Intra- and interlaboratory quality control for assay of amino acids in biological fluids: 14 years of the French experience

The functioning of an external quality-control scheme for amino acids set up in 1978 is described. Two measurements were made each month by participating laboratories on a control plasma sample provided by the quality-control center; freeze-dried samples were used from 1978 to 1989 and liquid samples since 1990. In addition, two "blind" samples were sent to the laboratories each year. Every 3 months, the overall results and those of the individual laboratories were analyzed statistically. The validity of the liquid sample control is demonstrated. The progressive improvement of results is commendable. In 1990, the coefficients of variation for all participants ranged from 8.4% for alanine to 23.5% for methionine. The standards used for calibration could contribute to the broad range of results, especially those for histidine and ornithine. The use of blind samples made it possible to detect problems of calibration, of linearity of measurement, of contamination, and of identification of unusual amino acids.

[Variation of amino acids in relation to age in Down syndrome subjects]

BACKGROUND

A preliminary study of plasma and urinary amino acid concentration in Down's syndrome subjects had shown some impairments.

PATIENTS AND METHODS

A comparative study of the variations of amino acid concentration with age in Down's syndrome subjects aged 0 to 60 years and in control subjects aged 0 to 94 years was made in order to determine whether these impairments could be explained by generalized premature aging, or by a specific gene dosage effect.

RESULTS

Two major changes (P < 0.001) were found in Down's syndrome: a decrease in plasma concentration of serine at any age, which could be due to a dosage effect of cytathionine-beta-synthase, and an increase in plasma lysine concentration in patients above 10 year's old, probably due to premature aging. Other minor changes were also present in plasma and urine, also possibly explained by premature aging.

CONCLUSIONS

Other studies are necessary to evaluate possible consequences of such changes in the amino acid profiles.

Amino acid handling in uremic rats: citrulline, a reliable marker of renal insufficiency and proximal tubular dysfunction

The kidney is involved in amino acid reabsorption and metabolism; consequently, in renal insufficiency, these important functions are disturbed, as has been reported in animals and patients. In a first experimental series, rats were subjected to degrees of nephrectomy (NX) varying between 10% and 90%. Three weeks later, amino acid levels were measured in plasma to correlate the levels with the degree of NX. The results indicate that in the range of 33% to 74% NX, the plasma concentration of only three to four amino acids was modified, whereas in rats with 84% NX, the concentration of 11 amino acids was disturbed, compared with sham-operated rats. Citrullinemia was enhanced in uremic rats and correlated with the degree of NX. More interestingly, citrullinemia was increased in the range of 10% to 33% NX without any changes in uremia and creatininemia, two well-known markers of uremic states. A second experimental series was designed to study the time course of changes in aminoacidemia to find a marker for the onset of renal failure. Rats were subjected to 36% NX for a period of 1 to 21 days. Uremia and creatininemia peaked 24 to 48 hours after NX, and creatinine clearance (Clcreat) concomitantly diminished. Unfortunately, these three markers of uremic states returned to control values during the next few days before increasing during the last 2 weeks. In contrast, citrullinemia increased twofold 48 hours after NX and plateaued over the next 20 days. We conclude that in rats, citrullinemia could be used (1) to detect acute and chronic renal failure, (2) as a specific marker of normal function of the proximal tubule, and (3) to estimate the degree of renal damage. From this study, renal insufficiency might be easily detected by measuring citrullinemia.

Gestational age-related reference values for amniotic fluid amino acids: a useful tool for prenatal diagnosis of aminoacidopathies

In this study we have measured the concentration of 24 amino acids and total homocysteine in amniotic fluids obtained between the tenth and 32nd week of gestation from pregnancies not at risk for metabolic diseases. These results are used as reference values to which are compared values obtained from pregnancies at risk for citrullinaemia, argininosuccinic aciduria, HHH (hyperornithinaemia, hyperammonaemia and homocitrullinaemia) syndrome, cobalamin metabolism disorders (CblC or CblD), and sulphite oxidase deficiency. We discuss the helpfulness of amino acid analysis in amniotic fluid for prenatal diagnosis of aminoacidopathies.

