Urinary excretion of l-carnitine and acylcarnitines by patients with disorders of organic acid metabolism: evidence for secondary insufficiency of l-carnitine

Urinary C6-C12 dicarboxylic acylcarnitines in Reye's syndrome

C6-C12 dicarboxylic acylcarnitines have been identified for the first time in urine from a 2-year-old girl presenting with Reye's syndrome. The acylcarnitines were extracted by ion-exchange chromatography and analysed, both underivatised and as methyl esters using high-resolution fast-atom-bombardment mass spectrometry and B/E-linked scanning. The acylcarnitines were quantified by capillary gas chromatography of the acids extracted after hydrolysis of the acylcarnitine esters. Dodecandioylcarnitine was present in the highest concentration (35.9 mmol/mol creatinine) which exceeded the urinary free dodecandioic acid concentration. The adipic, suberic and sebacic acylcarnitine concentrations were less than 10% of the respective free acid concentrations. It is possible that beta-oxidation of dicarboxylic acids is partially inhibited in Reye's syndrome leading to accumulation of precursor dodecandioyl CoA which is metabolised to dodecandioylcarnitine. The accumulation of these metabolic intermediates may be significant in the pathogenesis of Reye's syndrome.

Urinary acylcarnitines in a patient with neonatal multiple acyl-CoA dehydrogenation deficiency, quantified by a carboxylic acid analyzer with a reversed-phase column

A quantitative analysis for urinary acylcarnitines in a patient with neonatal multiple acyl-CoA dehydrogenation deficiency is described. This method (liquid chromatography) can quantify twelve acylcarnitines including glutarylcarnitine and 3 isomeric acylcarnitines (butyryl-1, valeryl- and octanoylisomer) in urine. Before and up to the 15th hour of DL-carnitine therapy, isovalerylcarnitine was the largest single component existing in urinary acylcarnitines. Its excretion increased approximately 10 times within 1 day of DL-carnitine therapy. However, the acetyl-, the isobutyryl- and the butyrylcarnitine values increased gradually. From the 8th day of the therapy, the isobutyrylcarnitine value exceeded the isovalerylcarnitine. The patient's dominant urinary specific acylcarnitine derived from amino acids oxidation deficiency was changed from isovalerylcarnitine(leucine) to isobutyrylcarnitine(valine) during the early period of DL-carnitine therapy. Glutarylcarnitine was a minor component in the urine. Its degree of increase was as small as that of octanoylcarnitine. 2-Methylbutyrylcarnitine and propionylcarnitine were not detected.

Secondary carnitine deficiency in hyperammonemic attacks of ornithine transcarbamylase deficiency

Carnitine status was evaluated in 12 patients with hyperammonemic attacks caused by a deficiency in ornithine transcarbamylase. We found decreased free carnitine and increased acylcarnitine levels in the serum, a decreased free carnitine content and an elevated acyl/free carnitine ratio in the liver, and increased excretion of free and acylcarnitine in the urine. Analyses of urinary acylcarnitine using the secondary ion mass spectrometry technique revealed increased amounts of acetylcarnitine and dicarboxylic acid derivatives. These data suggest that the patients had a secondary carnitine deficiency, possibly an aggravating factor in urea cycle dysfunction. After oral administration of L-carnitine (50 to 100 mg/kg/d) in two patients, hyperammonemic episodes were less frequent. Blood ammonia levels decreased significantly, accompanied by an increase in serum free carnitine levels.

The importance of recognizing secondary carnitine deficiency in organic acidaemias: case report in glutaric acidaemia type II

Secondary carnitine deficiency in a patient with glutaric acidaemia type II, due to deficient ETF-dehydrogenase activity, is described. The patient responded clinically to a pharmacological dose of riboflavin and a restricted protein diet. In the second year of her life she developed more frequent and severe exacerbations during intercurrent infections from which she did not fully recover. Hypotonia and marked ataxia persisted. Plasma carnitine was entirely complexed as acylcarnitine with no free carnitine detected. Retrospective evaluation of several frozen urine specimens obtained since the age of 10 months revealed undetectable free carnitine with elevated acylcarnitine levels. Marked clinical improvement was observed following L-carnitine supplementation. The hypotonia and ataxia disappeared. The frequency and the severity of the exacerbations were noticeably decreased. The role of L-carnitine in preventing the accumulation of acyl-CoA compounds in inborn errors of organic acid metabolism is further emphasized by this patient. The necessity to evaluate free carnitine, acylcarnitine and acyl/free ratio in the assessment, follow-up and management of patients with inborn errors of organic acid metabolism is discussed.

