New formamidase having substrate specificity for o-formylaminoacetophenone in pig liver

Species Differences in Tryptophan Metabolism and Disposition

Major species differences in tryptophan (Trp) metabolism and disposition exist with important physiological, functional and toxicity implications. Unlike mammalian and other species in which plasma Trp exists largely bound to albumin, teleosts and other aquatic species possess little or no albumin, such that Trp entry into their tissues is not hampered, neither is that of environmental chemicals and toxins, hence the need for strict measures to safeguard their aquatic environments. In species sensitive to toxicity of excess Trp, hepatic Trp 2,3-dioxygenase (TDO) lacks the free apoenzyme and its glucocorticoid induction mechanism. These species, which are largely herbivorous, however, dispose of Trp more rapidly and their TDO is activated by smaller doses of Trp than Trp-tolerant species. In general, sensitive species may possess a higher indoleamine 2,3-dioxygenase (IDO) activity which equips them to resist immune insults up to a point. Of the enzymes of the kynurenine pathway beyond TDO and IDO, 2-amino-3-carboxymuconic acid-6-semialdehyde decarboxylase (ACMSD) determines the extent of progress of the pathway towards NAD⁺ synthesis and its activity varies across species, with the domestic cat (Felis catus) being the leading species possessing the highest activity, hence its inability to utilise Trp for NAD⁺ synthesis. The paucity of current knowledge of Trp metabolism and disposition in wild carnivores, invertebrates and many other animal species described here underscores the need for further studies of the physiology of these species and its interaction with Trp metabolism.

Deuterium-Labeling Studies Reveal the Mechanism of Cytochrome P450-Catalyzed Formation of 2-Aminoacetophenone from 3-Methylindole (Skatole) in Porcine Liver Microsomes

2-Aminoacetophenone, a product of the hepatic 3-methylindole (skatole) clearance in pigs, contributes to the specific aroma of fat from tainted boar meat. Surprisingly, high formation rates for 2-aminoacetophenone from skatole in microsomal preparations from Pietrain × Baden-Württemberg hybrid type boars have been previously demonstrated, but the mechanism of this cytochrome P450-mediated reaction remained unknown. Therefore, microsomal fractions from boars were incubated with deuterium-labeled skatole and with possible reaction intermediates. 3-Methyloxindole and 3-hydroxy-3-methyloxindole were identified as degradation products of skatole en route to 2-aminoacetophenone. Additionally, the labeling studies provided further evidence for a cytochrome P450-mediated lyase reaction that leads to the cleavage of the indole heterocyclic ring system in 3-hydroxy-3-methyloxindole and demonstrated the involvement of several cytochrome P450-isoforms by employing isotopically sensitive branching experiments.

Effect of arylformamidase (kynurenine formamidase) gene inactivation in mice on enzymatic activity, kynurenine pathway metabolites and phenotype

The gene coding for arylformamidase (Afmid, also known as kynurenine formamidase) was inactivated in mice through the removal of a shared bidirectional promoter region regulating expression of the Afmid and thymidine kinase (Tk) genes. Afmid/Tk -deficient mice are known to develop sclerosis of glomeruli and to have an abnormal immune system. Afmid-catalyzed hydrolysis of N-formyl-kynurenine is a key step in tryptophan metabolism and biosynthesis of kynurenine-derived products including kynurenic acid, quinolinic acid, nicotinamide, NAD, and NADP. A disruption of these pathways is implicated in neurotoxicity and immunotoxicity. In wild-type (WT) mice, Afmid-specific activity (as measured by formyl-kynurenine hydrolysis) was 2-fold higher in the liver than in the kidney. Formyl-kynurenine hydrolysis was reduced by approximately 50% in mice heterozygous (HZ) for Afmid/Tk and almost completely eliminated in Afmid/Tk knockout (KO) mice. However, there was 13% residual formyl-kynurenine hydrolysis in the kidney of KO mice, suggesting the existence of a formamidase other than Afmid. Liver and kidney levels of nicotinamide plus NAD/NADP remained the same in WT, HZ and KO mice. Plasma concentrations of formyl-kynurenine, kynurenine, and kynurenic acid were elevated in KO mice (but not HZ mice) relative to WT mice, further suggesting that there must be enzymes other than Afmid (possibly in the kidney) capable of metabolizing formyl-kynurenine into kynurenine. Gradual kidney deterioration and subsequent failure in KO mice is consistent with high levels of tissue-specific Afmid expression in the kidney of WT but not KO mice. On this basis, the most significant function of the kynurenine pathway and Afmid in mice may be in eliminating toxic metabolites and to a lesser extent in providing intermediates for other processes.

Genetics of formamidase-5 (brain formamidase) in the mouse: localization of the structural gene on chromosome 14

A single formamidase, which is different from the formamidases found in other tissues occurs in the brains of mice. This enzyme is here called formamidase-5 and the gene symbol is designated For-5. Two alleles are recognized on the basis of their differential heat sensitivity:For-5b is relatively heat stable and is present in strain C57BL/6J, while For-5d is relatively heat sensitive and is present in strain DBA/2J. The heat sensitivity of formamidase-5 in 44 other inbred strains and substrains was tested and found to resemble that of C57BL/6J or DBA/2J. Thirty-six recombinant inbred strains derived from progenitors that differed at For-5 were studies to test for single-gene inheritance and linkage with other loci. Complete concordance was found with the esterase-10 locus (Es-10), indicating close linkage. The 99% upper confidence limit of the distance between For-5 and Es-10 is 3.7 centimorgans (cM). Es-10 is located on chromosome 14 about 19 cM from the centromere. An independent demonstration of linkage of For-5 with Es-10 and another chromosome 14 marker, hairless (hr), is provided by the finding that the HRS/J strain, which has been sibmated for 60 generations with forced heterozygosity at the hr locus, is cosegregating at For-5 and Es-10. A survey of 32 inbred strains and substrains revealed that the For-5d allele is associated with the Es-10b allele, and that the For-5b allele is associated with Es-10a and Es-10c. Formamidase-5 segregates as expected in the F2 generation of crosses between strains bearing For-5b and For-5d alleles. It is possible that this unique formamidase of the brain is involved in the metabolism of a neurotransmitter substance.

New formamidase having substrate specificity for o-formylaminoacetophenone in pig liver

The supernatant (13 000 x g, 20 min) of pig liver homogenate was filtered with Sephadex G-200 and formamidase (aryl-formylamine amidohydrolase, EC 3.5.1.9)activity in each fraction was measured. When formylkynurenine was used as substrate, two peaks of formamidase activity were observed but, with formylaminoacetophenone as substrate, only one peak was observed. Formamidase in the lower molecular weight fraction is known as kynurenine formamidase (FA I), formamidase found here in the higher molecular weight fraction has not been previously reported. This form, designated FA II has been purified about 160-fold from pig liver. The formamidase obtained has substrate specificity for o-formylaminoacetophenone only and could not hydrolyze formylkynurenine. The optimal pH was 8.5 and the Km for o-formylaminoacetophenone was 1.66-10(-3) M. This formamidase was considered to be a new enzyme and was different from FA I in molecular weight and substrate specificity. This new formamidase was present in pig, rabbit and guinea pig liver and not present in rat or mouse liver.