Clinical outcome and long-term management of 17 patients with propionic acidaemia

A retrospective study was performed on the clinical outcome and long-term treatment of 17 patients with propionic acidaemia diagnosed during the last 20 years in our hospital. The study group consisted of 12 patients with early onset type of disease and 5 patients with late onset. Seven (41%) patients died, five with early onset and two with late onset. The deceased early onset patients had a median survival of 0.4 years while the deceased late onset patients died at the age of 2.8 and 4 years respectively. Median age of the living early onset patients was 5.2 (1-9.25) years, the late onset patients were 4, 7 and 23 years old. Patients were all treated with natural protein restriction and in most cases carnitine and metronidazole were added. The early onset patients were almost all treated with daily home tube feeding. The mean natural protein intake of early onset patients (6.3 +/- 1.5 g/day) was significantly lower than the natural protein intake of late onset patients (17.6 +/- 5.3 g/day). Supplemental protein intake was higher in early onset patients. The general neurological outcome of our study group was satisfactory with a better outcome for early onset patients. As to growth, many patients showed a failure to thrive, this was particularly for height. The strong protein restriction during the first years of life probably contributed to this.

CONCLUSION

The prognosis for patients with propionic acidaemia appeared to be satisfactory in terms of survival and outcome characteristics such as neurological and mental development. Despite these results the authors feel that the prognosis and quality of life of these patients might be improved with liver transplantation or possibly somatic gene therapy in the future.

Arginine metabolism in cat kidney

1. Arginine is essential for growth in the kitten and, because of the resulting hyperammonaemia, in the adult cat an arginine-free diet is life threatening. 2. The kidney is the main site of arginine synthesis. 3. This study was performed to determine whether the cat kidney synthesizes arginine and to establish which factors, such as low citrullinaemia, defects of argininosuccinate synthase and lyase activities or high renal arginase activity, might limit renal arginine production. 4. Identified nephron segments were isolated by microdissection from collagenase-treated cat kidney. 5. Arginine metabolism was studied by incubating the nephron segments with either physiological concentrations of L-[ureido-14C]citrulline (anabolism) or L-[guanido-14C]-arginine (catabolism). Arginine and urea were measured by a micro-enzymatic method. Amino acids were measured by HPLC. 6. In cat blood, the citrulline, but not the arginine, concentration was very low by comparison with other species. 7. Arginine synthesis occurred almost entirely in the proximal tubule, the highest rate occurring in the proximal convoluted tubule and the lowest in the medullary straight proximal tubule. 8. Arginase activity was restricted to the proximal tubule. Urea production increased from the convoluted towards the medullary straight tubule. 9. The limited capacity of the cat kidney to produce arginine in vivo may result from the low blood concentration of citrulline and from the high arginase activity in the various proximal cells with the ability to synthesize arginine.

Excess extracellular and low intracellular glutamate in poorly differentiating wobbler astrocytes and astrocyte recovery in glutamine-depleted culture medium

The wobbler mouse develops an inherited motoneuronal degeneration of unknown origin in the spinal cord. Primary cultures of adult wobbler spinal cord astrocytes display abnormal morphological characteristics with fewer processes and paucity of cell-cell contacts. We have searched for a possible involvement of glutamate and glutamine intra- and extracellular accumulations in vitro in the abnormal differentiation of mutant astrocytes. We have found significantly higher glutamate and glutamine concentrations in the culture media of mutant astrocytes over a 3-day period compared with normal control astrocytes. Moreover, intracellular glutamate concentrations decreased substantially in mutant astrocytes, but intracellular glutamine concentrations remained unchanged. Furthermore, decreasing initial glutamine concentrations in the culture medium (glutamine-depleted medium) led to the recovery of normal extra- and intracellular concentrations of glutamate and recovery of quasi-normal morphological differentiation and increased cell-cell contacts, leading to an essentially normal looking astrocyte network after 3 days of culture. Under these conditions, which lead to recovery, the only remaining abnormality was the higher glutamine extracellular concentration attained in the originally depleted glutamine media. These findings suggest that mechanisms regulating glutamate/glutamine synthesis and/or influx/efflux are defective in wobbler astrocytes, leading to metabolic imbalance and possible cytotoxic effects characterized by disturbed intercellular networks and poor differentiation.

A scheme for the interpretation of primary and secondary disturbances of plasma and urinary amino acid profiles. A possible way to an expert system

A general scheme for the interpretation of primary and secondary abnormalities of plasma and urine amino acid concentrations is described. The key steps of this scheme are: analytical assessment of the measurements, comparison of results obtained with the reference values expressed in absolute and/or relative concentrations and identification of abnormally increased ninhydrin-positive compounds. The interpretation of results takes account of the various abnormalities induced by drugs or diet. The origins of these abnormalities are ordered by their frequency. A part of the proposed scheme is now computerized as the first step in the development of an expert system.

