Tryptophan transaminase activity

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). 

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. 

Tjo osine-a-ketoglutarate transaminase (210) is an inducible enzyme much like tryptophan pyrrolase (see Section XIII). The ability to increase the transaminase activity in liver by tyrosine administration depends on the integrity of adrenal cortical function. 

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. 

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. 

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. 

But in assessing the state of vitamin B nutrition, it is often advantageous to quantitate the urinary excretion of as many tryptophan metabolites as possible (P7). For example, in severe deficiency, as occurs in tuberculosis patients treated with isoniazid, the activities of both 3-hydroxykynureninase and 3-hydroxykynurenine transaminase are apparently markedly reduced, such that xanthurenic acid excretion may be normal and excretion of kynurenine and 3-hydroxykynurenine markedly increased (B20, P7). 

Estrogens may modify the activity of enzymes in the kynurenine pathway other than the rate-limiting enzyme tryptophan pyrrolase. Mason and co-workers (M9, M12) have presented in vitro and in vivo evidence that estrogens may effect binding of PLP to the apoenzyme of kynurenine transaminase. 

The activity of this mechanism depends upon the presence of pyri-doxine. Thus pyridoxine plays still another role in connection with the metabolism of tryptophan, in addition to being a part of the coenzyme of kynureninase and kynurenine transaminase. 

The most widely used method of assessing vitamin B status is by the activation of erythrocyte transaminases by pyridoxal phosphate added in vitro, expressed as the activation coefficient (section 11.6.4.1). The ability to metabolize a test dose of tryptophan (section 11.9.5.1) or methionine (section 11.9.5.2) has also been used. 

Comparatively recently a sharp increase in activity of a number of liver enzymes in rats and other animals is produced by cortisone (Knox et al., 1956). This stimulation of enzyme (glutamate-tyrosine transaminase, tryptophan-pyrrolase, glucose-6-phosphatase, etc.) activity by cortisone in experiments on adrenalectomized rats was associated with the synthesis of these enzymes de novo (Kenney, 1962 Alievskaya, 1965). As might be expected, this increase in enz5mie synthesis was preceded by a sharp increase in synthesis of RNA, particularly RNA of the cell nuclei (Fig. 99) (Kenney and Kull, 1963). The RNA fraction which was stimulated corresponded in its sedimentation characteristics to fast-labeled (messenger) RNA. 

The oxidative pathway of tryptophan metabolism is shown in Figure 3. Kynureninase is a pyridoxal phosphate-dependent enzyme, and in deficiency its activity is lower than that of tryptophan dioxygenase, so that there is an accumulation of hydroxy-kynurenine and kynurenine, resulting in greater metabolic flux through kynurenine transaminase and increased formation of kynurenic and xanthurenic acids. Kynureninase is exquisitely sensitive to vitamin Bg deficiency because it undergoes a slow inactivation as a result of catalysing the half-reaction of transamination instead of its normal reaction. The resultant enzyme with pyridoxamine phosphate at the catalytic site is catalytically inactive and can only be reactivated if there is an adequate concentration of pyridoxal phosphate to displace the pyridoxamine phosphate. 

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