Defect In Amino Acid Transport Research Articles

The Great Masquerade: Varying Manifestations of Lysinuric Protein Intolerance.

Lysinuric protein intolerance (LPI) is an inborn metabolic error caused by cationic amino acid transport defects. The disease has a significant degree of phenotypic variation, with no confirmed genotype-phenotype correlation. Because it presents with symptoms similar to far more common diseases, the diagnosis is often missed, resulting in increased morbidity and mortality. This case series describes three examples of LPI with pulmonary, neurological, and immunological manifestations, emphasising the importance of keeping this disorder on the differential list. Appropriate metabolic and genetic testing is important in providing the correct diagnosis and timely care in such cases.

Amino Acid Transport Defects in Human Inherited Metabolic Disorders.

Amino acid transporters play very important roles in nutrient uptake, neurotransmitter recycling, protein synthesis, gene expression, cell redox balance, cell signaling, and regulation of cell volume. With regard to transporters that are closely connected to metabolism, amino acid transporter-associated diseases are linked to metabolic disorders, particularly when they involve different organs, cell types, or cell compartments. To date, 65 different human solute carrier (SLC) families and more than 400 transporter genes have been identified, including 11 that are known to include amino acid transporters. This review intends to summarize and update all the conditions in which a strong association has been found between an amino acid transporter and an inherited metabolic disorder. Many of these inherited disorders have been identified in recent years. In this work, the physiological functions of amino acid transporters will be described by the inherited diseases that arise from transporter impairment. The pathogenesis, clinical phenotype, laboratory findings, diagnosis, genetics, and treatment of these disorders are also briefly described. Appropriate clinical and diagnostic characterization of the underlying molecular defect may give patients the opportunity to avail themselves of appropriate therapeutic options in the future.

PREVALENCE OF CYSTINURIA IN SERVALS ( LEPTAILURUS SERVAL) IN THE UNITED STATES.

Cystinuria is a condition caused by defects in amino acid transport within the kidneys and small intestines. It has been reported in humans, dogs, domestic cats, ferrets, nondomestic canids, and nondomestic felids, including servals ( Leptailurus serval). Genetic mutations have been identified in dogs, humans, and domestic cats. Cystinuria usually follows an autosomal recessive inheritance, although it can be autosomal dominant and sex linked. The primary objective of this study was to screen urine samples dried on filter paper from captive servals in the United States for cystinuria by using the cyanide-nitroprusside screening test. A second objective was to determine whether cystinuria is inheritable in servals. Servals were initially recruited for the study by survey. Owners and institutions interested in participating were sent a second survey and filter paper for collecting urine samples. Samples were collected from 25 servals. One additional serval with confirmed cystine urolithiasis was added for a total sample size of 26 servals. Twenty-seven percent (7/26) were positive, 54% (14/26) were weakly positive, and 19% (5/26) were negative. Sex, reproductive status, and urine collection method had no significant association with test results. This condition is likely underreported in servals and should be ruled out in any serval with nonspecific signs of illness; neurologic signs such as lethargy, ataxia, or seizures; ptyalism; or signs of lower urinary tract disease such as dysuria, hematuria, stranguria, pollakiuria, or urethral obstructions.

Functional characterization of tyrosine transport in fibroblast cells from healthy controls

Human fibroblast cells are an advantageous model to study the transport of amino acids across cell membranes, since one can control the environmental factors. A major problem in all earlier studies is the lack of precise and detailed knowledge regarding the expression and functionality of tyrosine transporters in human fibroblasts. This motivated us to perform a systematic functional characterization of the tyrosine transport in fibroblast cells with respect to the isoforms of system-L (LAT1, LAT2, LAT3, LAT4), which is the major transporter of tyrosine. Ten (n=10) fibroblast cell lines from healthy volunteers were included in the study. Uptake of L-[U-14C] tyrosine in fibroblasts was measured using the cluster tray method in the presence and absence of excess concentrations of various combinations of inhibitors. This study demonstrated that LAT1 is involved in 90% of total uptake of tyrosine and also around 51% of alanine. Not more than 10% can be accounted for by LAT2, LAT3 and LAT4 isoforms. LAT2 seems to be functionally weak in uptake of tyrosine while LAT3 and LAT4 contributed around 7%. 10% could be contributed by system-A (ATA2 isoform). Alanine consequently inhibited the tyrosine transport by up to 60%. Tyrosine transport through the LAT1 isoform has a higher affinity compared to system-L. In conclusion, the LAT1 isoform is the major transporter of tyrosine in human fibroblast cells. Competition between tyrosine and alanine for transport is shown to exist, probably between LAT1 and LAT2 isoforms. This study established fibroblast cells as a suitable experimental model for studying amino acid transport defects in humans.

