Kynurenine-anthranilate

However, a more recent study exists [33], also based on the use of isotopically labeled anthranilic acid, which demonstrated that the substance is unable to increase HA levels in the liver and in the urines, when administered to rats on the contrary, such an increase was observed when either kynurenine or HA were administered, suggesting that the pathway, operating in mammals, should be kynurenine - HK - HA, and not kynurenine - anthranilic acid - HA. In that study, it was also concluded that the preferred route of kynurenine further metabolism was through... 

In addition to the pathway just outlined, tryptophan can result from the transamination of indole-pyruvic acid, but it seems unlikely that this reaction makes any important contribution to the biosynthesis. A trypto-phan-kynurenine-anthranilate-indole-tryptophan cycle has also been proposed i.e. the reverse of the catabolic pathway described in Fig. 58. However this sequence of reactions is only traversed if an excess of tryptophan is present and its function appears to be purely degradative. 

Role of Riboflavin. Riboflavin deficiency has been found to produce abnormaUties in the metabolism of tryptophan (307-311). The deficiency leads to an increased excretion in the urine of kynurenine, anthranilic acid, and kynurenic acid and its conjugates (308, 309). In liver and kidney slices riboflavin deficiency leads to a decrease in indolepyruvic acid accumulation and an increase in the production of kynurenic acid and anthranilic acid from L-kynurenine (310, 311). The deficiency was also found to... 

Phenylalanine hydroxylase occurs only in mammalian liver (that is, in the rat, guinea-pig, rabbit, d<, chicken, and human) (see also 259). No activity has been observed in (rat) lung, kidney, brain, or muscle. The system is quite speciOc for L-phenylalanine. Tjrro-sine is not formed from n-phenylalanine, nor are the corresponding p-phenols formed from N-acetyl- or N-chloroacetyl-L-phenylalanine, L-phenylalanine ethyl ester, DL-phenylglycine, phenylserine, phenylpyruvic acid, phenylethylamine, benzoic acid, hippuric acid, cinnamic acid, or mandelic acid (768), or from aniline, acetanilide, tryptophan, kynurenine, anthranilic acid, or phenylacetate (557). This specificity is a distinguishing character of the enzyme, which occurs in the same tissue as the nonspecific aromatic hydroxylase described above. 

The kynurenine pathway metabolites are kynurenine, kynurenic acid, xahthurenic acid, 3-hydroxykynurenine, anthranilic acid and quinolinic acid. The more important are kynurenine (Kyn) and 3-hydroxykynurenine (30HKyn) (Fig 1). 

The biosynthesis and metabolism of nicotinic acid in disease has received little attention metabolic studies deal mainly with normal animals and man (01, R5). After a tryptophan load dose, the main catabolites in the urine are nicotinuric acid, N1-methylnicotinamide, nicotinamide, quinolinic acid, kynurenine, 6-pyridone, anthranilic acid, and 3-hydroxyanthranilic acid. These excretory products were estimated... 

This pyridoxal-phosphate-dependent enzyme [EC 3.7.1.3] catalyzes the hydrolysis of kynurenine to produce anthranilate and alanine. 3 -Hydroxykynurenine and some other (3-arylcarbonyl)alanines can also be acted upon by this enzyme. 

Another related reaction that goes through a ketimine is the conversion of the amino acid kynurenine to alanine and anthranilic add.225 It presumably depends upon hydration of the carbonyl group prior to P cleavage (Eq. 14-35). An analogous thiolytic cleavage utilizes CoA to convert 2-amino-4-ketopentanoate to acetyl-CoA and alanine.226... 

Returning to the major tryptophan catabolic pathway, marked by green arrows in Fig. 25-11, formate is removed hydrolytically (step c) from the product of tryptophan dioxygenase action to form kynurenine, a compound that is acted upon by a number of enzymes. Kynureninase (Eq. 14-35) cleaves the compound to anthranilate and alanine (step d), while transamination leads to the cyclic kynurenic acid (step e). Hie latter is dehydroxylated in an unusual reaction to quinaldic acid, a prominent urinary excretion product. 

