Studies in maple syrup urine disease

Growing older: the adult metabolic clinic

Improvements in screening programmes, diagnostic tests and therapeutic interventions have all led to increasing numbers of children with inherited metabolic diseases surviving through childhood into adolescence and adulthood. These individuals are often able to integrate into society, but many have complex, multisystem problems that require ongoing care. Understanding of the long-term outcome of these disorders is scanty. Awareness of these conditions within the adult medical community is sadly limited and much of the good work of the paediatricians potentially could go to waste. This is often prevented by paediatricians remaining involved in patient care into the third decade and beyond. Appropriate resources to deal with the transition from the children's services to the adult sector are necessary, and appropriately trained medical and dietetic personnel are required to run these services. An adult inborn errors service needs to have close links with a feeding paediatric unit, as well as to integrate with other adult medical and surgical specialties, and to be supported by metabolic biochemical and molecular laboratories. This paper discusses the issues around this growing problem based upon experience gained working within a single centre dedicated to the care of adults and adolescents with inborn errors of metabolism.

Formation of L-alloisoleucine in vivo: an L-[13C]isoleucine study in man

L-alloisoleucine (2S, 3R), a diastereomer of L-isoleucine (2S, 3S), is a normal constituent of human plasma. Considerable amounts accumulate in maple syrup urine disease, in which the branched-chain 2-oxo acid dehydrogenase step is impaired. The mechanism of L-alloisoleucine formation, however, is unclear. We addressed this issue by performing oral L-[1-13C]isoleucine loading (38 micromol/kg body wt, 50% 1-13C) in overnight-fasted healthy subjects (n = 4) and measuring the 3-h kinetics of 13C-label incorporation into L-isoleucine plasma metabolites. Compared with L-isoleucine, the time course of 13C-enrichment in the related 2-oxo acid, S-3-methyl-2-oxopentanoate, was only slightly delayed. Peak values, amounting to 18+/-4 and 17+/-3 mol percent excess, respectively, were reached within 35 and 45 min, respectively. The kinetics of 13C-enrichment in S- and R-3-methyl-2-oxopentanoate enantiomorphs were similar and linearly correlated (p << 0.001). In L-alloisoleucine, however, 13C-label accumulated only gradually and in minor amounts. Our results indicate that R-3-methyl-2-oxopentanoate is an immediate and inevitable byproduct of L-isoleucine transamination and further suggest that alloisoleucine is primarily formed via retransamination of 3-methyl-2-oxopenanoate in vivo.

Maple syrup urine disease: clinical, EEG, and plasma amino acid correlations with a theoretical mechanism of acute neurotoxicity

Classical Maple Syrup Urine Disease (MSUD) is a disease of infancy which is an inherited disorder of metabolism of branched-chain amino acids (BCAA). The BCAA are normally transaminated to branched-chain keto acids (BCKA). However, the enzyme required to metabolize the BCKA is deficient, resulting in elevation of both, the BCAA and the BCKA. One of the BCAA (isoleucine) produces a metabolite that causes the urine to smell like maple syrup. The elevations of the BCAA and BCKA are associated with an acute, critical neurotoxic condition often prior to the age of two weeks. The clinical state, the electroencephalogram-(EEG), and plasma BCAA levels were evaluated in 26 patients with classical and variant MSUD. Patients were seen from the time of diagnosis, often within a week after birth, and some were followed clinically for more than 20 years while on specific diet therapy. They were monitored by plasma BCAA (leucine, isoleucine and valine) levels and a total of 101 EEGs were performed during different phases of their illness. During periods of acute metabolic decompensation, there were marked clinical symptoms of neurotoxicity including opisthotonos, seizures, and coma with elevated BCAA plasma levels. The EEGs revealed spikes, polyspikes, spike-wave complexes, triphasic waves, severe slowing and bursts of periodic suppression. Occasionally paradoxical EEG arousal was noted while the patient was lethargic. During asymptomatic periods when the plasma BCAA were at low or normal levels, EEG abnormalities occurred in patients with and without residual neurological deficit. These observations included rolandic sharp waves (comb-like rhythm) which were observed in 7 of 15 patients less than two months of age. Additionally, paroxysmal spike and spike-wave response to photic stimuli were observed in 9 of 17 patients. Loading tests were performed on three patients. Clinical and EEG changes were most marked after leucine. Less dramatic EEG changes also occurred with the other two BCAA loads but without clinical manifestations. Elevation of the appropriate BCAA plasma level occurred after each load. These studies and a review of the literature suggest that one component of the pathophysiological mechanism for the acute neurotoxic effects in this disorder is related to a defect in glutamate, glutamine and gamma-aminobutyric acid (GABA) production. The BCAAs are transaminated to BCKAs. Further metabolism of the BCKAs are blocked because of enzyme deficiency required for decarboxylation.(ABSTRACT TRUNCATED AT 400 WORDS)

Maple syrup urine disease: branched-chain amino acid concentrations and metabolism in cultured human lymphoblasts