Characterization of new diagnostic acylcarnitines in patients with beta-ketothiolase deficiency and glutaric aciduria type I using mass spectrometry

Direct analysis of unpurified urine from patients with beta-ketothiolase deficiency and glutaryl-coenzyme A dehydrogenase deficiency was carried out by methylation and fast atom bombardment mass spectrometry. Previously unidentified signals consistent with unusual acylcarnitines were detected. In the former disease, thermospray liquid chromatography/mass spectrometry analysis confirmed the identification of tiglylcarnitine and differentiated it from a biological isomer, 3-methylcrotonylcarnitine. In glutaric aciduria, glutarylcarnitine was confirmed by detection of glutaric acid liberated upon base hydrolysis of a purified acylcarnitine fraction. The discovery of these metabolites suggests that L-carnitine therapy might be beneficial for the enhanced excretion of toxic metabolites that accumulate in patients with these disorders.

Pivampicillin-promoted excretion of pivaloylcarnitine in humans

Pivampicillin treatment of seven children (five boys and two girls) for 7 days significantly reduced the amounts of total acid-soluble carnitine, free carnitine, and long-chain acylcarnitines and increased the amounts of acid-soluble acylcarnitine in plasma. The fasting plasma levels of 3-hydroxybutyrate at the end of treatment were 15% of the control value. The levels of free fatty acids were decreased, whereas triglyceride levels were unaffected, indicating impaired fat metabolism. Daily urinary excretion of total carnitine was four to five times higher than controls after the first day of treatment, although the amounts of free carnitine and acetylcarnitine were decreased. The urinary acylcarnitines were isolated and characterized by gas chromatography/electron impact mass spectrometry and fast-atom bombardment mass spectrometry. Pivaloylcarnitine was the predominant urinary acylcarnitine; it represented greater than 96% of the increased excretion of total carnitine and 75-80% of the total conjugated pivalic acid. The renal clearance of acylcarnitines was comparable to that of creatinine, indicating no reabsorption of pivaloylcarnitine. These data suggest a detoxification function of carnitine for pivalic acid in humans.

The potentiation of ammonia toxicity by sodium benzoate is prevented by L-carnitine

Sodium benzoate has been recommended and even been used for the treatment of hyperammonemia in humans. More recently, a note of caution was raised since it has been shown that in experimental animals, sodium benzoate potentiates ammonia toxicity and inhibits urea synthesis in vitro. This has been further confirmed in the work presented here and the mechanism by which benzoate increases mortality and the levels of blood ammonia in mice given ammonium acetate have also been studied. In hyperammonemia, urea production and N-acetylglutamate levels were decreased by sodium benzoate. Pretreatment of mice with L-carnitine suppressed mortality following ammonium acetate plus sodium benzoate administration. Under these conditions L-carnitine lowered blood ammonia and increased urea production and N-acetylglutamate levels.

Urinary excretion of 2-methyl-2,3-butanediol and 2,3-pentanediol in patients with disorders of propionate and methylmalonate metabolism

Urine samples from patients with propionic acidemia and from a patient with methylmalonic acidemia contained unknown non-acidic metabolites by gas chromatography/mass spectrometry after ethyl acetate extraction. It could be demonstrated by mass spectrometric studies and by synthesis of reference compounds that the major metabolite was 2-methyl-2,3-butanediol, while smaller amounts of 2,3-pentanediol were also present. These diols were present in abnormal amounts in these patients during attacks of metabolic decompensation.

Identification of glutarylcarnitine in glutaric aciduria type 1 by carboxylic acid analyzer with an ODS reverse-phase column

A technique for the identification of glutarylcarnitine in urine from a patient with glutaric aciduria type 1 is described. The patient's urine sample was partially purified using an anion exchange column and analyzed by a carboxylic acid analyzer fitted with an ODS reverse-phase column. The chromatogram of the patient's urine sample revealed 3 different peaks, which corresponded respectively to those of carnitine with amino acids, acetylcarnitine and glutarylcarnitine. Following hydrolysis of the sample, the chromatogram had no peaks of acetylcarnitine and glutarylcarnitine but had remarkably amplified peaks of carnitine, acetic acid and glutaric acid. The eluent fraction of glutarylcarnitine from the non-hydrolyzed sample was hydrolyzed and analyzed again. It no longer had the glutarylcarnitine peak on the chromatogram, but had only two separate peaks of carnitine and glutaric acid. This technique simplifies the identification of glutarylcarnitine, in that it requires only removal of organic acids for preparation of samples, and does not require radioisotope or mass spectrometry.