A note on the rapid conversion of fevo-kynurenine to anthranilic acid, and the conjugation of the latter with glucuronic acid by riboflavin-deficient rat-liver and rat-kidney slices, has recently appeared.148... 

Kynureninase is involved in the oxidative metabolism of tryptophan. It catalyzes the conversion of L-kynurenine to anthranilic acid. The enzyme also converts L-3-hydroxykyneurenine to 3-hydroxyanthranilic acid. The latter compound has a high fluorescence, which is the basis for detection in this assay. 

Kynurenine Hydroxylase Kynurenine hydroxylase is an FAD-dependent mixed-function oxidase of the outer mitochondrial membrane, which uses NADPH as the reductant. The activity of kynurenine hydroxylase in the liver of riboflavin-deficient rats is only 30% to 50% of that in control animals, and deficient rats excrete abnormally large amounts of kynurenic and anthranilic acids after the administration of a loading dose of tryptophan, and, correspondingly lower amounts of quinolinate and niacin metabolites. Riboflavin deficiency may thus be a contributory factor in the etiology of pellagra when intakes of tryptophan and niacin are marginal (Section 8.5.1). 

The enzyme cleaving kynurenine to form anthranilic acid and alanine (W14) is kynureninase which also requires pyridoxal phosphate as coenzyme (B19). 

Tompsett (T3) achieved a separate elution from cation or anion resin columns of several tryptophan metabolites which were then determined colorimetrically. Finally, Boyland and Williams (B18) quantitatively adsorbed on inactivated charcoal anthranilic acid, kynurenine, 3-hy-droxyanthranilic acid, 3-hydroxykynurenine, and the sulfuric acid ester derivatives of the two latter compounds from urine of normal controls and of patients with bladder cancer. After elution, the compounds were separated by gradient chromatography on Celite columns and determined colorimetrically or spectrophotometrically. 

Quagliariello et al. (Ql) followed qualitatively (by means of onedimensional paper chromatography) the spontaneous excretion of kynurenines and anthranilic acids in 47 bladder tumor patients. They found that 3-hydroxyanthranilic acid was usually excreted and other metabolites only occasionally. Furthermore, Quagliariello (Q4) in an interesting review, reports that in urine of 15 bladder tumor patients 3-hydroxyanthranilic acid is excreted in amounts that are usually accepted as normal, whereas normal controls do not excrete 3-hydroxyanthranilic acid at all. 

This observation is related to that made by us (P4a) that kynurenine alone, or sometimes together with 3-hydroxykynurenine and 3-hydroxy-anthranilic acid, was found to be abnormally elevated in 17 of cases with renal tumors, in 3 of 6 cases with pelvic tumors, and in 1 of 2 cases with cystic renal disease. 

We carried out several experiments by loading with 100 or 50 mg/kg body weight of L-tryptophan and determining kynurenine, N-a-acetyl-kynurenine, 3-hydroxykynurenine, kynurenic acid, xanthurenic acid, 3-hydroxyanthranilic acid, and free anthranilic acid. These tests were performed in a variety of conditions and the results are discussed in the following sections. 

Boyland and Williams (B18) loaded 10 normal controls with 10 g DL-tryptophan. In spite of the fact that it is essential to use L-tryptophan in studies of this type and although data on excretion of kynurenic and xanthurenic acids are lacking, at least 5% was recovered as increased kynurenine, 3-hydroxykynurenine, anthranilic acid, and 3-hydroxyan-thranilic acid, free and conjugated. It should be pointed out that in these studies only a low percentage of the ingested dose of tryptophan was found in the form of urinary metabolites. The remainder, approximately 93-94%, of the load could not be recovered. 

