Amino acid maple syrup urine disease

DISORDERS OF BRANCHED-CHAIN AMINO ACIDS MAPLE SYRUP URINE DISEASE 671... 

As the name implies, the odor of urine in maple syrup urine disease (brancbed-chain ketonuria) suggests maple symp or burnt sugar. The biochemical defect involves the a-keto acid decarboxylase complex (reaction 2, Figure 30-19). Plasma and urinary levels of leucine, isoleucine, valine, a-keto acids, and a-hydroxy acids (reduced a-keto acids) are elevated. The mechanism of toxicity is unknown. Early diagnosis, especially prior to 1 week of age, employs enzymatic analysis. Prompt replacement of dietary protein by an amino acid mixture that lacks leucine, isoleucine, and valine averts brain damage and early mortality. 

The catabolism of leucine, valine, and isoleucine presents many analogies to fatty acid catabolism. Metabolic disorders of branched-chain amino acid catabolism include hypervalinemia, maple syrup urine disease, intermittent branched-chain ketonuria, isovaleric acidemia, and methylmalonic aciduria. 

Maple syrup urine disease involves a congenital failure to oxidize the three branched-chain amino acids 671... 

Effective treatment of maple syrup urine disease involves the restriction of dietary branched-chain amino acids 672... 

Imbalances of brain amino acids may hinder the synthesis of brain lipids, leading to a diminution in the rate of myelin formation. Decreases of lipids, proteolipids and cerebrosides (Ch. 3) have been noted in several of these syndromes, e.g. maple syrup urine disease, when intra-myelinic edema is a prominent finding, particularly during the acute phase of metabolic decompensation [9]. Pathological changes in brain myelin are common, especially in infants who die early in life. The fundamental... 

Effective treatment of maple syrup urine disease involves the restriction of dietary branched-chain amino acids. Long-term treatment entails the dietary restriction of the BCAAs. This is accomplished by administration of a special formula from which these amino acids are removed. The outlook for intellectual development is favorable in youngsters in whom diagnosis is made early and who do not suffer recurrent, severe episodes of metabolic decompensation [17]. 

Wagner, M., Coelho, D. M., Barschak, A. G. et al. Reduction of large neutral amino acid concentrations in plasma and CSF of patients with maple syrup urine disease. J. Inker. Metab. Dis. 23 505-512,2000. 

Answer C. Maple syrup urine disease substrates are branched chain a-ketoacids derived from the branched chain amino acids. 

Table 2.1.9 Changes of blood amino acids in various primary inherited defects and as a result of secondary changes. ASA Argininosuccinic acid, CPS carbamoyl phosphate synthase, LPI Lysinuric protein intolerance, MAD multiple acyl-CoA dehydrogenation, MSUD maple syrup urine disease, NAGS N-acetylglutamate synthase, NKH nonketotic hyperglycinemia, NTBC 2-(2-nitro-4-3 trifluoro-methylbenzoyl)-1,3-cyclohexanedione, OCT Ornithine carbamoyltransferase,...

T There is a relatively rare genetic disease in which the three branched-chain a-lceto acids (as well as their precursor amino acids, especially leucine) accumulate in the blood and spill over into the urine. This condition, called maple syrup urine disease because of the characteristic odor imparted to the urine by the a-lceto acids, results from a defective branched-chain a-lceto acid dehydrogenase complex. Untreated, the disease results in abnormal development of the brain, mental retardation, and death in early infancy. Treatment entails rigid control of the diet, limiting the intake of valine, isoleucine, and leucine to the minimum required to permit normal growth. ... 

Maple syrup urine disease (MSUD) is a recessive disorder in which there is a partial or complete deficiency in branched-chain o-ketoacid dehydrogenase, an enzyme that decarboxylates leucine, isoleucine, and valine (see Figure 20.10). These amino acids and their corre sponding a-keto acids accumulate in the blood, causing a toxic effect that interferes with brain functions. The disease is characterized by feeding problems, vomiting, dehydration, severe metabolic acidosis, and a characteristic maple syrup odor to the urine. If untreated, the disease leads to mental retardation, physical disabilities, and death. 