Abnormal alpha-aminoadipic acid excretion in a newborn with a defect in platelet aggregation and antenatal cerebral haemorrhage

alpha-Aminoadipic acid (alpha AA) is an intermediate in lysine metabolism. We report a new case with alpha AA excess in urine and plasma, without alpha-ketoadipic acid, in a full-term male child born to unrelated parents; he presented at 24h of life with seizures that failed to respond to phenobarbital, clonazepam, and Vigabatrin and death occurred on the 38th day of life. Brain imaging suggested antenatal haemorrhage. Small quantities of alpha AA were also detected in the blood and urine of both parents and a healthy brother, all three of whom exhibited the same defect in platelet aggregation as the deceased child. Both parents had decreased levels of plasma neopterin, a finding that might be related to the immunodeficiency described in other cases.

Recurrent episodes of bizarre behavior in a boy with ornithine transcarbamylase deficiency: diagnostic failure of protein loading and allopurinol challenge tests

Recurrent episodes of bizarre behavior were the only clinical symptoms that finally led to the diagnosis of ornithine transcarbamylase deficiency in an 8-year-old boy. The suspected diagnosis could not be confirmed with the use of current challenge tests. The response to a high-protein diet for 24 hours appeared to be a helpful diagnostic aid.

Renal synthesis of arginine in chronic renal failure: in vivo and in vitro studies in rats with 5/6 nephrectomy

Synthesis of arginine (Arg) from citrulline (Cit) by the kidney is a major source of Arg for the body. The high level of plasma Cit in chronic renal failure is often thought to result from the impairment of the renal conversion of Cit to Arg. To verify this assumption, we performed two studies in Sprague-Dawley rats with 5/6 nephrectomy (CRF rats) and in sham-operated rats (CONT rats). In study I synthesis of Arg by isolated proximal convoluted tubules (PCT; the nephron segment exhibiting the highest Arg synthesis) was measured in vitro with two concentrations of Cit (200 or 50 microM) corresponding to those observed in plasma of rats with or without renal failure. In study II the net renal uptake of Cit and release of Arg were determined in vivo by measuring PAH clearance and arterial and renal venous Arg, and Cit concentrations in anesthetized rats. The in vitro results showed that Arg synthesis increased only in proportion to the hypertrophy of remnant PCT (+50%), and was highly and similarly dependent on Cit concentration in PCT of remnant and normal kidneys (Arg production with 200 microM Cit was 3 times higher than with 50 microM Cit for both CONT and CRF). The in vivo results showed that renal Cit uptake and Arg release were not altered in CRF: -286 +/- 28 versus -326 +/- 16 nmol Cit.min-1 (NS), and + 390 +/- 47 versus + 399 +/- 22 nmol Arg.min-1 (NS) in CONT and CRF rats, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Intra- and interlaboratory quality control for assay of amino acids in biological fluids: 14 years of the French experience

The functioning of an external quality-control scheme for amino acids set up in 1978 is described. Two measurements were made each month by participating laboratories on a control plasma sample provided by the quality-control center; freeze-dried samples were used from 1978 to 1989 and liquid samples since 1990. In addition, two "blind" samples were sent to the laboratories each year. Every 3 months, the overall results and those of the individual laboratories were analyzed statistically. The validity of the liquid sample control is demonstrated. The progressive improvement of results is commendable. In 1990, the coefficients of variation for all participants ranged from 8.4% for alanine to 23.5% for methionine. The standards used for calibration could contribute to the broad range of results, especially those for histidine and ornithine. The use of blind samples made it possible to detect problems of calibration, of linearity of measurement, of contamination, and of identification of unusual amino acids.

Molecular characteristics of uronic-acid-rich protein, a strong inhibitor of calcium oxalate crystallization in vitro

Uronic-acid-rich protein (UAP) is a new urinary macromolecule which strongly inhibits calcium oxalate crystal formation. It is a glycoprotein with a molecular weight of about 35,000 Da, and its carbohydrate content is 8.5%. This inhibitor is composed of two polypeptidic chains crosslinked by chondroitin sulfate. It exhibits partial structural homology with alpha 1-microglobulin. The inhibitory activity seems to be supported by peptidic chains as determined by enzymatic assay.