Continuous Renal Replacement Therapy in the Initial Management of Neonatal Hyperammonemia Due to Urea Cycle Defects

After completing this article, readers should be able to: 1. Describe the metabolic acidosis that may be seen in urea cycle defects. 2. Delineate the most important factor in determining neurologic outcome in urea cycle defects. 3. Explain the factors that affect the blood ammonia concentration in continuous renal replacement therapy (CRRT). 4. Delineate the most difficult issue in pursuing CRRT in neonates. 5. Describe the morbidities associated with CRRT. MP was born at 39 weeks' estimated gestational age by vaginal delivery to a 35-year-old gravida 3 para 2 woman who had an uncomplicated pregnancy. The 3- and 6-year-old female siblings of the patient were healthy. Apgar scores were 8 at 1 minute and 9 at 5 minutes. The newborn was discharged to home after a 2-day stay in the well baby nursery during which time she appeared to be alert and was breastfeeding appropriately. She presented to the emergency department less than 1 day later after her parents noted increasing lethargy and very poor feeding at home. She was admitted to the neonatal intensive care unit (NICU). Results of a complete blood count with differential count and C-reactive protein evaluation were unremarkable, but the patient progressed to worsening lethargy, respiratory failure, and abdominal distension requiring intubation within hours of admission. The anterior fontanelle was soft on admission physical examination. Further testing revealed a base deficit of 6.1 on arterial blood gas, serum ammonia level of 810 mcmol/L (1,134 mcg/dL), serum lactate level of 5.6 mmol/L (5.6 mEq/L), and serum pyruvate level of 0.06 mmol/L. Results of liver function tests were abnormal, and the infant had a prothrombin time of 35.9 seconds, a partial thromboplastin time of 54.3 seconds, and a fibrinogen concentration of 1.75 g/L (175 mg/dL). Genetic and nephrology consultations were requested immediately. Additional metabolic studies were obtained, including measurement of plasma amino …

Blood Levels of Ammonia and Nitrogen Scavenging Amino Acids in Patients with Inherited Hyperammonemia

Plasma levels of glutamine (456 determinations), alanine (434 determinations), and asparagine (431 determinations) and corresponding ammonia levels (260 determinations) were retrospectively analyzed in 30 patients with hyperammonemia secondary to urea cycle disorders (including 3 patients with amino acid transport defects) and 5 patients with propionic acidemia (PA). All patients had elevated glutamine levels on one or more testing except for 2 patients with severe PA and 1 patient with a mild urea cycle disorder. All but 4 patients with urea cycle disorders showed a maximal glutamine level higher than 100 μmol/dl, and 3 patients had a maximal glutamine level of higher than 200 μmol/dl. The only exceptions were 2 asymptomatic ornithine transcarbamylase (OTC)-deficient females, 1 male with mild OTC deficiency, and 1 patient with citrullinemia (CIT) whose plasma glutamine levels were never above 100 μmol/L. Patients with CIT and argininosuccinic aciduria (ASA) showed statistically significant lower levels of glutamine than patients with other urea cycle disorders. However, the maximal glutamine level did not directly correlate with severity of the disorder and within disorders correlated inversely with severity of outcome. Patients with PA showed statistically significant lower glutamine, alanine, and asparagine levels than patients with urea cycle disorders and the severity of this disorder correlated inversely with plasma glutamine levels. Plasma ammonia levels showed a positive correlation with glutamine in patients with carbamyl phosphate synthetase I and OTC deficiency and a negative correlation in patients with PA. Although, most patients also showed elevated levels of alanine and asparagine, their levels generally did not show a good correlation with glutamine (R2= 0.25 and 0.34, respectively).