Chizhova and Ivanova (C7) studied 20 children, aged 1-12 years, under therapy for leukemia and 10 healthy children as control. A total of 15-20 g of tryptophan was administered during 5-10 days (1.5-3 g/day) to 7 children whereas 13 were given a single dose of 2-3 g. Daily determinations of urinary metabolites by paper chromatography demonstrated a disturbance of tryptophan metabolism in 19 of the 20 leukemic children before and after tryptophan loading. Kynurenine, 3-hydroxykynurenine, and anthranilic and 3-hydroxyanthranilic acids appeared in urine, whereas 5-hydroxyindoleacetic acid was absent in the majority of the young patients. The disturbances of tryptophan metabolism were similar in all of them. Administration of vitamin Be restored tryptophan metabolism to normal in the majority of the patients. 

Alloxan-diabetic animals, in which the conversion of tryptophan to methylnicotinamide was greatly impaired, excreted increased amounts of methylnicotinamide when very large doses of tryptophan were given, indicating that the defect may be due to changes in the kynurenine pathway of tryptophan (M5). Moreover, diabetic rats excreted much more xanthurenic acid and less anthranilic and 3-hydroxyanthranilic acids than did nondiabetics, following large doses (200-400 mg) of tryptophan. 

Among the active compounds were xanthurenic and 3-hydroxyanthra-nilic acids, whereas tryptophan, kynurenine, and kynurenic, anthranilic, and nicotinic acids were unable to cause deiodination. These findings support, according to the authors, the view that the deiodination of the thyroid hormone may be closely associated with its biological action. 

The spontaneous urinary pattern during viral hepatitis in an acute phase seems to be abnormal, showing kynurenine in a high percentage of cases together with 3-hydroxykynurenine, 3-hydroxyanthranilic acid, and in a few instances anthranilic acid. Some discrepancies appear for the excretion in clinically recovered subjects, since Quagliariello (Q3) found a normalized output of metabolites, whereas Piazza and Tancredi (P5) found that about 60% showed an abnormal excretion of the same substances. 

Interest then moved to animals. Both isotopic and nutritional experiments showed that the pathway established in microorganisms applied equally to mammals. Thus hydroxyanthranilic acid was converted to nicotinic acid (9, 604), which it could replace as a growth factor (944), whereas there was no similar conversion of anthranilic acid (343). An outstanding series of isotopic experiments, especially by Heidelberger and co-workers, showed that the 8-carbon atom of the tryptophan side chain became the 8-carbon atom of the kynurenine side chain and that the side chain was lost in conversion of kynurenine to nicotinic acid (369, 371, 427). Moreover the carbon in the 3-position of the indole nucleus became the carboxyl carbon of nicotinic acid (370 this experiment proved conclusively the reality of the tryptophan-nicotinic acid conversion) and the indole nitrogen appeared with only slight dilution in kynurenine, kynurenic acid, and xanthurenic acid (759). All these relations are those to be expected for the pathway tryptophan —+ kynurenine —> hydroxykynurenine (or its phosphate) —> hydroxyanthranilic acid (or its phosphate) — nicotinic acid, illustrated in diagrams 17 and 18. 

Kynurminase, Kynurenine Transaminase, and the Formation of Anthranilic, Kynurenic, Hydroxyanthranilic, and Xanthurenic Acids... 

Knox et al. (176) found that formation of anthranilic acid from kynurenine was always accompani by formation of kynurenic acid, and they considered that the keto acid (o-aminobenzoylpyruvic acid, diagram 20) might be a common intermediate in formation of both substances. This was soon disproved, and it is now clear that two independent reactions are involved, as illustrated in diagram 20. 

Wiss (937, 938) fractionated crude liver extracts to give a kynureninase fraction which would form anthranilic acid, but not kynurenic acid, and a transaminase fraction which would not form anthranilic acid, but formed kynurenic acid provided an a-keto acid was present. o-Aminobenzoyl-pyruvic acid, the keto acid corresponding to kynurenine, is known to cyclize spontaneously to kynurenic acid (622), and the absence of ammonia production and requirement for an a-keto acid (c/. also 434) suggests that... 

Side Reactions of Kynurenine, Hijdroxykynurenine, Anthranilic Acid, and Hydroxyanthranilic Acid... 

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