3-Hydroxyanthranilic Acid as an Intermediate in the Oxidation of the Indole Nucleus of Tryptophan

Schizophrenia as an inflammation-mediated dysbalance of glutamatergic neurotransmission

This overview tries to bridge the gap between psychoneuroimmunological findings and recent results from pharmacological, neurochemical and genetic studies in schizophrenia. Schizophrenia is a disorder of dopaminergic neurotransmission, but modulation of the dopaminergic system by glutamatergic neurotransmission seems to play a key role. This view is supported by genetic findings of the neuregulin- and dysbindin genes, which have functional impact on the glutamatergic system. Glutamatergic hypofunction, however, is mediated by the N-methyl-D-aspartate (NMDA)-receptor antagonism. The only endogenous NMDA receptor antagonist identified up to now is kynurenic acid (KYNA). Despite the NMDA receptor antagonism, KYNA also blocks, in lower doses, the nicotinergic acetycholine receptor, i.e., increased KYNA levels can explain psychotic symptoms and cognitive deterioration. KYNA levels are described to be higher in the cerebrospinal fluid (CSF) and in critical central nervous system (CNS) regions of schizophrenics as compared to controls. Another line of evidence suggests that a (prenatal) infection is involved in the pathogenesis of schizophrenia. Due to an early sensitization process of the immune system or to a (chronic) infection, which is not cleared through the immune response, an immune imbalance between the type-1 and the type-2 immune responses takes place in schizophrenia. The type-1 response is partially inhibited, while the type-2 response is over-activated. This immune constellation is associated with inhibition of the enzyme indoleamine dioxygenase (IDO), because IDO - located in astrocytes and microglial cells - is inhibited by type-2 cytokines. IDO catalyzes the first step in tryptophan metabolism, the degradation from tryptophan to kynurenine, as does tryptophan 2,3-dioxygenase (TDO). Due to the inhibition of IDO, tryptophan-kynurenine is predominantly metabolized by TDO, which is located in astrocytes, not in microglial or other CNS cells. In schizophrenia, astrocytes in particular are activated, as increased levels of S100B appear. Additionally, they do not have the enzymatic equipment for the normal metabolism-route of tryptophan. Due to the lack of kynurenine hydroxylase (KYN-OHase) in astrocytes, KYNA accumulates in the CNS, while the metabolic pathway in microglial cells is blocked. Accordingly, an increase of TDO activity has been observed in critical CNS regions of schizophrenics. These mechanisms result in an accumulation of KYNA in critical CNS regions. Thus, the immune-mediated glutamatergic-dopaminergic dysregulation may lead to the clinical symptoms of schizophrenia. Therapeutic consequences, e.g., the use of anti-inflammatory cyclo-oxygenase-2 inhibitors, which can also decrease KYNA directly, are discussed.

Tryptophan Metabolism Along the Kynurenine Pathway in Rats

Enzyme activities along the kynurenine pathway, liver tryptophan 2,3-dioxygenase, small intestine indole 2,3-dioxygenase, liver and kidney kynurenine 3-monooxygenase, kynureninase, kynurenine-oxoglutarate transaminase, 3-hydroxyanthranilate 3,4-dioxygenase, and aminocarboxymuconate-semialdehyde decarboxylase, involved in the catabolism of tryptophan, were studied in male adult Wistar albino rats. Intestine superoxide dismutase and serum tryptophan were also determined. Hepatic tryptophan 2,3-dioxygenase is present both as holoenzyme and apoenzyme, but the total activity is inferior to that of intestine indole 2,3-dioxygenase which, therefore, actively oxidizes tryptophan in rats. However, this activity is inhibited by scavengers for the superoxide anion, such as superoxide dismutase, which also shows to be active in small intestine of rat. However, the more active enzymes appeared to be kynurenine 3-monooxygenase and 3-hydroxyanthranilate 3,4-dioxygenase. The former is equally active in both liver and kidney, the latter is more active in liver. Kynurenine-oxoglutarate transaminase is much more active in kidney than in liver, and much more active than kynureninase, which shows similar activities in both tissues. In contrast to the high activity of 3-hydroxyanthranilate 3,4-dioxygenase, aminocarboxymuconate-semialdehyde decarboxylase is 30-35 times less active, showing the efficiency of conversion of tryptophan to NAD. These data demonstrate that rat is a useful animal model for studying tryptophan metabolism along the kynurenine pathway. Serum tryptophan appeared to be 90% bound to proteins. Results demonstrate that, in rat, tryptophan is mainly metabolised along the kynurenine pathway. Therefore, rat is a suitable animal model for studying tryptophan metabolism in the pathological field.

