Histidine transaminase

The histidinemic urine is sometimes only weakly positive or negative on ferric chloride testing, which reflects either the instability of imidazole-pyruvic acid, the relation of its excretion to protein intake, immaturity of histidine transaminase, or a combination of these factors. Even after histidine loading, histidinemic neonates excrete much smaller quantities of imi-dazolepyruvic acid than do older children. In patients with urocanase deficiency, the levels of blood histidine and urine imidazolepyruvic acid are normal or slightly elevated. Therefore, it is necessary to do a quantitative analysis of blood histidine and urine ferric chloride test in order to overlook the patients with this condition. 

Figure 9-22. Sedimentation of histidinal dehydrogenase (O) and imidazoleacetol phosphate transaminase (9) as a function of time. [From R. G. Martin and B. N. Ames, J. Biol. Chem., 236 1372 (1961).]...

All of the amino acids except lysine, threonine, proline, and hydroxyproline participate in transamination reactions. Transaminases exist for histidine, serine, phenylalanine, and methionine, but the major pathways of their metabolism do not involve transamination. Transamination of an amino group not at the a-position can also occur. Thus, transfer of 3-amino group of ornithine to a-ketoglutarate converts ornithine to glutamate-y-semialdehyde. 

Steps in the formation of classical neurotransmitters. AADC, amino acid decarboxylase AChE, acetylcholinesterase CAT, choline acetyltransferase COMT, catechol-O-methyltransfeiase DBH, dopamine P-hydroxylase DA, dopamine DOPA, dibydroxyphenylalanine GABA-T, GABA transaminase GAD, glutamic acid decarboxylase HD, histidine decarboxylase 5-HTP, 5-hydroxytrytophan MAO, monoamine oxidase PNMT, phenylethanolamine N-methyltransferase TH, tyrosine hydroxylase TPH, tryptophan hydroxylase. 

Enzymes with pyridoxine coenzymes in human metabolism (examples, several hundred enzymes). (1) Transaminases (aspartate-aminotransferase (EC 2.6.1.1), alanine-aminotransferase (EC 2.6.1.2) aromatic-amino acid aminotransferase (EC 2.6.1.57)) (2) decarboxylas (histidine-decarboxylase (EC 4.1.1.22) aromatic-amino acid decarboxylase (EC 4.1.1.28)) (3) a,jS-elimination (t-serine-... 

Imidazolone propionate hydrolase catalyzes the enzymatic cleavage of the imidazole ring to yield formi-minoglutamate. The rat liver enzyme has been partially purified. In addition to the enzymic conversion, two nonenzymic spontaneous reactions yield N-formyl-isoglutamine and 4-oxoglutamic acid. In addition to the oxidative pathways for histidine, there exist three other pathways for its use protein synthesis, decarboxylation to yield histamine (see Inflammation), and transaminase. The activity of histidine pyruvic transamination in rat liver is three times that of histidase. The product of the transaminase reaction is imidazole pyruvic acid, which in turn is converted to imidazole acetic acid. 

A transaminase isolated from Neurospora reacts with certain substrates which do not contain carboxyl groups, but have phosphate esteri-fied to the carbon adjacent to that bearing the amino group (11). Histidinol phosphate, histidine, arginine, glutamic acid and their keto analogs react with this enzyme. 

Brief reference will be made to the following papers which have appeared since this chapter was written. Partially purified Neurospora L-serine dehydrase has been studied. Both L-serine and L-threonine appear to be deaminated by the same enzyme. The enzymatic pathway of histidine degradation in liver has been investigated. Soluble gluta-minase I has been prepared. Additional eiddence for the Avide scope of transamination has appeared. The presence of an ornithine-a-keto-glutarate transaminase in neurospora has been demonstrated. Transamination of non-a-amino acids has been demonstrated in brain in... 

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