Tryptophan transaminase

Indolmydn.—Previous evidence on the biosynthesis of indolmycin (88) in Strepto-myces griseus cultures accords with the pathway shown in Scheme 4. The first two steps in the pathway have been carried out using cell-free extracts of 5. griseus - and recent work has led to the isolation of two enzymes which can effect these transformations. The first, tryptophan transaminase, catalysed the pyridoxal phosphate-dependent transamination of L-tryptophan, but not D-trptophan, and in common with some other microbial transaminases, a-ketoglutarate was an efficient amino-group acceptor. L-Phenylalanine, tyrosine, and 3-methyltryptophan (this compound inhibited enzyme function) also underwent transamination. 

Speedie (4S) has obtained a cell-free preparation from S. flocculus which catalyzes the formation of (31) from L-tryptophan and S-adenosyl-L methionine. The crude enzyme has been purified 2-fold by ammonium sulfate fractionation, and preliminary results with this preparation after dialysis indicated that pyridoxal phosphate is not required, but may cause some stimulation of enzyme activity. At this stage of purification tryptophan transaminase activity was also present, and it has not yet been possible to determine whether the true methylase substrate is an activated tryptophan, or indole pyruvic acid (33), as has been demonstrated to be the case in the biosynthesis of indolmycin (34) (46). 

This enzyme [EC 2.6.1.1] (also known as transaminase A, glutamicioxaloacetic transaminase, and glutamic aspartic transaminase) catalyzes the reversible reaction of aspartate with a-ketoglutarate to produce oxaloace-tate and glutamate. Pyridoxal phosphate is a required cofactor. The enzyme has a relatively broad specificity, and tyrosine, phenylalanine, and tryptophan can all serve as substrates. 

Vitamin B6 occurs naturally in three related forms pyridoxine (6.26 the alcohol form), pyridoxal (6.27 aldehyde) and pyridoxamine (6.28 amine). All are structurally related to pyridine. The active co-enzyme form of this vitamin is pyridoxal phosphate (PLP 6.29), which is a co-factor for transaminases which catalyse the transfer of amino groups (6.29). PLP is also important for amino acid decarboxylases and functions in the metabolism of glycogen and the synthesis of sphingolipids in the nervous system. In addition, PLP is involved in the formation of niacin from tryptophan (section 6.3.3) and in the initial synthesis of haem. 

For two transaminases the remaining unknown stereochemical parameter was determined by demonstrating an internal transfer of tritium (dialkyl amino acid transaminase) [28] or deuterium (pyridoxamine-pyruvate transaminase) [27] from the a-position of the substrate L-alanine to C-4 of the cofactor. Internal hydrogen transfer from the a-position of the substrate amino acid to C-4 of PLP has also been demonstrated for two of the abortive transamination reactions, those catalyzed by tryptophan synthase fi2 protein [32] and by aspartate-/8-decarboxylase [31]. In addition, the same phenomenon must occur in alanine transaminase, as deduced from the observation that the enzyme catalyzes exchange of the /8-hydrogens of... 

Under normal conditions, the rate-limiting enzyme of the pathway is tryptophan dioxygenase (Section 8.3.2), and there is hide accumulation of intermediates. Kynurenine transaminase, the enzyme which catalyzes the transamination and ring closure of kynurenine to kynurenic acid, and of hydroxykynurenine to xanthurenic acid, has a high relative to the normal steady-state concentrations of its substrates in the liver. Kynureninase and kynurenine hydroxylase have lower values of K, so that there is normally litde accumuladon of kynurenine or hydroxykynurenine. 

Induction of extrahepatic mdoleamine dioxygenase (which catalyzes the same reaction as tryptophan dioxygenase, albeit by a different mechanism) by bacterial lipopolysaccharides and mterferon-y may result in the production of relatively large amounts of kynurenine and hydroxykynurenine in tissues that lack the enzymes for onward metabolism. Kidney has kynurenine transaminase activity, and therefore extrahepatic metabolism of tryptophan may result in significant excretion of kynurenic and xanthurenic acids, even when vitamin Bg nutrition is adequate. 

Early studies of vitamin Be requirements used the development of abnormalities of tryptophan or methionine metabolism during depletion, and normalization during repletion with graded intakes of the vitamin. Although tryptophan and methionine load tests are unreliable as indices of vitamin Be status in epidemiological studies (Section 9.5.4 and Section 9.5.5), under the controlled conditions of depletion/repletion studies they do give a useful indication of the state of vitamin Be nutrition. More recent studies have used more sensitive indices of status, including the plasma concentration of pyridoxal phosphate, urinary excretion of 4-pyridoxic acid, and erythrocyte transaminase activation coefficient. 

Estimation of the vitamm Be requirements of infants presents a problem, and there is a clear need for further research. Human mUk, which must be assumed to be adequate for infant nutrition, provides only 2.5 to 3.5 //g of vitamin Be per g of protein-lower than the requirement for adults. Although their requirement for catabolism of amino acids may be lower than in adults (because they have net new protein synthesis), they must also increase their body content of the vitamin as they grow. Coburn (1994) noted that the requirement for growth in a number of animal species was less than that to maintain saturation of transaminases or rniriimum excretion of tryptophan metabolites after a test dose and was about 15 nmol per g of body weight gain across a range of species. 

Kim JH and Miiier LL (1969) The functional significance of changes in activity of the enzymes, tryptophan pyrroiase and tyrosine transaminase, after induction in intact rats and in the isolated, perfused rat livei. Journal of Biological Chemistry 244,1410-16. 

Urinary excretions of nicotinic acid metabolites and 2-pyridone, as well as of 4-pyridoxic and xanthurenic acids were determined in 15 South African Bantu pellagrins before and after tryptophan administration (P13). Red blood cell riboflavine levels and serum glutamic-oxalacetic transaminase levels were also measured. The authors discussed the apparent inability of the pellagra patients to convert tryptophan to nicotinic acid as indicated by their low excretion of nicotinic acid metabolites before and after tryptophan load. The possibility that the subjects were also suffering from a riboflavine deficiency was also discussed. 

INDOLMYCIN (20) is formed from pyruvate, and two enzymes active in initial stages of Its biosynthesis have been studied. They are a transaminase and aC-methyltransferase. The hypothetical route to indolmycin is by indole pyruvate, 3-methyl-indolepyruvate, indolmycenic acid (reduced alpha oxo group) and finally indolmycin which probably takes its amidine group from an arginine molecule 79. The closely related [pyrrolo (1,4) benzodiazepines] 80>81,82 antitumor antibiotics, anthramycin, tomaymycin and sibiromycin are formed from tryptophan (via the kynurenine pathway ), tyrosine and methionine-derived methyl groups 80.si.sz. 

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. 

Inhibits binding of penoxybenzamine in aorta, permeability of water, pigmentation, serotonin levels, tryptophan levels, tryptophan 2,3-dioxygenase, tyrosine hydroxylase, tyrosine transaminase. 

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