Formylkynurenine formamidase

Figure 8.4. Pathways of tryptophan metaholism. Tryptophan dioxygenase, EC 1.13.11.11 formylkynurenine formamidase, EC 3.5.1.9 kynurenine hydroxylase, EC 1.14.13.9 kynureninase, EC 3.7.1.3 3-hydroxyanthranilate oxidase, EC 1.10.3.5 picolinate carboxylase, EC 4.1.1.45 kynurenine oxoglutarate aminotransferase, EC 2.6.1.7 kynurenine glyoxylate aminotransferase, 2.6.1.63 tryptophan hydroxylase, EC 1.14.16.4 and 5-hydroxytryptophan decarboxylase, EC 4.1.1.26. Relative molecular masses (Mr) tryptophan, 204.2 serotonin, 176.2 kynurenine, 208.2 3-hydroxykynurenine, 223.2 kynurenic acid, 189.2 xanthurenic acid, 205.2 and quinolinic acid 167.1. CoA, coenzyme A.

Samples of (3R)- and (3S)-[3- Hi]tryptophans 331 have been converted to the corresponding kynurenines 336 using the enzymes tryptophan dioxygenase (EC 1.13.11.11) and formylkynurenine formamidase (EC 3.5.1.9). These have been used to investigate the fission of kynurenine 336 to anthranilic acid 300 and alanine 337 in H20 (337) (Scheme 85). Conversion of the alanine to acetate and assessment of sense of chirality indicated that... 

The enzyme called formylase by Knox and Mehler (490, 591) and ky-nurenine formamidase by Jakoby (437) is present in liver in a considerable excess relative to tryptophan peroxidase-oxidase (e.g., 491), and formylky-nurenine is therefore not normally found in tissues or excreted in urine (e.g., 171). Partially purified tryptophan peroxidase-oxidase, from which formylase activity has been removed, accumulates formylkynurenine, shown (591) to be identical with synthetic (947 or better, 172) material. Formylase occurs widely in bacteria, and has been partially purified from Neurospora (437). In both higher and lower organisms the enzyme shows considerable specificity. 

Kynurenine formamidase catalyzes the hydrolysis of formylkynurenine. A variety of aromatic forma-mides will react, but the enzyme is more active with its natural than with the synthetic substrates. The product of the reaction, kynurenine, may then either be hydroxylated or decarboxylated. 

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-... 

The experiments discussed above on the biosynthesis of a pseudan have also shown that kynurenic acid is not the precursor of the 2-alkylquinolin-4(lH)-ones. Based on the putative function of the genes of the qbs operons (176), a pathway was proposed for the biosynthesis of kynurenic acid (I), xanthurenic acid [10(8)], and quinolobactin [IO(8) j in Pseudomonas fluorescens ATCC 17400 (Scheme 5). The first step, the oxidation of tryptophan to N-formylkynurenine, is likely to be catalyzed by the enzyme tryptophan 2,3-dioxygenase (TDO) (QbsF), which is a heme-dependent enzyme. The second step, the deformylation of N-formyl-kynurenine to L-kynurenine is catalyzed by kynurenine formamidase (KFA). The product of qbsH, a metal-dependent hydrolase found also in other bacterial genomes, is the likely candidate. 

The tryptophan-oxidizing activity can be assayed by various determinations of the substrate, but more rapid and convenient assays have been used based on the appearance of the product. The initial product of tryptophan oxidation is A -formylkynurenine, but this is rapidly hydrolyzed by an enzyme, kynurenine formylase (Mehler and Knox, 1951) or formamidase, that is much more active than tryptophan pyrrolase. The kynurenine that is produced in the second reaction accumulates... 

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