Isoleucine excretion

Stein et al. found in the course of experiments dealing with free and conjugated urinary amino acids in Wilson s disease (S9) that besides a marked aminoaciduria, almost a twofold increase in the excretion of all bound amino acids could be observed. As compared with normal urine (S8), unusual amounts of conjugated leucine, isoleucine, and valine are excreted in cases of Wilson s disease. Also the increase of glutamic acid, aspartic acid, and phenylalanine after urine hydrolysis is much more distinct in this disease than in normal conditions. Other bound amino acids are at or below normal levels. 

Methylmalonyl-CoA mutase is a cobalamin-linked enzyme of mitochondria that catalyzes the isomerization of methylmalonyl-CoA to succinyl-CoA. A reduction of this enzyme due to vitamin B12 deficiency will result in a metabolic block with the urinary excretion of methylmalonic acid, and the measurement of this metabolite has been used to confirm a deficiency of vitamin B12. The test has also been useful in investigating rare abnormalities of this enzyme that result in the excretion of methylmalonic acid in the presence of adequate vitamin B12. Given an oral loading dose of valine or isoleucine will increase the urinary excretion of methylmalonic acid in patients with a vitamin B12 deficiency (G4). However, Chanarin and his colleagues (CIO) found that one-quarter of their patients with pernicious anemia excreted a normal concentration of methylmalonic acid even after a loading dose of valine. Normal subjects excrete up to 15 mg of methylmalonic acid in their urine over a 24-hour period (Cll). 

In known metabolic states and disorders, the nature of metabolites excreted at abnormal levels has been identified by GC-MS. Examples of this are adipic and suberic acids found in urine from ketotic patients [347], 2-hydroxybutyric acid from patients with lactic acidosis [348], and methylcitric acid (2-hydroxybutan-l,2,3-tricarboxylic acid) [349] in a case of propionic acidemia [350,351]. In the latter instance, the methylcitric acid is thought to be due to the condensation of accumulated propionyl CoA with oxaloacetate [349]. Increased amounts of odd-numbered fatty acids present in the tissues of these patients due to the involvement of the propionyl CoA in fatty acid synthesis, have also been characterised [278]. A deficiency in a-methylacetoacetyl CoA thiolase enzyme in the isoleucine pathway prevents the conversion of a-methylacetoacetyl CoA to propionyl CoA and acetyl CoA [352,353]. The resultant urinary excretion of large amounts of 2-hydroxy-3-methylbutanoic acid (a-methyl-/3-hydroxybutyric acid) and an excess of a-methylacetoacetate and often tiglyl glycine are readily detected and identified by GC-MS. 

Breakdown of isoleucine, valine, threonine, and methionine results in the production of propionyl-CoA. Propionyl-CoA, in turn, is catabolized to succinyl-CoA via the intermediate methylmalonyl-CoA. Methylmalonyl-CoA is a compoimd of imusual interest to nutritional scientists. This compound accumulates in the cell during a vitamin B12 deficiency. Vitamin B12 deficiency is not a rare disease, as it appears in a common autoimmune disease called pernicious anemia. Vitamin B12 deficiency also occurs in strict vegetarians who avoid meat, fish, poultry, and dairy products. Methylmalonyl-CoA can also build up with rare genetic diseases that involve the production of defective, mutant forms of methylmalonyl-CoAmutase. Most of the methylmalonyl-CoAthat accumulates to abnormally high levels in the cell is hydrolyzed to methylmalonic acid (MMA), which leaves the cell for the bloodstream and eventual excretion in the urine. Some of the MMA is converted back to propionyl-CoA, resulting in the production and accumulation of propionic acid in the cell. The measurement of plasma and urinary MMA has proven to be a method of choice for the diagnosis of vitamin B12 deficiency, whether induced by pernicious anemia or by dietary deficiency. 

During the course of a day, one might consume about 1(X) mmol of isoleucine and valine, the two major precursors of methyhnalonyl-CoA. Why is it that oidy about 4.0 mmol of MMA is normally excreted p>er day (See Rasmussen, 1989.)... 

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

Liver plays a major role, since it can oxidize all amino acids except leucine, isoleucine, and valine (see Chapter 22). It also produces the nonessential amino acids from the appropriate carbon precursors. Ammonia formed in the gastrointestinal tract or from various deaminations in the liver is converted to urea and excreted in urine (discussed later). 

Ethylmalonic Acid. Stalder (S35) found ethylmalonic acid in normal urine of rat and man and showed that its excretion by the rat increases when isoleucine is fed. The increase is analogous to that of methylmalonic acid when valine is fed. 

