Valine excretion

The amounts of single amino acids excreted in urine in the conjugated form, as determined independently by Stein and Muting, are given in Tables 1 and 2. According to Stein, glycine, glutamic acid, aspartic acid, histidine, and proline are quantitatively the most important amino acids liberated in the course of urine hydrolysis. Serine, lysine, tyrosine, cysteine and cystine, threonine, alanine, valine, phenylalanine, and leucine are... 

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). 

As discussed in Section 10.8.2, moderate vitamin B12 deficiency results in increased accumulation of methylmalonyl CoA, and methylmalonic aciduria and methylmalonic acidemia. This can be exploited as both a means of detecting subclinical deficiency and monitoring vitamin B12 status in patients with pernicious anemia who have been treated with parenteral vitamin. As they become depleted, the excretion of methylmalonic acid, especially after a loading dose of valine, will provide a sensitive index of depletion of vitamin Bi2 reserves. 

Methylmalonyl CoA mutase is especially sensitive to vitamin B12 depletion, so methylmalonic aciduria is the most sensitive index of vitamin B12 status. Folate deficiency does not cause methylmalonic aciduria. However, up to 25% of patients with confirmed pernicious anemia excrete normal amounts of methylmalonic acid, even after a loading dose of valine (Chanarin et al., 1973). 

During the course of a day, one might consume about 100 mmol of isoleuctnq and valine, the two major precursors of methylmalonyl-CoA, Why is it that only about 4-0 mmol of MM A is normally excreted per day (See Rasmussen, 1989.)... 

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. 

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). 

Conservation of amino acids filtered at the glomerulus is made possible by the existence of four main transport systems for specific amino acids that facilitate active reabsorption of these amino acids from the proximal tubule. A lack or deficiency of the transport system responsible for the absorption of valine, alanine, cystine, and tryptophan, and of the transport system for arginine, lysine, cystine, and ornithine, leads to excretion of these specific amino acids in urine, which is characterized as renal aminoaciduria to distinguish it from overflow aminoaciduria. In the latter situation, the production of amino acids far exceeds the proximal tubular reabsorption capacity, thus leading to overflow of amino acids into urine. This can occur due to defective metabolism of amino acids, as is the case when phenylalanine cannot be metabolized due to the deficiency of the enzyme phenylalanine hydroxylase, or to the inability to deaminate amino acids in liver disease. 

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. 

Little is known about the transport of amino acids across the intestine of nematodes. In vitro studies using isolated segments of intestine from A. suum indicate that uptake of methionine, glycine, histidine and valine is stereospecific and non-linear with respect to concentration, indicative of a mediated transport process (12). Most nematodes excrete a wide range of amino acids (149), and though some of these, such as alanine and proline, are true metabolic end-products, others must be derived from ingested materials. 

C. glutamicum Strain dapA copies Synthase activity (U mg proteim ) Growth rate (h->) Intra- cellular Threonine (mmoh ) Intra- cellular Valine (mmoh ) Lysine excretion rate (nmol min mg dry wV )... 

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). 

The production of oxoisovaleric acid and oxoisocaproic acids, intermediates in the biosynthesis of valine and leucine respectively, is shown in Fig. 17.16. The decarboxylation and reduction (NAD+ dependent) of these oxo-acids yields the fusel alcohols isobutanol and isoamyl alcohol (Fig. 17.16). Presumably, some oxo-acids in beer result from excretion from the pool and some aldehydes by excretion prior to reduction to the corresponding alcohol. Table 17.7 shows the chemical relationships between alcohols, aldehydes, oxo-acids and corresponding amino acids. 

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