3-Hydroxy-3-methylglutaric aciduria: response to carnitine therapy and fat and leucine restriction

A female infant, born to first cousin parents, lapsed into coma with severe metabolic acidosis on day three of life. The gas chromatographic/mass spectrometric urinary organic acid profile showed marked elevation of the leucine metabolites 3-hydroxy-3-methylglutaric, 3-methylglutaconic, 3-methylglutaric and 3-hydroxy-isovaleric acids. Less than 5% of the normal activity of the enzyme 3-hydroxy-3-methylglutaryl CoA lyase was detected in cultured skin fibroblasts. The patient's total and free carnitine was initially low but rose to normal levels after placing her on DL-carnitine (100 mg kg-1 d-1). On a diet providing 87 mg kg-1 d-1 of leucine and only 25% of total calories as fat and 2 g kg-1 d-1 protein, the concentration of the urinary organic acids fell markedly. She is now 15 months old with normal growth and development. This regimen appears effective in the early treatment of 3-hydroxy-3-methylglutaric aciduria.

Effect of alpha-ketobutyrate on palmitic acid and pyruvate metabolism in isolated rat hepatocytes

alpha-Ketobutyrate, an intermediate in the catabolism of threonine and methionine, is metabolized to CO2 and propionyl-CoA. Recent studies have suggested that propionyl-CoA may interfere with normal hepatic oxidative metabolism. Based on these observations, the present study examined the effect of alpha-ketobutyrate on palmitic acid and pyruvate metabolism in hepatocytes isolated from fed rats. alpha-Ketobutyrate (10 mM) inhibited the oxidation of palmitic acid by 34%. In the presence of 10 mM carnitine, the inhibition of palmitic acid oxidation by alpha-ketobutyrate was reduced to 21%. These observations are similar to those previously reported using propionate as an inhibitor of fatty acid oxidation, suggesting that propionyl-CoA may be responsible for the inhibition. alpha-Ketobutyrate (10 mM) inhibited 14CO2 generation from [14C]pyruvate by more than 75%. This inhibition was quantitatively larger than seen with equal concentrations of propionate. Carnitine (10 mM) had no effect on the inhibition of pyruvate oxidation by alpha-ketobutyrate despite the generation of large amounts of propionylcarnitine during the incubation. alpha-Ketobutyrate inhibited [14C]glucose formation from [14C]pyruvate by more than 60%. This contrasted to a 30% inhibition caused by propionate. These results suggest that alpha-ketobutyrate inhibits hepatic pyruvate metabolism by a mechanism independent of propionyl-CoA formation. The present study demonstrates that tissue accumulation of alpha-ketobutyrate may lead to disruption of normal cellular metabolism. Additionally, the production of propionyl-CoA from alpha-ketobutyrate is associated with increased generation of propionylcarnitine. These observations provide further evidence that organic acid accumulation associated with a number of disease states may result in interference with normal hepatic metabolism and increased carnitine requirements.

Inhibition of oxidative metabolism by propionic acid and its reversal by carnitine in isolated rat hepatocytes

The present study was designed to study the interaction of propionic acid and carnitine on oxidative metabolism by isolated rat hepatocytes. Propionic acid (10 mM) inhibited hepatocyte oxidation of [1-14C]-pyruvate (10 mM) by 60%. This inhibition was not the result of substrate competition, as butyric acid had minimal effects on pyruvate oxidation. Carnitine had a small inhibitory effect on pyruvate oxidation in the hepatocyte system (210 +/- 19 and 184 +/- 18 nmol of pyruvate/60 min per mg of protein in the absence and presence of 10 mM-carnitine respectively; means +/- S.E.M., n = 10). However, in the presence of propionic acid (10 mM), carnitine (10 mM) increased the rate of pyruvate oxidation by 19%. Under conditions where carnitine partially reversed the inhibitory effect of propionic acid on pyruvate oxidation, formation of propionylcarnitine was documented by using fast-atom-bombardment mass spectroscopy. Propionic acid also inhibited oxidation of [1-14C]palmitic acid (0.8 mM) by hepatocytes isolated from fed rats. The degree of inhibition caused by propionic acid was decreased in the presence of 10 mM-carnitine (41% inhibition in the absence of carnitine, 22% inhibition in the presence of carnitine). Propionic acid did not inhibit [1-14C]palmitic acid oxidation by hepatocytes isolated from 48 h-starved rats. These results demonstrate that propionic acid interferes with oxidative metabolism in intact hepatocytes. Carnitine partially reverses the inhibition of pyruvate and palmitic acid oxidation by propionic acid, and this reversal is associated with increased propionylcarnitine formation. The present study provides a metabolic basis for the efficacy of carnitine in patients with abnormal organic acid accumulation, and the observation that such patients appear to have increased carnitine requirements ('carnitine insufficiency').

