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Heteromeric amino acid transporters: biochemistry, genetics, and physiology.

The heteromeric amino acid transporters (HATs) are composed of two polypeptides: a heavy subunit (HSHAT) and a light subunit (LSHAT) linked by a disulfide bridge. HSHATs are N-glycosylated type II membrane glycoproteins, whereas LSHATs are nonglycosylated polytopic membrane proteins. The HSHATs have been known since 1992, and the LSHATs have been described in the last three years. HATs represent several of the classic mammalian amino acid transport systems (e.g., L isoforms, y(+)L isoforms, asc, x(c)(-), and b(0,+)). Members of the HAT family are the molecular bases of inherited primary aminoacidurias cystinuria and lysinuric protein intolerance. In addition to the role in amino acid transport, one HSHAT [the heavy subunit of the cell-surface antigen 4F2 (also named CD98)] is involved in other cell functions that might be related to integrin activation. This review covers the biochemistry, human genetics, and cell physiology of HATs, including the multifunctional character of CD98.

Amino Acid Sequence↗

Cutaneous manifestations associated with aminoaciduria. Report of two cases.

This is a report of 2 cases revealing photosensitivity in association with aminoaciduria. The first is a patient with psoriasis and the second, a patient with Ehlers-Danlos syndrome, mitis type. The display of the above combinations seems peculiar. The psoriatic lesions become confined mainly to the exposed surface and intermingled with those of photodermatitis forming lesions which are a mixture of the two processes. In the case with Ehlers-Danlos, the disease presented rupioid plaque-like erythematous oozing lesions which seem somewhat different from those of the photodermatosis yet known.

Adolescent↗

High liver glycogen in hereditary fructose intolerance.

A case of hereditary fructose intolerance is reported in a girl aged 2 years at the time of her death. She had apparently progressed normally until the age of 14 months. At 19 months she was admitted to hospital with failure to thrive, hepatomegaly, and superficial infections. Investigations revealed hypoglycaemia, persistent acidosis, aminoaciduria, and a high liver glycogen level which suggested that she had glycogen storage disease. There was also some evidence of malabsorption. At necropsy the liver enzyme estimations showed that fructose 1-phosphate aldolase activity was absent and that fructose 1,6-diphosphate aldolase activity was reduced. Hereditary fructose intolerance and glycogen storage disease have been confused in the past on clinical grounds, but a high liver glycogen level has not previously been reported in hereditary fructose intolerance.

Acidosis↗

The molecular basis of neutral aminoacidurias.

Recent success in the molecular cloning and identification of apical neutral amino acid transporters has shed a new light on inherited neutral amino acidurias, such as Hartnup disorder and Iminoglycinuria. Hartnup disorder is caused by mutations in the neutral amino acid transporter B(0) AT1 (SLC6A19). The transporter is found in kidney and intestine, where it is involved in the resorption of all neutral amino acids. The molecular defect underlying Iminoglycinuria has not yet been identified. However, two transporters, the proton amino acid transporter PAT1 (SLC36A1) and the IMINO transporter (SLC6A20) appear to play key roles in the resorption of glycine and proline. A model is presented, involving all three transporters that can explain the phenotypic variability of iminoglycinuria.

Amino Acid Transport Systems, Neutral↗

Beta-aminoaciduria in patients with Burkitt's lymphoma.

The massive amounts of beta-aminoisobutyric acid (beta-AIBA) in the urine of Burkitt's lymphoma patients were measured along with other alpha-amino acids and beta-alanine present in normal and decreased levels. The ratios of the amount of beta-AIBA to beta-alanine, in mumoles/kg urine collected in 24 hours, were elevated for all patients. The degree of elevation of beta-AIBA excretion and the ratio of the two beta-amino acids appeared to be related to the amount of tumor mass present. These analyses may have possible value in monitoring patients with Burkitt's lymphoma during their disease course.

Alanine↗

[An enteral modular formula in dibasic amino aciduria].

