Formamidase

In this section, we summarize information about five Ci enzymes involved in metabolism of 5-formyl-THF (5-CHO-THF) and formate, whose existence was predicted from genomics data. 5-Formyltetrahydrofolate cycloligase (5-FCL) and S-formylglutathione hydrolase have since been shown to catalyze the anticipated reactions the other three enzymes (10-formyl-THF deformylase, formamidase, and sarcosine oxidase) are still putative. 

WYBORN, N.R., SCHERR, D.J., JONES, C.W., Purification, properties and heterologous expression of formamidase from Methylophilus methylotrophus, Microbiology, 1994,140,191-195. 

Eto M, Seifert J, Engel JL, et al. 1980. Organophosphorus and methylcarbamate teratogens structural requirements for inducing embryonic abnormalities in chickens and kynurenine formamidase inhibition in mouse liver. Toxicol Appl Pharmacol 54(l) 20-30. 

Seifert J, Casida JE. 1978. Relation of yolk sac membrane kynurenine formamidase inhibition to certain teratogenic effects of organophosphorus insecticides and of carbaryl and eserine in chicken embryos. Biochem Pharmacol 27 2611-15. 

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.

The second (functional) aspect of this problem eoneems the development of pathologic symptoms, the moleeular causes of which are not due to AChE inhibition. One of the first cases of OP nonanticholinesterase effeets (when the association between the OP molecular target and the functional disturbance has been proved) involved inhibition of kinurenin formamidase of the hen egg yolk sac membrane responsible for teratogenic effects (Seifert and Casida, 1978). Another example of OP teratogenic effect is abnormal development of the conjunctival tissue of Xen-opus embryos due to inhibition of lysyloxidase and incomplete post-translational modification of collagen (Snawder and Chambers, 1993). 

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. 

IDO is a key enzyme in the degradation of tryptophan in extra-hepatic tissues [37], through the generation of AAformyl kynurenine which is further degraded to kynurenine (l-KYN) by formamidase. In addition to its potential role in neurodegeneration, inhibition of IDO has been implicated as an important new therapeutic target for the treatment of cancer through tumor immunosuppression [3, 38]. 

Formation of Qa via aerobic degradation ofTrp (Kyn pathway) includes five enzymatic steps (1) oxidation of Trp to N-formyl kynurenine (FKyn) by Trp 2,3-dioxygenase (TRDOX), (2) deformylation of FKyn by kynurenine formamidase (KYNFA), (3) oxidation of Kyn to 3-hydroxykynurenine (HKyn) by kynurenine 3-monooxygenase (KYNOX), (4) conversion of HKyn into 3-hydroxyanthranilate (HAnt) by kynureninase (KYNSE), and (5) oxidation of HAnt by 3-hydroxyanthranilate 3,4-dioxygenase (HADOX) to a-amino-/3-carboxymuconic semialdehyde (ACMS) followed by its spontaneous cyclization to Qa (Scheme 2). This pathway and all respective... 

Substrate specificity of the purified indoleamine 2,3-dioxygenase from rabbit intestine was examined spectrophotometrically at 24°C. The spectra of the reaction products in either the absence or presence of formamidase were compared with those of authentic compounds. A single enzyme protein catalyzed the oxygenative ring cleavage of d- and L-tryptophan, 5-hydroxy-D- and -L-tryptophan, tryptamine, and serotonin (10). The maximal turnover number was obtained with L-tryptophan (99 mol min -mor of enzyme at 24°C), and the lowest value was with 5-hydroxy-L-tryp-tophan (20 p.Af). A marked substrate inhibition is observed by the L isomers of tryptophan and 5-hydroxytryptophan above 0.2 and 0.06 mM, at pH 6.6, respectively. The compounds including skatole, indole, in-doleacetic acid, 5-hydroxyindoleacetic acid, N-acetyltryptophan, melatonin, and a-methyl-DL-tryptophan, are all inert as substrate. 

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... 

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