Identification of 3-methylglutarylcarnitine. A new diagnostic metabolite of 3-hydroxy-3-methylglutaryl-coenzyme A lyase deficiency

Deficiency of 3-hydroxy-3-methylglutaryl-coenzyme A (CoA) lyase affects the metabolism of leucine as well as ketogenesis. This disorder is one of an increasing list of inborn errors of metabolism that presents clinically like Reye's Syndrome or nonketotic hypoglycemia. Four patients with proven 3-hydroxy-3-methylglutaryl-CoA lyase deficiency were shown to excrete a new diagnostically specific metabolite. The technique of fast atom bombardment and tandem mass spectrometry revealed that only 3-methylglutaryl-CoA is a substrate for acylcarnitine formation. Neither 3-methylglutaconyl-CoA nor 3-hydroxy-3-methylglutaryl-CoA are excreted as acylcarnitines. The excretion of 3-methylglutarylcarnitine may explain, in part, the apparent secondary carnitine deficiency in this disorder. Carnitine supplementation with moderate dietary restrictions may be a useful treatment strategy for this disorder.

Carnitine reduces fasting ketogenesis in patients with disorders of propionate metabolism

Patients with disorders of propionate metabolism have low plasma levels of free carnitine and excrete higher than normal quantities of esterified carnitine. The response to a 19 h fast was assessed as a physiological index of carnitine deficiency. In patients with propionic acidaemia and methylmalonic acidaemia a substantial ketogenesis developed in response to fasting. Supplementation with L-carnitine significantly reduced this ketogenic response.

The response to L-carnitine and glycine therapy in isovaleric acidaemia

The profound metabolic disturbances which occur in isovaleric acidaemia are due to the intramitochondrial accumulation of isovaleryl coenzyme A (CoA) with a consequent reduction in the availability of free CoA. Secondary carnitine insufficiency is also a feature of this and other disorders of organic acid metabolism. A patient who presented at 2.5 years of age was diagnosed using capillary GC-MS as having isovaleric acidaemia. She showed the full spectrum of abnormal organic acids previously associated with the 'neonatal' form of the disease despite her late presentation, indicating that it is inappropriate to refer to acute early and late onset forms of isovaleric acidaemia. Instead, a spectrum of disease exists, determined by environmental factors, residual enzyme activities and modifying effects of different phenotypes in different individuals. She also showed evidence of carnitine insufficiency. An oral challenge with L-carnitine resulted in the excretion of large amounts of urinary acylcarnitines which were shown by use of fast atom bombardment mass spectrometry to be primarily isovalerylcarnitine. Regular glycine supplementation caused no significant increase in urinary isovalerylglycine and had to be stopped because of side-effects after 5 days. An oral L-carnitine challenge during glycine supplementation resulted in a marked increase in isovalerylglycine excretion, again associated with the excretion of large amounts of isovalerylcarnitine. Carnitine acts by removing (detoxifying) intramitochondrial isovaleryl groups and, in the presence of glycine, it promotes the formation of isovalerylglycine. We believe L-carnitine supplementation is of value in the treatment of isovaleric acidaemia and that, in the present case, L-carnitine together with a moderate dietary restriction has proved to be the optimum form of therapy.

Direct identification of propionylcarnitine in propionic acidaemia: biochemical and clinical results of oral carnitine supplementation

Urinary short-chain acylcarnitine in a patient with propionic acidaemia and low levels of free carnitine was found to consist mainly of propionylcarnitine. The compound was isolated by sequential paper and thin layer chromatography and identified by ammonia desorption chemical ionization mass spectrometry. Treatment of the patient with oral carnitine supplements led to a near-normalization of the plasma free carnitine concentrations and an increase in his muscle tone. The propionylcarnitine excretion rose and there was a simultaneous decrease in the methylcitrate output. Carnitine treatment did not prevent the occurrence of an episode of metabolic decompensation.