Lysinuric protein intolerance (LPI) or dibasic amino acid aminoaciduria is an unusual metabolic illness which, in countries where it is most common, affects one individual for every 60,000-80,000 births, and which is characterised by the inability to transport dibasic amino acids to the interior of the different cells of the organism. This paper sets out the design for an enteral modular formula for a pediatric patient very probably suffering from LPI. The formula has a very limited protein content, and incorporates L-citruline, a non-proteinogenic amino acid which intervenes in the urea cycle.

Amino Acid Metabolism, Inborn Errors↗

Dipropylacetate and aminoaciduria.

Dipropylacetate (DPA) is an anticonvulsant, which has been successfully used in the treatment of several types of epilepsy. The mode of action has not yet been definitely elucidated, although evidence of influence on gamma aminobutyric acid (GABA) turnover in brain has been presented. Several recent reports of the occurrence of hyperglycinemia in association with DPA-treatment indicate that this agent also influences other areas of amino acid metabolism. In the present study of 10 patients receiving DPA for epilepsy, high concentrations of glycine in plasma and CSF were observed, whereas the levels of all other amino acids remained virtually unchanged. The effect of DPA on urinary excretion of amino acids seems to be of considerable significance as marked elevation of urine concentrations of alanine, asparagine, cystine, glycine, histidine, isoleucine and leucine, phenylalanine and tyrosine were observed. This secondary hyperglycinemia could be due to suppression of glycine conjugation reactions, whereas DPA or its metabolites might interfere with tubular reabsorption of various amino acids, thereby causing hyperaminoaciduria.

Adolescent↗

Inherited defect in hereditary pancreatitis.

Definite inherited defect in hereditary pancreatitis (HP) is not known. A new kindred with 3 definite and 6 suspected cases of HP was investigated for possible inherited abnormalities. No aminoaciduria (except for a slight rise in urinary histidine in one patient) and no hyperparathyroidism, hyperlipidemia, or chromosomal abnormality was present. An increase in serum IgM level of a polyclonal type was noted in 3 definitely affected sisters and also in 2 nonaffected members. Serum alpha-1-antitrypsin and serum trypsin inhibition were normal. However, very marked dilatation and ectasia of the pancreatic duct were found in the propositus. Reviewing the data from this family and previously described kindreds, it is postulated that the genetic abnormality in HP encompasses a wide variety of structural and anatomical defects in the sphincter of Oddi or the pancreatic ductal system. These predispose to intermittent obstruction of the duct with concomitant activation of enzymes and ductal metaplasia. In suspected cases an early effort should be made to outline the pancreatic duct as the defect may be amenable to surgery.

Adult↗

Antenatal diagnosis of hereditary fetal growth retardation with aminoaciduria, cholestasis, iron overload, and lactic acidosis in the newborn infant.

OBJECTIVE: A prenatal diagnosis of the fetus for a mother of two previously deceased infants who died from the recently described autosomal recessive disease (OMIM 603358). The infants presented with intrauterine growth retardation, aminoaciduria, cholestasis, iron overload, severe lactic acidosis, and early death (GRACILE syndrome). STUDY DESIGN: DNA was extracted from the fibroblasts and tissue samples of the deceased infants, parental leukocytes, and from a chorion villus biopsy in the next pregnancy. Haplotypes were determined using the relevant markers flanking the disease-associated region of chromosome 2. RESULTS: Both deceased infants were homozygous for the four critical markers. The fetal haploptypes were identical to those of the siblings and the pregnancy was terminated. The iron content of the fetal liver was increased (5000 microg/g) compared with the controls, with a marked iron accumulation in the Kupffer cells. CONCLUSIONS: Antenatal diagnosis can be performed based on linkage analysis in families with at least one affected child because the disease locus has been assigned to a restricted chromosomal region. Typical histological abnormalities may be present in early fetal life.

