Isoleucine metabolism

Figure 55-10 Electron impact positive ion mass spectra of isomeric acylglycines detected by GC/MS analysis of organic acid trimethyisily (IMS) derivatives. A, 3-liethylcrotonylglycine monO TMS ester (leucine metabolism). B, Tiglylglycine mono-TMS ester (isoleucine metabolism). As their retention times are relatively close in most chromatographic systems, proper differentiation betv een the two compounds is best achieved by evaluation of the fragment ion at m/z 82 (arrow) which is prominent in the spectrum of 3-methylcrotonylglydne but not tigiylgiycine.

Pyrrolizidine Alkaloids.— Two molecules of L-isoleucine are used for the biosynthesis of the senecic acid component of senecionine (15). In order to understand how the two isoleucine fragments are linked together (C-6 of one joins to C-4 of the other), further work has been undertaken. First, 2-methyl-3-oxobutanoic acid and the five-carbon intermediates in isoleucine metabolism, i.e. 2-methylbutanoic acid and angelic acid (17), were examined as precursors for the senecic acid fragment of senecionine (15), with negative results [angelic acid rather than the isomeric tiglic acid, see (13), was examined since its stereochemistry is the same as that around C-15-C-20 in (15)]. 

C22H20O10, Mr 444.39, mp. 171-173 °C (other reports 163-167 °C), [a]o° -10.5° (CH3OH). V. is an antibiotic from Streptomyces species with activities against Gram-positive and Gram-negative bacteria, especially intestinal bacteria. V. exhibits synergistic effects with streptomycin and chloramphenicol it also has her-bicidal properties and influences the isoleucine metabolism. 

The disorders of valine and isoleucine metabolism comprise quite distinct diseases. 

Gibson, K. M., Burlingame, T. G., Hogema, B., et al. (2000) 2-Methylbutyryl-coen-zyme A dehydrogenase deficiency A new inborn error of L-isoleucine metabolism, Pediatr. Res., 47, 830. 

Zschocke, J., Ruiter, J. P. N., Brand, J., et al.. (2000) Progressive infantile neuro-de-generation caused by 2-methyl-3-hydroxybutyryl-CoA dehydrogenase deficiency A novel inborn error of branched-chain fatty acid and isoleucine metabolism. Pediatr. Res., 48, 852. 

Ogawa, J., Kodera, T., Smirnov, S. V, Hibi, M., Samsonova, N. N., Koyama, R., Yamanaka, H., Mano, J., Kawashima, T., Yokozeki, K., Shimizu, S. (2011). A novel L-isoleucine metabolism in Bacillus thuringiensis generating (2S,3R,4S)-4-hydroxyisoleucine, a potential insu-linotropic and anti-obesity amino acid. Appl. Microbiol. Biotechnol., 89,1929-1938. 

The mechanism by which these two isoleucine untis are coupled in senecic acid biosynthesis is not obvious. Crout et al. (25) tested a number of Cs-intermediates of isoleucine metabolism as possible precursors for this coupling process. Eventually, they found that 2-amino-[3- H2]-methylenepentanoic acid (98) was incorporated specifically into senecic acid (97) with about the same efficiency as for L-isoleucine. Unfortunately, the low radioactivity in the necic acid precluded further degradation to establish whether one or both halves of the senecic acid were labelled. There is also the possibility that the precursor (98) could be converted into isoleucine before incorporation. Thus, the status of (98) as an intermediate in senecic acid biosynthesis clearly requires further investigation. Nevertheless, Crout (73) has postulated a mechanism for activation of (98) into a pyridoxal bound mesomeric nucleophile (99), which could then attack an unspecified electrophilic metabolite of isoleucine to form senecic acid (Scheme 24). 

The 0X0 acids have been discussed in part in the section on the Krebs cycle acids, however, the importance of branched-chain aliphatic 2-oxo acids in valine, leucine and isoleucine metabolism and of oxo acids in other disorders prompts further comment of their chromatographic analysis. 

Witten et al. (1973) identified adipic and 3-methyladipic acids and also reported the presence in urine, using GC-MS, of aconitic and isocitric acids in addition to citrate. Mamer et al, (1971) reported the occurrence of several hydroxyaliphatic acids in addition to those already identified by other workers, and Mamer and Tjoa have identified 2-ethylhydracrylic acid in urine derived from isoleucine metabolism (Mamer and Tjoa, 1974). Urine from healthy children and adults may contain low amounts of aliphatic dicarboxylic acids of chain length C4-C8 (Lawson et ai, 1976). Pettersen and Stokke (1973) reported a series of 3-methyl-branched C4-C8 dicarboxylic acids in urine from normal subjects, and Lindstedt and co-workers have identified other dicarboxylic acids with cyclopropane rings and acetylenic bonds as well as a series of cis and trans mono-unsaturated aliphatic dicarboxylic acids (Lindstedt et al., 1974,1976 Lindstedt and Steen, 1975). 

In the metabolism of L-leucine, the isovaleryl-CoA produced by the oxidative decarboxylation step is further metabolized by a series of enzyme-catalysed steps to acetoacetate and acetyl-CoA and thence into the tricarboxylic acid cycle. Specific enzyme deficiencies at every stage of this metabolic pathway are known and are described in Section 10.3. In contrast, only one disorder of L-isoleucine metabolism subsequent to the oxidative decarboxylation step has been recognized (Section 10.4), and no disorders of the L-valine pathway from isobutyryl-CoA have been described. This may be due to their relative rarity but possibly also to greater difficulty in their detection. The metabolism of valine and leucine is, however, of particular interest in the organic acidurias, since both are major precursors of propionyl-CoA and methylmalonyl-CoA, defects in the metabolism of which lead to propionic acidaemia and methylmalonic aciduria (Chapter 11). 

Gompertz, D., Saudubray, J.M., Charpentier, C., Bartlett, K., Goodey, P.A. and Draffan, G.H. (1974), A defect in L-isoleucine metabolism associated with a-methyl-/S-hydroxybutyric and a-methylacetoacetic aciduria Quantitative in vivo and in vitro studies. Clin. Chim. Acta, 57,269. 

Keating, J.P., Feigin, R.D., Tenenbaum, S.M. and Hillman, R.E. (1972), Hyperglycinemia with ketosis due to a defect in isoleucine metabolism A preliminary report. Pediatrics, 50, 890. 

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