Selective metabolism of kynurenine in the spleen in the absence of indoleamine 2,3-dioxygenase induction

The kynurenine pathway of L-tryptophan degradation is differentially regulated dependent on the level of immune system activation. During inflammation and disease, activity of the hepatocellular enzyme tryptophan 2,3-dioxygenase (TDO) decreases and a second enzyme, indoleamine 2,3-dioxygenase (IDO), is induced in extrahepatic sites. Substantial formation of a metabolise downstream of this step, quinolinic acid (Quin), subsequently occurs only in select regions of the lymphoid tissues, such as spleen, in a temporally restricted manner. The goal of this study was to determine the localization of Quin in unstimulated mice under conditions where rate-limiting control of the pathway by both TDO and IDO was by-passed. Supplementation of drinking water with L-kynurenine, a pathway intermediate that lies between tryptophan and Quin, resulted in a dose-dependent increase in Quin immunoreactivity in the follicles and discontinuous regions of the marginal zones of the spleen. Strongly immunoreactive cells in the periarteriole lymphoid sheaths adopted a highly reactive morphology despite the lack of immunostimulation and IDO induction. In contrast, a patchy to diffuse pallor of staining was observed in the liver parenchyma with 1 and 10 mM L-kynurenine ingestion, respectively. These data show that selective tryptophan metabolism can occur in discrete subcompartments of the lymphoid tissues beyond the level of IDO. In vivo manipulation of Quin synthesis in the absence of IDO induction may serve as a model for studying regulation and function of the kynurenine pathway activation in the immune system.

Nicotinamide deamidation by microorganisms in rat stomach

We have extended our investigation of nicotinamide deamidation in the stomach of conventional rats. The bacterial species in the pars preventricularis were identified as Flavobacterium peregrinum, Escherichia coli, Streptococcus faecalis, and Lactobacillus acidophilus, listed in order of decreasing deamidase activity. Nicotinamide-7-(14)C ingested into rat stomach was rapidly deamidated to nicotinic acid. These results contribute to the accumulated evidence that microorganisms present in the pars preventricularis of rat stomach are responsible for the deamidation of nicotinamide to nicotinic acid, a known precursor of mammalian pyridine nucleotides.

TRYPTOPHAN-NIACIN RELATIONSHIP IN XANTHOMONAS PRUNI

Wilson , R. G. (Oklahoma State University, Stillwater) and L. M. Henderson . Tryptophan-niacin relationship in Xanthomonas pruni . J. Bacteriol. 85: 221–229. 1963.—The observation that Xanthomonas pruni , a bacterial pathogen for the peach, requires niacin for growth and can use tryptophan or 3-hydroxyanthranilic acid as a substitute was confirmed. To determine whether niacin is synthesized via the tryptophan-3-hydroxyanthranilic acid pathway, experiments using labeled metabolites were undertaken. Labeled tryptophan, 3-hydroxyanthranilic acid, quinolinic acid, and nicotinic acid were supplied in the basal medium in amounts sufficient to insure maximal growth. Nicotinic and quinolinic acids were isolated from the cells after the growth period. The isotope was incorporated from the first three labeled compounds into niacin with dilutions approximately the same in all cases, ranging from 7.6 to 17.1. The dilution of isotopic niacin was 3.1- to 5.9-fold. Only labeled quinolinic acid gave rise to labeled quinolinic acid in the cell, but this acid gave rise to niacin with 10- to 12-fold reduction in specific activity. The results indicate that if quinolinate participates as an obligatory intermediate in the synthesis of niacin from tryptophan, its concentration within the cell is very small and it does not equilibrate readily with exogenous quinolinate. The results confirm the conclusion, drawn from growth studies, that niacin is needed to permit tryptophan synthesis at a sufficient rate to promote growth. In the absence of an external source of niacin, tryptophan or some of its metabolites can promote growth by acting as precursors of niacin.