A candidate mouse model for Hartnup disorder deficient in neutral amino acid transport.

The mutant mouse strain HPH2 (hyperphenylalaninemia) was isolated after N-ethyl-N-nitrosourea (ENU) mutagenesis on the basis of delayed plasma clearance of an injected load of phenylalanine. Animals homozygous for the recessive hph2 mutation excrete elevated concentrations of many of the neutral amino acids in the urine, while plasma concentrations of these amino acids are normal. In contrast, mutant homozygotes excrete normal levels of glucose and phosphorus. These data suggest an amino acid transport defect in the mutant, confirmed in a small reduction in normalized values of 14C-labeled glutamine uptake by kidney cortex brush border membrane vesicles (BBMV). The hyperaminoaciduria pattern is very similar to that of Hartnup Disorder cases also show niacin deficiency symptoms, of Hartnup Disorder cases also show niacin deficiency symptoms, which are thought to be multifactorially determined. Similarly, the HPH2 mouse exhibits a niacin-reversible syndrome that is modified by diet and by genetic background. Thus, HPH2 provides a candidate mouse model for the study of Hartnup Disorder, an amino acid transport deficiency and a multifactorial disease in the human.

Variant of canine erythrocytes with high potassium content and lack of glutathione accumulation

Summary Although the cation composition of mature erythrocytes in clinically normal dogs comprises low K concentration and high Na concentration because of lack of a Na/K pump, a dog was found that had erythrocytes with high K and reduced glutathione (gsh) concentrations attributable to the existence of a Na/K pump (hk/hg cells). However, 2 dogs were subsequently found that had erythrocytes with high K concentration and increased Na/K pump activity, but without high gsh accumulation (hk/lg cells). In those hk/lg cells, Na-dependent glutamate and aspartate influxes were about a sixth of the values in hk/hg cells, despite a steep Na gradient, and cellular glutamate concentration was not accumulated in hk/lg cells as it was in hk/hg cells. In the latter cells, glutamate and aspartate accumulated because of high activity of Na-dependent amino acid influxes. Therefore, low concentration of glutamate may be the reason for the low gsh concentration in hk/lg cells. In such cells, however, aspartate and glutamine were accumulated similarly as they were in hk/hg cells. The hk/lg cells also had a defect in amino acid metabolism. This defect was different from that in hk/hg cells. Thus, hk/lg cells might be a new model for study of defects in amino acid transport and amino acid metabolism.

Diminished Brush Border Membrane Na‐Dependent L‐Alanine Transport in Acute Viral Enteritis in Piglets

We studied sodium‐dependent uptake of L‐alanine into small intestinal brush border membrane vesicles (BBMV) isolated from piglets 40 h after infection with transmissible gastroenteritis (TGE) virus. Vesicles from TGE‐infected pigs and uninfected litter‐mate controls showed comparable degrees of enrichment and purity. In BBMV prepared by conventional techniques, [3H]L‐alanine “overshoot” (peak uptake/equilibrium uptake) in the presence of a Na gradient was preserved in TGE BBMV, unlike [3H]D‐glucose “overshoot,” which was reduced. When these experiments were repeated using vesicles of greater purity, initial rates of Nadependent L‐alanine influx were reduced in BBMV from infected piglets under voltage clamped conditions with valinomycin. These studies demonstrate a specific amino acid transport defect in the small intestinal epithelium during acute viral diarrhea. They demonstrate too that brush border L‐alanine‐Na co‐transport, although reduced, is present after viral damage, confirming previous studies that showed additive effects of amino acid and glucose on jejunal epithelial Na+ transport in transmissible gastroenteritis. Our findings support the concept that, in viral enteritis, oral rehydration solutions containing amino acid and glucose have a theoretical advantage over glucose electrolyte solutions because they facilitate brush border Na+ entry by two carrier mechanisms.

