as well as alkalinization of urine and hydration to make cystine more soluble.

Transport of glycyl-L-proline by human intestinal brush border membrane vesicles

This study characterizes the transport of [1-14C]glycyl-L-proline into purified brush border membrane vesicles prepared from human small intestine. Time-course uptake curves of glycyl-L-proline were similar under sodium thiocyanate or potassium thiocyanate gradient conditions (extravesicular greater than intravesicular) and did not show any overshoot phenomena. The transport of glycine and proline, however, was stimulated by the presence of sodium gradient. Measurement of peptide uptake with increasing medium osmolarity showed that glycyl-L-proline was transported into an osmotically reactive intravesicular space with insignificant binding to the surface of the vesicles. Only 2% of the glycyl-L-proline in the incubation media was hydrolyzed after 10 min of incubation. Also, there was no hydrolysis of peptide transported into the intravesicular space. The effects of increasing concentrations of glycyl-L-proline on uptake showed that uptake of the peptide was saturable and conformed to Michaelis-Menten kinetics with a Km of 4.1 +/- 0.5 mM and a Vmax of 1.53 +/- 0.07 nmol/mg protein X 0.5 min. Free amino acids did not inhibit the transport of glycyl-L-proline while dipeptides and tripeptides exerted appreciable inhibition (up to 60%). Our results show that human small intestinal brush border membrane vesicles transport glycyl-L-proline as an intact peptide by a carrier-mediated, Na+-independent process.

A third dipeptide carrier system typified by l-prolyl-lhydroxyproline and independent of l-leucyl and beta-alanyl dipeptides in rat gut loops

1. The intestinal transport of L-prolyl-L-hydroxyproline (10 mmol/l) was investigated in rat small gut loops in vivo under pentobarbitone anaesthesia. Osmolality of test solutions was adjusted to eliminate any positive effect of solvent drag on disappearance of solutes from the lumen. 2. L-Leucylglycine and beta-alanyl-L-histidine (carnosine), representative members of two distinctly different dipeptide transport groups previously delineated, were tested for competitive action on L-prolyl-L-hydroxyproline uptake at ten times equimolar concentration (100 mmol/l), but were found to have no effect on the carrier system. 3. L-Prolyl-L-hydroxyproline uptake was markedly blocked by other L-prolyl dipeptides, indicating that they shared a common carrier system. Disappearance of L-prolyl-L-hydroxyproline from the gut lumen was reduced from 48% 15 min-1 10 cm-1 (control, containing 70 mmol/l mannitol) to 11% or 20% in the presence of L-prolylglycine (100 mmol/l) or L-prolyl-L-leucine (25 mmol/l), respectively. 4. It was concluded that at least three separate dipeptide carrier protein systems exist in the rat small gut, the disappearance of L-prolyl-L-hydroxyproline from the gut lumen being inhibited by two other L-prolyl dipeptides but not by L-leucyl or beta-alanyl dipeptides.

Amino acid imbalance in cystinuria

After oral ingestion of a free amino acid mixture by three cystinuric patients, plasma increments of lysine and arginine were lower and those of many other amino acids were significantly higher than those found in control subjects. Similar results were obtained in control subjects after amino acid imbalance had been artificially induced by the omission of cystine, lysine, and arginine from the amino acid mixture. Especially high increments of alanine and proline provided the best evidence of amino acid imbalance caused by a temporary lysine and, to a lesser extent, arginine and cystine deficit. No such amino acid imbalance was found to occur in the cystinuric patients after ingestion of whole protein, indicating that absorption of oligopeptides produced by protein digestion provided a balanced physiological serum amino acid increment. This is considered to explain the lack of any unequivocal nutritional deficit in cystinuric patients despite poor absorption of the essential free amino acid, lysine.

Functional characterization of dipeptide transport system in human jejunum

The present studies were performed to determine whether dipeptide absorption in human jejunum exhibits the characteristics of carrier-mediated transport. 15-cm jejunal segments from human volunteers were perfused with test solutions containing varying amounts of either glycylglycine, glycylleucine, glycine, leucine, glycylglycine with leucine or glycine, glycylglycine with glycylleucine, or glycylleucine with an equimolar mixture of free glycine and leucine. Jejunal absorption rates of both glycylglycine and glycylleucine followed the kinetics of a saturable process. The K(m) value in millimoles/liter of glycylglycine was significantly greater than the K(m) value of glycylleucine (43.3+/-2.6 vs. 26.8+/-5.9, P < 0.05); and the K(m) value of glycine was also significantly greater than the K(m) value of leucine (42.7+/-7.5 vs. 20.4+/-5.4, P < 0.05). While overlapping occurred among the K(m) values of free amino acids and dipeptides, the transport kinetics of dipeptides were characterized by higher V(max) values (in micromoles per minute per 15 centimeters) than those of free amino acids. For example, the V(max) values for glycylglycine and glycine were 837+/-62 and 590+/-56, respectively (P < 0.02). While jejunal absorption rates of glycylglycine were not significantly affected by free leucine or free glycine, they were competitively inhibited by glycylleucine. The jejunal absorption rate of glycylleucine was not significantly altered by an equimolar mixture of free glycine and leucine. The selective absorption of dipeptides was investigated by infusing three equimolar mixtures, each containing two different dipeptides. Among the three dipeptides examined, glycylglycine was the least absorbed. There was no significant difference between the absorption of glycylleucine and leucylglycine. The above studies suggest that absorption of both glycylglycine and glycylleucine is mediated by a carrier which is not shared with free neutral amino acids; and that both COOH- and NH(2)-terminal amino acids appear to be influential in imposing the affinity of a dipeptide for the absorption sites.