Abortion, Induced↗

Maleic acid induced aminoaciduria, studied by free flow micropuncture and continuous microperfusion.

The injection of 200 mg/kg BW maleic acid was found to be a suitable dose for exploring the experimental Fanconi syndrome by micropuncture techniques in rats. In clearance experiments, the fractional excretion of glycine, L-alanine, L-aspartate and taurine was measured. After intraperitoneal administration of maleic acid the excretion of these amino acids was increased in the range between the 20-fold and the 230-fold. Free flow micropuncture experiments showed that the reabsorption of these amino acids is reduced drastically along the whole proximal tubule. Continuous microperfusion experiments lead to the result that, in maleic acid pretreated rats, the reabsorption of 14C-glycine from the proximal convolution was strongly inhibited. It was found, furthermore, that after blocking the saturable glycine transport by L-phenylalanine, the remaining reabsorption of glycine (corresponding to passive diffusion) was exactly the same with and without maleic acid. Microinfusion experiments with 8 mumol.1(-1) L-3H-alanine into the early distal tubule showed a fractional recovery of 103 +/- 4.2% (S.D.) in the control and of 101 +/- 6.5% in presence of maleic acid. It is concluded that maleic acid inhibits the saturable reabsorption mechanism of amino acids along the proximal tubule. Passive permeability of the tubular membrane does not seem to be altered by maleic acid.

Animals↗

ABCB6 (MTABC3) excluded as the causative gene for the growth retardation syndrome with aminoaciduria, cholestasis, iron overload, and lactacidosis.

GRACILE syndrome (growth retardation, aminoaciduria, cholestasis, iron overload, lactacidosis, and early death; OMIM 603358) is a rare metabolic disorder with autosomal recessive mode of inheritance. So far it has been diagnosed only in patients with Finnish ancestors. The GRACILE locus has been positioned to a restricted region of chromosome 2q33-37, but the causative gene remains to be identified. The ABCB6 gene, involved in iron homeostasis, mitochondrial respiratory function, and maintenance of the stability of mitochondrial DNA, has been positioned to this same chromosomal region, and advocated in literature as a highly probable candidate gene for the syndrome on both functional and positional grounds. We carried out sequence and quantitative expression analyses to detect potential disease-associated mutations in the ABCB6 gene. No mutations in the coding region of ABCB6 were found, and the expression level of ABCB6 in patient fibroblasts was found to be comparable to controls. Haplotype analysis of the critical DNA region provided evidence for positional exclusion also. Based on these data, ABCB6 is not the causative gene for GRACILE syndrome.

ATP-Binding Cassette Transporters↗

The oculo-cerebral-renal syndrome of Lowe.

Clinical, light microscopical, ultrastructural, and biochemical studies were done on nerve and muscle biopsy specimens from five patients with the oculo-cerebral-renal syndrome of Lowe. Four patients were American Indians, a racial group in whom this disease has not previously been recognized. The hypotonia, areflexia, and diffuse atrophy of muscles are associated with slowed motor nerve conduction velocities, and the morphologic changes in sensory nerves are attributed to a "dying-back" phenomenon probably resulting from an unknown metabolic derangement.

Abnormalities, Multiple↗

Hyperdibasicaminoaciduria and hyperammonemia in familial protein intolerance.

A 3-year-old boy with hyperdibasicaminoaciduria and hyperammonemia showed characteristics of familial protein intolerance (FPI). Oral loading tests of lysine and arginine disclosed a remarkably reduced capability for intestinal absorption of these amino acids. Because urinary excretion and renal clearance of dibasic amino acids were only moderately elevated in the patient, the conspicuously decreased serum concentration of lysine, arginine, and ornithine was attributed to the defect in internal absorption. A possible explanation for elevated blood ammonia levels in FPI is that it is due to a deficiency of arginine and ornithine in the urea cycle that in turn results from a severe impairment in absorption of the amino acids by the gut mucosa.

Amino Acid Metabolism, Inborn Errors↗