Kynurenine from formylkynurenine

Formation of kynurenine from tryptophan was discovered in liver extracts by Kotake and Masayama 289). These authors proposed the name tryptophan pyrrolase for the enzyme. Knox and Mehler 290) subsequently showed that the formation of kynurenine consisted of two enzyme reactions, an initial oxidation to formylkynurenine followed by hydrolysis to kynurenine. Because the reaction was stimulated by HsO produced in situ it was assumed that there was an intermediate formation and utilization of peroxide in the oxidation. For this reason the enzyme was renamed tryptophan oxidase-peroxidase by Knox. Experiments with O showed that molecular O2 was incorporated into the reaction products 291). One mole of O2 per mole of tryptophan was contained in the formylkynurenine. When H20 was tested, very little 0 was utilized. This observation led Tanaka and Knox 292) to return to the use of the original name, tryptophan pyrrolase. 

Phenoxazines — The two main types of phenoxazines are the ommochromes and the microbial phenoxazines. The biosynthesis of ommochromes occurs via the kynurenine pathway. The tryptophan amino acid is converted to formylkynurenine and then to kynurenine and 3-hydroxykynurenine. Not all the steps of ommochrome synthesis are completely elucidated yet. Ommatins are dimers and ommins are oligomers of 3-hydroxykynurenine. - The papiliochromes are derived from tyrosine as well as from the tryptophan pathway. The key intermediate in the formation of papiliochromes is N-beta-alanyldopamine (NBAD). Papiliochromes are synthesized in special wing scale cells, before melanins. " "... 

Alkaloids derived from nicotinic acid contain a pyridine nucleus. Nicotinic acid itself is synthesized from L-tryptophan via A-formylkynurenine, L-kynurenine, 3-hydroxykynurenine, 3-hydroxyanthranilic acid and quinolinic acid. 

Figure U.1D. Nicotinamide analogs tested in pellagra or animal models of pellagra. The active compounds include (1,2, 6, 11, 12, 13,15,18, and 20). Compounds active in some models or in pellagrainclude (7,17,21), pyrazine-2,3-dicarboxylic acid, NAD, pyridyl-3-aldehyde,pyridyl-3-carbi-nol, tryptophan, and 3-hydroxyanthranilicacid. The inactive compounds are (3,4,5,8,9,10,14,16, 19), 3-arninopyridine,thiazole-5-carboxylicacid, 2-methylpyridine, 3-methylpyridine, 2,6-dimethyl-pyridine-3,5-dicarboxylic acid, pyridine-3,5-dicarboxylic acid, kynurenine, 3-hydroxykynurenine, and formylkynurenine. This figure is from Ref 33 and is used with permission.

Tryptophan is an essential amino acid involved in synthesis of several important compounds. Nicotinic acid (amide), a vitamin required in the synthesis of NAD+ and NADP+, can be synthesized from tryptophan (Figure 17-24). About 60 mg of tryptophan can give rise to 1 mg of nicotinamide. The synthesis begins with conversion of tryptophan to N-formylkynurenine by tryptophan pyrrolase, an inducible iron-porphyrin enzyme of liver. N-Formylkynurenine is converted to kynurenine by removal of formate, which enters the one-carbon pool. Kynurenine is hydroxylated to 3-hydroxykynurenine, which is converted to 3-hydroxyanthranilate, catalyzed by kynureninase, a pyridoxal phosphate-dependent enzyme. 3-Hydroxyanthranilate is then converted by a series of reactions to nicotinamide ribotide, the immedi-... 

The heme dioxygenase enzymes involved in tryptophan oxidation catalyse the first and rate-limiting step in the kynurenine pathway—the 02-dependent oxidation of 1-tryptophan to AT -formylkynurenine. In the past 10 years, there have been substantial new developments, including new structural information, bacterial expression systems for a number of dioxygenases, contributions from computational chemistry, and emerging... 

Kynurenine Formylase. Hydrolysis of formylkynurenine to kynurenine is catalyzed in liver preparations by an enzyme (formylase or kynurenine formamidase) with rather low specificity for aromatic formamides. The reaction with formylkynurenine, however, is faster than that with any analogs. A very similar enzyme has been obtained from Neurospora. In contrast, an enzyme from insects does not hydrolyze formylanthranilic acid, a model substrate for formylase from other sources, or other analo-... 

This enzyme has been studied in liver SIS), Neuroapara (S14), and Pseudomonas fluorescens (SIB). It is able to cleave a variety of compounds containing the COCH2CHNH1COOH group. Liver kynureninase (SIS) splits 3-hydroxyk3murenine about twice as rapidly as kynurenine the enzyme from Pseudomonas (315) splits kynurenine five times as rapidly as the hydroxy compound. Jacoby and Bonner (SI4) reported that kynureninase from Neurospora also attacks formylkynurenine. 

Irradiation of both soluble and fibrous proteins with ultraviolet light leads to considerable destruction of cystine and aromatic acid residues, in particular tryptophan. However, apart from a small degree of conversion to formylkynurenine and/or kynurenine residues, all attempts to ascertain the nature of the tryptophan photoproducts have so far been unsuccessful (72, 63, 303, 304). Pirie and Dilley 304) have proposed that photo-oxidation of tryptophan to formylkynurenine is followed by splitting out of N-formylanthranilic acid (60). Rivett s... 

Asquith and Rivett (72) irradiated tryptophan in oxygenated aqueous solution with a medium pressure mercury lamp under neutral, acidic and basic conditions and identified several degradation products including formylkynurenine, kynurenine, aspartic acid, serine, glycine, alanine and (3-alanine. They suggest that the main degradative pathway of tryptophan is conversion to kynurenine via the indolenine hydroperoxide (see Section III. 1.1). The formation of amino acids could arise from further degradation of kynurenine as shown below. 

Apart from the results reported by Jori et al. 211-214) who claimed quantitative conversion of tryptophan to formylkynurenine residues when proteins are photooxidized in aqueous buffered solution with proflavine as sensitizer, no other author has reported similar quantitative conversion. On the other hand it has been found that loss of tryptophan clearly does not match the formation of formylkynurenine 63, 304). Gomyo and Fujimaki 148) detected kynurenine and 3-hydroxykynurenine as products of the lumiflavine sensitized photooxidation of lysozyme in buffered aqueous solution. Kravchenko and Lapuk 229, 232), careful purified a photooxidized sample of lysozyme and isolated a group of monooxidized enzymes in which a single tryptophan entity was oxidized. Even this relatively homogeneous product could be separated into different species of enzyme with different degrees of residual activity and different ultraviolet spectra (229). 

Jori et al. indicate that the estimation of formylkynurenine can be made by acid hydrolysis to kynurenine followed by analysis with an amino acid analyzer, but it is not clear from the paper (272) which of the two methods were used to estimate the actual figures reported. In addition, Jori et al. have as yet given no evidence in support of their claim that formylkynurenine is the single end-product of tryptophan photooxidation, but merely quote (272) in a footnote unpublished experi-... 

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