Diminished brush border membrane Na-dependent L-alanine transport in acute viral enteritis in piglets.

We studied sodium-dependent uptake of L-alanine into small intestinal brush border membrane vesicles (BBMV) isolated from piglets 40 h after infection with transmissible gastroenteritis (TGE) virus. Vesicles from TGE-infected pigs and uninfected litter-mate controls showed comparable degrees of enrichment and purity. In BBMV prepared by conventional techniques, [3H]L-alanine "overshoot" (peak uptake/equilibrium uptake) in the presence of a Na gradient was preserved in TGE BBMV, unlike [3H]D-glucose "overshoot," which was reduced. When these experiments were repeated using vesicles of greater purity, initial rates of Na-dependent L-alanine influx were reduced in BBMV from infected piglets under voltage clamped conditions with valinomycin. These studies demonstrate a specific amino acid transport defect in the small intestinal epithelium during acute viral diarrhea. They demonstrate too that brush border L-alanine-Na co-transport, although reduced, is present after viral damage, confirming previous studies that showed additive effects of amino acid and glucose on jejunal epithelial Na+ transport in transmissible gastroenteritis. Our findings support the concept that, in viral enteritis, oral rehydration solutions containing amino acid and glucose have a theoretical advantage over glucose electrolyte solutions because they facilitate brush border Na+ entry by two carrier mechanisms.

大腸菌のペニシリン誘導フィラメント細胞におけるアミノ酸能動輸送能変化

最少発育阻止濃度以下のペニシリンGにより誘導した大腸菌フィラメント細胞のアミノ酸能動輸送能の測定を行った.各種アミノ酸の能動輸送能は細胞の伸長に伴い低下したが,特にosmotic shock resistant基質においてその低下が著しかった. α-MGやTPMP+の取り込み能は,ペニシリン処理の影響をほとんど受けなかった.このことから,アミノ酸能動輸送能の低下は,そのエネルギー源の消失によるのではなく,原形質膜中のアミノ酸能動輸送担体の分布またはその量的変化に起因することが示唆された.

Amino acid transport properties of erythrocytes from normal newborn lambs and lambs with an inherited defect in amino acid transport

An amino acid transport defect which occurs in the erythrocytes of adult sheep is also present in foetal erythrocytes from newborn lambs which have inherited the lesion. The transport defect in erythrocytes from adult sheep is associated with high intracellular levels of ornithine and lysine and a markedly diminished GSH concentration. Although the lesion in foetal cells also results in the accumulation of ornithine and lysine, the intracellular GSH concentration is only moderately diminished.

Some physiological aspects of genetic variation in the blood of sheep.

The principal genetic variants in sheep red cells and plasma are listed. Current hypotheses as to how the L blood group antigen affects active potassium transport across the red cell membrane are summarized. Recent work on an inherited defect in amino acid transport which results in a red cell GSH deficiency is also described.

Localization of the membrane defect in transepithelial transport of taurine by parallel studies in vivo and in vitro in hypertaurinuric mice.