Clinical aspects of mitochondrial disorders

Mitochondrial disorders have long been regarded as neuromuscular diseases only. In fact, owing to the ubiquitous nature of the oxidative phosphorylation, a broad spectrum of clinical features should be expected in mitochondrial disorders. Here, we present eight puzzling observations which give support to the view that a disorder of oxidative phosphorylation can give rise to any symptom in any organ or tissue with any apparent mode of inheritance. Consequently, we suggest giving consideration to the diagnosis of a mitochondrial disorder when dealing with an unexplained association of symptoms, with an early onset and a rapidly progressive course involving seemingly unrelated organs. Determination of lactate/pyruvate and ketone body molar ratios in plasma can help to select patients at risk for this condition.

Glycine/serine ratio and the prenatal diagnosis of non-ketotic hyperglycinaemia

We describe our experience of prenatal diagnosis of non-ketotic hyperglycinaemia in four at-risk pregnancies using the glycine/serine ratio in amniotic fluid obtained between 18 and 20 weeks of gestation. All glycine levels were in the normal range. Serine levels were normal in two patients and borderline in the others. Glycine/serine ratios were normal in two patients, moderately increased in one patient (+3 SD), and highly increased in one patient (+8 SD). All the children were perfectly normal at birth. Because of this false-positive prediction and the false-negative prediction recently reported, we suggest that this unreliable method should not be used.

Early alterations of plasma free amino acids in chronic renal failure

In order to assess the influence of renal failure and nutritional status on the fasting concentrations of free plasma amino acids, we studied 81 ambulatory adult patients with varying degrees of chronic renal failure. Each of the patients was in good general and nutritional condition. Compared to 33 healthy controls, patients with mild renal failure (Ccr greater than 25 ml/mn) exhibited significantly (p less than 0.01, Student's t test) raised concentrations of cystine, citrulline, ornithine, taurine and 3-methyl-histidine and low level of serine. Concentrations of cystine, citrulline, and 3-methyl-histidine in plasma but not of taurine or ornithine rose in parallel with the progression of renal failure. A significant, but moderate decrease in valine, leucine and isoleucine concentrations was observed in patients with the most marked degree of renal failure (Ccr less than 10 ml/mn). We conclude that changes in the plasma concentration of several non essential amino acids are already present in the early stage of renal failure in patients with no sign of protein malnutrition: these may result from altered metabolic pathways of amino acids related to uremia and/or nephron loss per se whereas the moderate decrease in branched-chain amino acids that is observed only in the advanced stage of renal failure may be, at least in part, nutritional in origin.

Clinical approach to inherited metabolic diseases in the neonatal period: a 20-year survey

Every newborn with unexplained neurological deterioration, ketosis, metabolic acidosis or hypoglycaemia should be suspected of having an inherited error of intermediary metabolism. Many of these conditions can be diagnosed clinically with the aid of simple laboratory investigations. Since a substantial number of these diseases respond well to treatment but may otherwise be fatal, and in order to assure adequate prenatal diagnosis in subsequent pregnancies, a high index of suspicion and rapid diagnosis are necessary in the face of the clinical presentations described. According to three major clinical presentations observed in 218 neonates with inborn errors of intermediary metabolism (neurological distress 'intoxication' type, neurological distress 'energy-deficiency' type and hypoglycaemia with liver dysfunction) and according to the proper use of few laboratory investigations, we propose a method of diagnosis which groups these children into five categories. Initial therapy, and sophisticated investigations can be planned on the basis of this grouping.

Potentiation by ammonia of the metabolic effects of pent-4-enoate in isolated rat hepatocytes

The metabolic effects of pent-4-enoate were studied in isolated rat hepatocytes; 1 mM-pent-4-enoate did not significantly inhibit gluconeogenesis from lactate, alanine and glycerol, but significantly decreased glucose synthesis from pyruvate. The addition of 1 mM-NH4Cl led to a drastic inhibition of glucose synthesis from all these substrates. In hepatocytes incubated with 10 mM-alanine and 1 mM-oleate, pent-4-enoate at 0.05-1 mM slightly inhibited glucose synthesis and ketogenesis. The addition of ammonia resulted in a dramatic potentiation of the metabolic effects of pent-4-enoate. Half-maximum effect of ammonia was observed at 0.2 mM concentration. Concomitant cellular concentrations of ATP and acetyl-CoA were also decreased by the addition of ammonia, as were lactate/pyruvate ratio and beta-hydroxybutyrate/acetoacetate ratio. These data suggest that ammonia seriously interferes with the cellular metabolism of pent-4-enoate and leads to a dramatic potentiation of its effects.