People can suffer from a variety of inborn metabolic errors that can result in various amino acids exhibiting toxic effects if ingested above certain threshold levels. Specialty products, intended to be absent these amino acids, must be rigorously tested for verification that threshold levels are not exceeded. Examples of this situation include phenylketoneuria (intolerance of phenylalanine) and maple syrup urine disease (intolerance for leucine, isoleucine, and valine). 

Maple Syrup Urine Disease Figure 18-28 shows the pathway for the degradation of branched-chain amino acids and the site of the biochemical defect that causes maple syrup urine disease. The initial findings that eventually led to the discovery of the defect in this disease were presented in three papers published in the late 1950s and early 1960s. This problem traces the history of the findings from initial clinical observations to proposal of a biochemical mechanism. 

Figure 20.20 Pathways of branched-chain amino acid metabolism. A, B, C, D, E, and F indicate defects in valinemia, maple syrup urine disease, isovaleric acidemia, /3-hydroxyisovaleric aciduria, a-methyl-j3-hydroxybutyric aciduria, and methylmalonic aciduria, respectively.

Branched-Chain Oxo-acid Decarboxylase and Maple Syrup Urine Disease The third oxo-add dehydrogenase catalyzes the oxidative decarboxylation of the branched-chain oxo-acids that arise from the transamination of the branched-chain amino acids, leucine, isoleuctne, emd vtdine. It has a similEU subunit composition to pyruvate and 2-oxoglutarate dehydrogenases, and the E3 subunit (dihydrolipoyl dehydrogenase) is the stune protein as in the other two multienzyme complexes. Genetic lack of this enzyme causes maple syrup urine disease, so-called because the bremched-chain oxo-acids that are excreted in the urine have a smell reminiscent of maple syrup. 

Various mutations tiffecting either the El or the E2 suhunit of the dehydrogenase are involved in different forms of maple syrup urine disease. Acute infantile disease is caused by near complete lack of activity of the enzyme. The intermittent form of the disease is associated with mtirgincdly adequate residual activity of the enzyme that is able to cope with the brcmched-chciin oxo-acids arising from the metabolism of modest cunounts of branched-chciin amino acids, but not relatively large cunounts. 

C-11) (1-9) Maple Syrup Urine Disease. There is a block in the degradation of the branched chain amino acids. Leucine, isoleucine, valine, and their ketoic acids are elevated in the blood and urine. Assays for these chemicals can be done in the laboratory. The urine acquires a maple syrup aroma. Infants with the condition have a variety of neurologic problems, including mental retardation. The condition is treated by dietary restriction of the affected amino acids. 

Although alcaptonuria is a relatively harmless condition, such is not the case with other errors in amino acid metabolism. In maple syrup urine disease, the oxidative decarboxylation of a-ketoacids derived from valine, isoleucine, and leucine is blocked because the branched-chain dehydrogenase is missing or defective. Hence, the levels of these a-ketoacids and the branched-chain amino acids that give rise to them are markedly elevated in both blood and urine. Indeed, the urine of patients has the odor of maple syrup—hence the name of the disease (also called branched-chain ketoaciduria). Maple syrup urine disease usually leads to mental and physical retardation unless the patient is placed on a diet low in valine. 

The procedure has been applied to the analysis of control urine and that from a patient with maple-syrup-urine disease. The results from 5 separate analyses in each case gave standard deviations of less than 10% of the mean. This was routinely achieved where the level of an amino acid was about 1 ng. 

In maple syrup urine disease, the enzyme complex that decarboxy-lates the transamination products of the branched-chain amino acids is defective (see Figure 7-11). Valine, isoleucine, and leucine accumulate. Urine has the odor of maple syrup. Mental retardation occurs. 

The plasma and urine of patients with maple syrup urine disease contain elevated levels of each of the following amino acids EXCEPT... 

C. In maple syrup urine disease, the branched-chain amino acids (valine, leucine, and isoleucine) can be transaminated but not oxidatively decarboxylated because the a-keto acid dehydrogenase is defective. 

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