Alteration of mice L-tryptophan metabolism by the organophosphorous acid triester diazinon

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.

Pesticides Exposure-Induced Changes in Brain Metabolome: Implications in the Pathogenesis of Neurodegenerative Disorders

Pesticides have been used in agriculture, public health programs, and pharmaceuticals for many decades. Though pesticides primarily target pests by affecting their nervous system and causing other lethal effects, these chemical entities also exert toxic effects in inadvertently exposed humans through inhalation or ingestion. Mounting pieces of evidence from cellular, animal, and clinical studies indicate that pesticide-exposed models display metabolite alterations of pathways involved in neurodegenerative diseases. Hence, identifying common key metabolites/metabolic pathways between pesticide-induced metabolic reprogramming and neurodegenerative diseases is necessary to understand the etiology of pesticides in the rise of neurodegenerative disorders. The present review provides an overview of specific metabolic pathways, including tryptophan metabolism, glutathione metabolism, dopamine metabolism, energy metabolism, mitochondrial dysfunction, fatty acids, and lipid metabolism that are specifically altered in response to pesticides. Furthermore, we discuss how these metabolite alterations are linked to the pathogenesis of neurodegenerative diseases and to identify novel biomarkers for targeted therapeutic approaches.

Embryo yolk sac membrane kynurenine formamidase of l-tryptophan to NAD+ pathway as a primary target for organophosphorus insecticides (OPI) in OPI-induced NAD-associated avian teratogenesis

The objective of this study was to provide in ovo evidence for the proposed role of kynurenine formamidase of l-tryptophan to NAD⁺ pathway in embryo yolk sac membranes as a primary target for organophosphorus insecticide (OPI) teratogens in OPI-induced NAD-associated avian teratogenesis. Slices prepared from yolk sac membranes or embryo livers of chicken eggs treated with the OPI dicrotophos and/or methyl parathion were incubated with l-tryptophan. Yolk sac membrane slices metabolized l-tryptophan in the pathway to NAD⁺ before that function was established in livers. OPI interfered in ovo with the second step of l-tryptophan to NAD⁺ biosynthesis by inhibiting kynurenine formamidase. Its inhibition due to the teratogen dicrotophos occurred in yolk sac membranes during the period of embryo highest susceptibility to OPI teratogens in contrast to delayed and lower inhibition caused by the nonteratogen methyl parathion. Both OPI affected liver kynurenine formamidase in a similar manner. The onsets of liver enzyme inhibition, however, were delayed by about two days and occurred at the time of the reduced embryo susceptibility to teratogens. The early disruption of l-tryptophan metabolism and higher inhibition of kynurenine formamidase in yolk sac membranes may be the factors that determine action of OPI as teratogens in chicken embryos.

Alteration of hepatic cells glucose metabolism as a non-cholinergic detoxication mechanism in counteracting diazinon-induced oxidative stress

The aim of this study was to evaluate effects of acute exposure to various doses of diazinon, a widely used synthetic organophosphorus (OP) insecticide on plasma glucose, hepatic cells key enzymes of glycogenolysis and gluconeogenesis, and oxidative stress in rats. Diazinon was administered by gavage at doses of 15, 30 and 60 mg/kg. The liver was perfused and removed under anaesthesia. The activities of glycogen phosphorylase (GP), phosphoenolpyruvate carboxykinase (PEPCK), thiobarbituric acid reactive substances (TBARS) and total antioxidant capacity (TAC) were analysed in liver homogenate. Administration of diazinon (15, 30 and 60 mg/kg) increased plasma glucose concentrations by 101.43% (P=0.001), 103.68% (P=0.000) and 160.65% (P=0.000) of control, respectively. Diazinon (15, 30 and 60 mg/kg) increased hepatic GP activity by 43.5% (P=0.05), 70.3% (P=0.00) and 117.2% (P=0.02) of control, respectively. In addition, diazinon (30 and 60 mg/kg) increased hepatic PEPCK by 77.3% (P=0.000) and 93.5% (P=0.000) of control, respectively. Diazinon (30 and 60 mg/kg) decreased liver TAC by 38% (P=0.046) and 48% (P=0.000) of control, respectively. Also diazinon (30 and 60 mg/kg) increased hepatic cell liver lipid peroxidation by 77% (P=0.05) and 280% (P=0.000) of control. The correlations between plasma glucose and hepatic cells TBARS (r2=0.537, P=0.02), between plasma glucose and ChE activity (r2=0.81, P=0.049) and between plasma glucose and hepatic cells GP activity (r2=0.833, P=0.04) were significant. It is concluded that the liver cells are a site of toxic action of diazinon. Diazinon increases glucose release from liver into blood through activation of glycogenolysis and gluconeogenesis as a detoxication non-cholinergic mechanism to overwhelm diazinon-induced toxic stress. The results are in accordance with the hypothesis that OPs are a predisposing factor of diabetes.