We investigated the mechanism of taurinuria in three inbred strains of mice: A/J, a normal taurine excretor (taut+); and two hypertaurinuric (taut-) strains, C57BL/6J and PRO/Re. Plasma taurine is comparable in the three strains (approximately 0.5 mM), but taurinuria is 10-fold greater in taut- animals. Fractional reabsorption of taurine is 0.967 +/- 0.013 (mean +/- SD) in A/J); and 0.839 +/- 0.08 and 0.787 +/- 0.05 in C57BL/6J and PRO/Re, respectively. Taurine concentration in renal cortex intracellular fluid (free of urine contamination) is similar in the three strains. Taurine reabsorption is inhibited by beta-alanine, in taut+ and taut- strains. These in vivo findings reveal residual taurine transport activity in the taut- phenotype and no evidence for impaired efflux at basilar membranes as the cause of impaired taurine reabsorption. Cortex slices provide information about uptake of amino acids at the antiluminal membrane. Taurine behaves as an inert metabolite in mouse kidney cortex slices. Taurine uptake by slices is active and, at less than 1 mM, is greater than normal in taut- slices. Concentration-dependent uptake studies reveal more than one taurine carrier in taut+ and taut- strains. The apparent Km values for uptake below 1 mM are different in taut- and taut+ slices (approximately 0.2 mM and approximately 0.7 mM, respectively); the apparent Km values above 1 mM taurine are similar in taut+ and taut- slices. Efflux from slices in all strains in the same (0.0105-0.0113 mumol-min-1-g-1 wet wt), but taut- tissue retains about 10% more radioactivity over the period of efflux. beta-Alanine is actively metabolized in mouse kidney. Its uptake in the presence of blocked transamination, is greater; its intracellular oxidation is attenuated; and its exchange with intracellular taurine is diminished in taut- slices. These findings indicate impaired beta-amino acid permeation on a low-Km uptake system at the luminal membrane in the taut- phenotype. beta-Amino acids are not reclaimed efficiently either from the innermost luminal pool in cortex slices or from the ultrafiltrate in the tubule lumen in vivo. The former leads to high uptake ratios in vitro, the latter to high clearance rates in vivo. In vitro and in vivo data are thus concordant. This is the first time that a hereditary defect in amino acid transport has been assigned to a specific membrane surface in mammalian kidney.

Amino acid transport defect in glutathione-deficient sheep erythrocytes.

THE tripeptide, reduced glutathione (GSH), is present in high concentration in mammalian red blood cells, where its major role is thought to be the protection of the cell against oxidative damage. Congenital red cell GSH deficiency has been reported in man, and attributed to a decreased activity of either of the two enzymes (γ glutamyl cysteine synthetase (GCS) or GSH synthetase (GSHS)) involved in GSH biosynthesis1. Such GSH-deficient red cells have a markedly diminished lifespan, and an increased susceptibility to the haemolytic action of oxidant drugs1.

Possibility of prenatal diagnosis of hereditary defect in amino acid transport

Possibility of prenatal diagnosis of hereditary defect in amino acid transport

Homoarginine in cystinuria.

1. By using ion-exchange columns coupled to a sensitive automated Sakaguchi reaction, in addition to the normal ninhydrin reaction for amino acids, homoarginine, a guanidino homologue of arginine, was found in the plasma and urine of both normal and cystinuric individuals. 2. In all seven cystinuric subjects studied, urinary excretion of homoarginine was approximately ten times that found in normal subjects; the plasma levels of this amino acid were considerably reduced relative to normal individuals. 3. Homoarginine in the plasma can be derived either metabolically from lysine or from dietary sources. 4. In normal subjects homoarginine was cleared at a higher rate than arginine. On the other hand, the clearance of arginine exceeded that for homoarginine in the majority of cystinuric subjects although the values obtained for homoarginine indicate that the defect in amino acid transport also affects this amino acid. 5. The defect in the proximal tubular reabsorption of homoarginine is less severe than that for arginine, suggesting that the reabsorptive site for homoarginine may not be the same as that for arginine. 6. In the design of models for the tubular reabsorption of amino acids in health and disease, the presence of homoarginine and its excessive loss in cystinuria must be accounted for.

Lowe's syndrome. Absence of amino acid transport defect in cultured fibroblasts.

SUMMARYTransport of lysine and glycine was studied in cultured fibroblasts of a patient with Lowe's oculocerebrorenal syndrome. Although patients with this syndrome demonstrate renal and intestinal amino acid transport defects, no abnormality was found in our study.

Small intestinal absorption of amino acids in scurvy

The concept of linked intestinal and renal amino acid transport defects was tested in scorbutic guinea-pigs. These animals developed a renal glycinuria when fed a diet deficient in Vitamin C for 3 to 4 weeks. At that time, no defect in intestinal glycine transport could be demonstrated. Uptake of alanine-C 14 and lysine C 14 in the gut was also normal.

The intestinal absorption defect in cystinuria

This paper brings forward evidence to suggest that the amino-acid transport defect known to occur in the kidneys of cystinurics is also present in the gut.