In a fashion similar to the lysine fermentation, mutation of E. coli to lysine and methionine auxotrophy results in excretion of up to 4 g/liter of threonine (Huang, 1961), while isoleucine auxotrophy in C. glutamicum leads to 11 g/liter valine production (Nakayama et al., 1961). 

By using colistine for the enrichment procedure, many auxotrophic mutants defective in the biosynthetic pathway of valine and isoleucine have been isolated. From an isoleucine-requiring mutant, defective in threonine desaminase, a prototrophic revertant has been isolated. The threonine desaminase of this revertant differs from the wild type enzyme in that its affinity for isoleucine is diminished. This revertant excretes isoleucine. Another revertant of an isoleucine-deficient mutant was obtained which formed the enzyme acetohydroxy add synthetase constitutively. During heterotrophic growth with fructose or lactate as substrates, valine, isoleucine and leucine were excreted into the culture medium. Approximately 0.6 g of amino acids were produced per liter suspension when lactate was supplied as a substrate under autotrophic conditions the excretion was negligible (Reh, 1970 Fig. 12). 

As expected, among the mutants excreting leucine there was a mutant constitutively derepressed with respect to the formation of the enzyme a-isopropylmalate synthetase, which is the first enzyme in the leucine biosynthetic pathway. In another mutant, this enzyme is insensitive to endproduct inhibition by leucine. However, contrary to our expectations, we found mutants carrying regulatory defects in the control of the valine-isoleucine biosynthetic pathway several mutants are constitutively derepressed with respect to the formation of aceto-hydroxy acid synthase and in one mutant this enzyme is insensitive to endproduct inhibition by valine. The selection and the existence of... 

Amino acids are used by the body to form proteins, hormones, and enzymes. Transamination reactions can convert one amino acid into another to meet immediate needs. However, just as there are essential fatty acids, there are also essential amino acids. These amino acids cannot be synthesized in the body and must come from external sources. Humans require phenylalanine, valine, tryptophan, threonine, lysine, leucine, isoleucine, and methionine as essential amino acids. All other amino acids in the body can be synthesized at rates sufficient to meet body needs. If any one of the amino acids necessary to synthesize a particular protein is not available, then the other amino acids that would have gone into the protein are deaminated, and their excess nitrogen is excreted as urea (Ganong, 1963). 

MSUD is an autosomal recessive disorder caused by deficiency of branched-chain a-ketoacid dehydrogenase (Pig. 47.1). The a-ketoadds derived from isoleucine, valine and leucine (branched-chain amino adds) accumulate and are excreted in the urine, giving it the peculiar odour of maple syrup. The branched-chain amino acids and the branched-chain a-ketoacids that accumulate in the blood are neurotoxic, causing severe neurological symptoms, cerebral oedema and mental retardation. A diet low in branched-chain amino acids is an effective treafinenL... 

Aramaki, S., Lehotay, D., Sweetman, L. et al (1991) Urinary excretion of 2-methylace-toacetate, 2-methyl-3-hydroxybutyrate and tiglylglycine after isoleucine loading in the diagnosis of 2-methylacetoacetyl-CoA thiolase deficiency. /. Inker. Metab. Dis. 14, 63. 

The neutral amino acids alanine, serine, threonine, asparagine, glutamine, valine, leucine, isoleucine, phenylalanine, tyrosine, tryptophan, histidine and citrulline share a common transporter at the luminal border of the epithelial cells in the renal tubuli and the epithelial cells in the small intestine [16]. In Hartnup disorder an impairment of this transporter leads to hyperexcretion of these neutral amino acids and to intestinal malabsorption. Excretion of tryptophan metabolites kynurenine and N-methyl-nico-tinamide is reduced. Plasma concentrations of the affected amino acids may be low normal or reduced. The inheritance is autosomal recessive. The hph2-deficient mouse has been postulated as a model for Hartnup disorder [17]. Affected persons may be asymptomatic, while some demonstrate pellagra-like photodermatitis or cerebellar ataxia due to a nicotinamide deficiency and respond well to the administration of nicotinamide [16]. 

Chromatography revealed excretion of a number of amino acids in his urine, with abnormally high concentrations of tryptophan, phenylalanine, tyrosine, leucine, isoleucine and valine. His urine also contained relatively high concentrations of a number of indolic compounds, including indoxyl sulphate (indican), indolyllactate, indolylacetate, indolylacetamide and indolylacetylglutamine, which are not detectable in the urine of normal subjects. 

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