Histidine metabolism

Valine, leucine, and isoleucine biosynthesis Lysine biosynthesis Lysine degradation Arginine and proline metabolism Histidine metabolism Tyrosine metabolism Phenylalanine metabolism Tryptophan metabolism Phenylalanine, tyrosine, and tryptophan biosynthesis Urea cycle and metabolism of amino groups... 

It plays a vital role in various intracellular reactions e.g. conversion of serine to glycine, synthesis of thymidylate, synthesis of purines, histidine metabolism etc. Due to folic acid deficiency these reactions are affected. 

Catabolism of histidine. The first steps of the major degradative pathway for histidine metabolism have already been discussed. Elimination of ammonia, followed by hydration and ring cleavage to formiminoglutamate, involves unusual reactions (Eq. 25-14)252 which have been discussed earlier. Transfer of the formimino group to tetrahydrofolic acid and its further metabolism have also been considered (Chapter 15). 

In experimental animals and with isolated tissue preparations and organ cultures, the test can be refined by measuring the production of G02 from [ C]histidine in the presence and absence of added methionine. If the impairment of histidine metabolism is the result of primary folate deficiency, the addition of methionine wUl have no effect. By contrast, if the problem is trapping of folate as methyl-tetrahydrofolate, the addition of methionine will restore normal histidine oxidation as a result of restoring the inhibition of methylene-tetrahydrofolate reductase by S-adenosylmethionine and restoring the activity of 10-formyl-tetrahydrofolate dehydrogenase, thus permitting more normal folate metabolism (Section 10.3.4.1). 

Histidine metabolism. H PteGlu also acts as an acceptor of aformimino group in the conversion of formiminoglutamic acid to glutamic acid. 

Histidine is the Cinderella of plant amino acids having been virtually ignored by plant biochemists a search of Chemical Abstracts over the last 10 years only revealed two papers relevant to histidine metabolism in higher plants. The probable reason for this is the very difficult biochemistry involved in this pathway and the lack of any usable mutants in higher plant systems. Our knowledge of the pathway is therefore solely based on bacterial metabolism and the elucidation of the pathway is a classical example of the use of auxotrophic mutants (see Brenner and Ames, 1971 Hartman ft al., 1971). 

Products of tryptophan and histidine metabolism Metabolism of amino acids and porphyrins 107... 

Imidazole alkaloids alkaloids of sporadic occurrence, possessing the imidazole ring system. Tlieir most important representative is Pilocarpine (see). The biosynthesis of the I. a. is coupled to histidine metabolism. 

Undoubtedly, the most intriguing aspect of the disease is the selected impairment of mental development that results from the metabolic block. Some children with histidinemia trail behind other children in learning to talk and when they talk, they exhibit speech difficulties. One patient communicated with only singlewords. The elucidation of the role of histidine metabolism in the development of speech could be an important landmark in our understanding of brain chemistry (see Fig. 3-24). 

Degradation. One pathway of degradation is started by decarboxylation to histamine by the enzyme previously discussed. This is a minor pathway in terms of quantities of histidine metabolized. The major pathway in both animals and microorganisms retains the 5-carbon chain and leads to glutamic acid. Older work established that histidine degradation occurs only in the livers of animals, but this work was carried out only with crude preparations, and led to the publication and general acceptance of hypothetical schemes that have little relation to reactions found more recently. 

The primary disorders of histidine metabolism are histidinemia and uroca-nase deficiency (urocanic aciduria). 

It is unlikely that the clinical signs and symptoms are associated with the disorders of histidine metabolism His, histidine. 

Fig. 5.2. Diagnostic flow chart for disorders of histidine metabolism...

Histidine Concentration to Rheumatoid Arthritis (Histidine. Metabolism, Clinical Aspects, Therapeutic Use, Eds. R. Kluthe and N. R. Katz) p. 90. Georg Thieme (1979). 

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