Kiyoshi Goda

Quinolinic Acid and Active Oxygens

Quinolinic acid (QA, pyridine-2,3-dicarboxylic acid) is an intermediate of the kynurenine pathway in the tryptophan metabolism, which has been detected in the central nervous system (Gal and Sherman, 1978; Speciale and Schwarcz, 1993), and behaves as an excitotoxin (Lapin, 1978; Schwarcz et al., 1983). Recently, it has been proposed that QA and other metabolites of tryptophan may be involved in the brain pathology accompanying neuroinflammatory conditions, and also implicated in the pathophysiology of brain ischemia (Jhamandas and Boegman, 1994). The neuroexcitatory and neurotoxic actions have been suggested to be mediated by NMDA receptor (recently, its subtypes) (Schwarcz et al., 1984; Nakanishi, 1992),. However, the proximate cause of cell death at metabolic level has remained elusive. In the brain, there are numerous sources of oxygen-derived free radicals and they may exert a large variety of effects upon importnat central nervous system functions. Several studies have pointed to the role of metal ions, especially iron, in forming oxygen-derived free radicals (Barber, 1966; Vladimirov et al., 1980), which introduce several reactions such as lipid peroxidation, DNA chain breakage and others in the brain. The purpose of this experiment was to see the interaction of QA with iron ion (complex formation), the electron transfer to oxygen molecules from the complex (superoxide formation) and superoxide-mediated reactions such as lipid peroxidation and DNA chain breakage in vitro. Also, tryptophan metabolites which are known to be antagonist for QA were examined in the QA-iron system.

Radical Scavenging Properties of Tryptophan Metabolites

Radical scavenging properties of tryptophan metabolites were estimated using their radical reactivity. Metabolites of the kynurenine and the melatonin biosynthesis pathway were mainly examined by use of a kinetical model. Their radical reactivity was determined as the reaction rate constant with a stable free radical, such as galvinoxyl; that is a phenoxy radical. The rate constants of the metabolites have a widely ranged spectrum, which can be divided into three groups. The first group (3-hydroxykynurenine, 3-hydroxyanthranilic acid, and indole-3-pyruvic acid) is more reactive than α-tocopherol; the reactivity of the second group (xanthurenic acid, serotonin, N-acetylserotonin) is similar to that of butylated hydroxytoluene (BHT); the third group (kynurenic acid, melatonin, and other ones) is less reactive than BHT.

NAD+-specific d-arabinose dehydrogenase and its contribution to erythroascorbic acid production in Saccharomyces cerevisiae

Erythroascorbic acid (eAsA) is a five‐carbon analog of ascorbic acid, and it is synthesized from d‐arabinose by d‐arabinose dehydrogenase (ARA) and d‐arabinono‐γ‐lactone oxidase. We found an NAD+‐specific ARA activity which is operative under submillimolar level of d‐arabinose in the extracts of Saccharomyces cerevisiae. The hypothetical protein encoded by YMR041c showed a significant homology to a l‐galactose dehydrogenase which plays in plant ascorbic acid biosynthesis, and we named it as Ara2p. Recombinant Ara2p showed NAD+‐specific ARA activity with K m = 0.78 mM to d‐arabinose, which is 200‐fold lower than that for the conventional NADP+‐specific ARA, Ara1p. Gene disruptant of ARA2 lost entire NAD+‐specific ARA activity and the conspicuous increase in intracellular eAsA by exogenous d‐arabinose feeding, while the double knockout mutant of ARA1 and ARA2 still retained measurable amount of eAsA. It demonstrates that Ara2p, not Ara1p, mainly contributes to the production of eAsA from d‐arabinose in S. cerevisiae.

NADP+-Dependent D -Arabinose Dehydrogenase Shows a Limited Contribution to Erythroascorbic Acid Biosynthesis and Oxidative Stress Resistance in Saccharomyces cerevisiae

The molecular aspects and physiological significance of NADP+-dependent D-arabinose dehydrogenase (ARA), which is thought to function in the biosynthesis of an analog of ascorbic acid, D-erythroascorbic acid in yeasts, were examined. A large subunit of ARA, Ara1p produced in E. coli, was purified as a homodimer, some of which was degraded at the N-terminus. It showed sufficient ARA activity. Degradation of Ara1p occurs naturally in yeast cells, and the small subunit of ARA previously thought as is, in fact, a naturally occuring degradation product of Ara1p. A deficient mutant of ARA1 lost almost all NADP+-ARA activity, but intracellular D-erythroascorbic acid was only halved. This mutant showed increased susceptibility to H2O2 and diamide but not to menadione or tert-butylhydroperoxide. Feeding D-arabinose to mutant cells led to increases in intracellular D-erythroascorbic acid, suggesting the presence of another ARA isozyme. The deficient mutant of ARA1 recovered resistance to H2O2 with feeding of D-arabinose. Our results suggest that the direct contributions of Ara1p both to D-erythroascorbic acid biosynthesis and to oxidative stress resistance are quite limited.

Kinetic studies of the reduction of methemoglobin by 5-hydroxyanthranilic acid, tryptophan metabolite.

Kinetic studies of the reduction of methemoglobin by 5-hydroxyanthranilic acid (5-HAT), a tryptophan metabolite, have been performed from the point of view of its electron-transfer ability. The reaction was found to follow a secondorder rate law: k, 2.8×10 M−1 min−1 [25°, μ 0.1 M, pH 7.8 (phosphate)]. The finding of reduction by 5-HAT, the first one by a metabolite of an amino acid, and the result of kinetic studies suggest that 5-HAT may play a physiological role as the reductant for methemoglobin in hereditary or drug-induced methemoglobinemia.

Photochemical Properties of Kynurenine Pathway Metabolites and Indoleamines

Photochemical damages to the biological system may occur through photodynamic action in the presence of photosensitive molecules. Photodynamic action contains the following processes; 1) photosensitisation and/or 2) electron transfer, in which singlet oxygen and superoxide radical production for each in the presence of oxygen molecules. We have studied those processes after the absorption of light by kynurenine pathway metabolites and indoleamine derivatives. We found that kynurenine and 3-hydroxykynurenine generate superoxide radical after electron transfer from their excited state molecules to oxygen molecules, and superoxide makes reduction reaction. On the other hand, it was found that kynurenic acid, melatonin, 5-methoxytryptamine and 5-methoxytryptophol work as photosensitisers with the detection of singlet oxygen production by using the N, N-dimethyl-4-nitrosoaniline bleaching method, while xanthurenic acid, serotonin and N-acetylserotonin generate no detectable amount of singlet oxygen. We have determined the photochemical quantum yields of singlet oxygen production for those photosensitisers, in which quantum yields are not so high except kynurenic acid (f3 = 0.101). In view of the multiple roles played by their metabolites in various systems, these results are relevant to taking into consideration of their photoeffect in the presence of light.

On Kynureninase Activity

1) In Mg-deficient rats, kynureninase activity is decreased. 2) p-Hydroxyphenylpyruvate inhibits kynureninase activity. 3) -SH groups in the apoenzyme of kynureninase play a very important role in the enzymatic reaction. 4) 3-Hydroxykynurenine may be a very important regulative metabolite in the 3-hydroxykynurenine----xanthurenic acid pathway.