The intracellular concentration of free leucine, isoleucine, and valine and their metabolism were studied in lymphoblast cultures established from peripheral blood of an individual with maple syrup urine disease (MSUD) and a control subject. Branched-chain alpha-keto acid decarboxylase activity in the MSUD cells was 10% or less of the control value as measured by the ability of the cells to release 14CO2 from the corresponding [1-14C]labeled branched-chain amino acid. The intracellular concentrations of free leucine and isoleucine were increased three-fold in MSUD lymphoblasts as compared to control cells. Free valine was present in only trace amounts of less than 0.1 mM in both cell lines. Exposure of normal and mutant cells to a 10 mM load of leucine, isoleucine, and valine revealed in a comparable concentration within cells after 24 hr. Concentrations returned to base values in normal cells 12 hr after removal of load, but leucine remained elevated in MSUD cells after 3 days. Leucine and its keto acid, alpha-ketoisocaproic acid, added to the culture medium gave significant growth inhibition of MSUD lymphoblasts but not of normal cells, in the millimolar range. Isoleucine, valine, and their keto acids had no effect.

Inhibition by the branched-chain 2-oxo acids of the 2-oxoglutarate dehydrogenase complex in developing rat and human brain

1. The effect of the branched-chain amino acids, namely leucine, isoleucine and valine and their corresponding 2-oxo acids on the metabolism of 2-oxoglutarate by developing rat and human brain preparations was investigated. 2. The decarboxylation of 2-oxo[1-(14)C]glutarate to (14)CO(2) by mitochondria from adult rat brain was inhibited by the branched-chain 2-oxo acids whereas the branched-chain amino acids had no inhibitory effect on this process. 3. The activity of 2-oxoglutarate dehydrogenase complex was about 0.2unit/g of brain from 2-day-old rats and increased by about fourfold reaching an adult value by the end of the third postnatal week. 4. The K(m) value for 2-oxoglutarate of the 2-oxoglutarate dehydrogenase complex in rat and human brain was 100 and 83mum respectively. 5. The branched-chain 2-oxo acids competitively inhibited this enzyme from suckling and adult rats brains as well as from foetal and adult human brains, whereas the branched-chain amino acids had no effect on this enzyme. 6. Approximate K(i) values for the branched-chain 2-oxo acids found for this enzyme were in the range found for these 2-oxo acids in plasma from patients with maple-syrup-urine disease. 7. The possible significance of the inhibition by the branched-chain 2-oxo acids of the 2-oxoglutarate dehydrogenase complex in brains of untreated patients with maple-syrup-urine disease is discussed in relation to the energy metabolism and the biosynthesis of lipids from ketone bodies.

Differential effects of 2-oxo acids on pyruvate utilization and fatty acid synthesis in rat brain

1. The effects of 2-oxo-4-methylpentanoate, 2-oxo-3-methylbutanoate and 2-oxo-3-methylpentanoate on the activity of pyruvate dehydrogenase (EC 1.2.4.1), citrate synthase (EC 4.1.3.7), acetyl-CoA carboxylase, (EC 6.4.1.2) and fatty acid synthetase derived from the brains of 14-day-old rats were investigated. 2. The pyruvate dehydrogenase enzyme activity was competitively inhibited by 2-oxo-3-methylbutanoate with respect to pyruvate with a K(i) of 2.04mm but was unaffected by 2-oxo-4-methylpentanoate or 2-oxo-3-methylpentanoate. 3. The citrate synthase activity was inhibited competitively (with respect to acetyl-CoA) by 2-oxo-4-methylpentanoate (K(i)~7.2mm) and 2-oxo-3-methylbutanoate (K(i)~14.9mm) but not by 2-oxo-3-methylpentanoate. 4. The acetyl-CoA carboxylase activity was not inhibited significantly by any of the 2-oxo acids investigated. 5. The fatty acid synthetase activity was competitively inhibited (with respect to acetyl-CoA) by 2-oxo-4-methylpentanoate (K(i)~930mum) and 2-oxo-3-methylpentanoate (K(i)~3.45mm) but not by 2-oxo-3-methylbutanoate. 6. Preliminary experiments indicate that 2-oxo-4-methylpentanoate and 2-oxo-3-phenylpropionate (phenylpyruvate) significantly inhibit the ability of intact brain mitochondria from 14-day-old rats to oxidize pyruvate. 7. The results are discussed with reference to phenylketonuria and maple-syrup-urine disease. A biochemical mechanism is proposed to explain the characteristics of these diseases.

DIETARY TREATMENT OF A CHILD WITH MAPLE SYRUP URINE DISEASE (BRANCHED-CHAIN KETOACIDURIA)

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HOMOCYSTINURIA: A NEW INBORN ERROR OF METABOLISM ASSOCIATED WITH MENTAL DEFICIENCY

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Recent work on phenylketonuria and maple syrup urine disease (leucinosis)

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