Role of N-acetylglutamate and acetyl-CoA in the inhibition of ureagenesis by isovaleric acid in isolated rat hepatocytes

1 and 10 mmol/l isovalerate strongly inhibited urea synthesis in isolated rat hepatocytes incubated with 10 mmol/l alanine and 3 mmol/l ornithine. Isovalerate also markedly decreased N-acetylglutamate levels, and the decrease correlated with the inhibition of urea synthesis by isovalerate. This compound also lowered cellular levels of acetyl-CoA, a substrate of N-acetylglutamate synthase (EC 2.3.1.1). Isovalerate did not significantly affect the cellular levels of ATP and had no direct effect on N-acetylglutamate synthase activity. These results suggest that the inhibition of urea synthesis by isovalerate is due to decrease in N-acetylglutamate levels.

Inhibition of ureagenesis by valproate in rat hepatocytes. Role of N-acetylglutamate and acetyl-CoA

Valproate (0.5-5 mM) strongly inhibited urea synthesis in isolated rat hepatocytes incubated with 10 mM-alanine and 3 mM-ornithine. Valproate at the same concentrations markedly decreased concentrations of N-acetylglutamate, an essential activator of carbamoyl-phosphate synthetase I (EC 6.3.4.16), in parallel with the inhibition of urea synthesis by valproate. This compound also lowered the cellular concentration of acetyl-CoA, a substrate of N-acetylglutamate synthase (EC 2.3.1.1); glutamate, aspartate and citrulline were similarly decreased. Valproate in a dose up to 2 mM did not significantly affect the cellular concentration of ATP and had no direct effect on N-acetylglutamate synthesis, carbamoyl-phosphate synthetase I and ornithine transcarbamoylase (EC 2.1.3.3) activities.

Characterization of enzymatic deficiencies of branched chain amino-acid catabolism in human fibroblasts by genetic complementation

Leucine and Isoleucine metabolism in cultured skin fibroblasts from patients with leucinosis, beta-Ketothiolase deficiency, propionic, methylmalonic and isovaleric acidemia, was compared with that in normal fibroblasts. A simple assay was developed using (U14C) Isoleucine and (U14C) Leucine as substrates. Radioactive incorporation into protein aminoacids were measured. The (U14C) branched chain aminoacid incorporation into proteins provides an estimation of the protein synthesis and the incorporation ratio (14C) Aspartate + (14C) Glutamate/(14C) branched chain aminoacid, measures the integrity of the metabolic pathway. Complementation tests permits to characterize the genetic defect. The incorporation ratio was significantly decreased in blocked pathways, namely in leucinosis and isovaleric acidemia in the presence of (U14C) Leucine and in Leucinosis, beta-Ketothiolase deficiency, propionic and methylmalonic acidemia in the presence of (U14C) Isoleucine. There was a significant restoration of activity in mutant strains with distinct genetic defects after polyethylene-glycol fusion. This assay provides a valuable tool to screen for enzymatic deficiencies of branched chain aminoacid catabolism.

Blood and urinary amino acids in children receiving total parenteral nutrition

Plasma concentrations and urinary outputs of amino acids were estimated in nineteen patients receiving intravenous hyperalimentation to evaluate the adequacy of dosage and composition of the infusates for the maintenance of normal blood concentrations of essential amino acids. The use of high concentrations of branched chain amino acids seems to be appropriate for valine and isoleucine but not for leucine. The high concentration of cysteine in the infusates used induces a very high urinary excretion of cysteine and cystine and are ineffective to bring the decreased plasma cystine levels back to normal.

Acute effects of glucagon on citrulline biosynthesis

Mitochondria isolated from livers of rats fed on different diets showed altered capacity to synthesize citrulline. Glucagon, 15 min after injection, increases citrulline biosynthesis, except after the high-protein diet. A significant correlation between citrulline biosynthesis and N-acetylglutamate content with and without glucagon treatment was shown when rats were fed on a standard or a carbohydrate diet. Different diets modified carbamoyl phosphate synthetase I (EC 6.3.4.16) and N-acetylglutamate synthase (acetyl-CoA:L-glutamate N-acetyltransferase, EC 2.3.1.1) activities. Glucagon did not modify these activities.