Changes in mouse liver and chicken embryo yolk sac membrane soluble proteins due to an organophosphorous insecticide (OPI) diazinon linked to several noncholinergic OPI effects in mice and chicken embryos

The objective of this study was to identify proteins in mouse livers and chicken embryo yolk sac membranes whose quantities were altered by an organophosphorous insecticide (OPI) treatment and which might be linked, based on their functionality, to the recognized noncholinergic effects of OPI. Mice and fertile chicken eggs were treated with an OPI representative diazinon. The quantitative changes in mouse liver and chicken embryo yolk sac membrane soluble proteins caused by diazinon were determined by two-dimensional electrophoresis. Proteins whose quantity was affected by diazinon were identified by the mass spectrometry. In mouse livers, the altered levels of several enzymes of glucose metabolism were considered with regards to amelioration of hyperglycemia due to diazinon; the reduced levels of 3-hydroxyanthranilate 3,4-dioxygenase to the changes in the l-tryptophan to NAD metabolism caused by pyrimidinyl and crotonamide OPI; the reduced levels of catalase, peroxiredoxin and superoxide dismutase to OPI-increased lipid and/or kynurenine oxidation, the latter effect resulting also in increased urinary excretion of xanthurenic and kynurenic acids; and an increase in glutathione S-methyltransferase to OPI detoxification. In chicken embryo yolk sac membranes, the reduced availability of procollagen-proline dioxygenase may be the factor in micromelia caused by OPI in chicken embryos.

Biochemical identification and crystal structure of kynurenine formamidase from Drosophila melanogaster

KFase (kynurenine formamidase), also known as arylformamidase and formylkynurenine formamidase, efficiently catalyses the hydrolysis of NFK (N-formyl-L-kynurenine) to kynurenine. KFase is the second enzyme in the kynurenine pathway of tryptophan metabolism. A number of intermediates formed in the kynurenine pathway are biologically active and implicated in an assortment of medical conditions, including cancer, schizophrenia and neurodegenerative diseases. Consequently, enzymes involved in the kynurenine pathway have been considered potential regulatory targets. In the present study, we report, for the first time, the biochemical characterization and crystal structures of Drosophila melanogaster KFase conjugated with an inhibitor, PMSF. The protein architecture of KFase reveals that it belongs to the α/β hydrolase fold family. The PMSF-binding information of the solved conjugated crystal structure was used to obtain a KFase and NFK complex using molecular docking. The complex is useful for understanding the catalytic mechanism of KFase. The present study provides a molecular basis for future efforts in maintaining or regulating kynurenine metabolism through the molecular and biochemical regulation of KFase.

Ebselen reduces hyperglycemia temporarily-induced by diazinon: a compound with insulin-mimetic properties

The present study investigated the effect of ebselen (EB) against hyperglycemia induced by the organophosphate (OPI) diazinon (DI) in rats. The insulin-mimetic properties of EB were investigated in vitro with the aim of better understanding the hypoglycemic effect of this compound. The protective effect of EB against pancreatic and hepatic damage caused by DI in rats was also appraised. In the in vivo experiments, rats were pre-treated with a single injection of EB (50mg/kg, intraperitoneal, i.p.). Afterward, animals were treated with a single injection of DI (200 mg/kg, i.p.). The parameters indicative of pancreatic and hepatic damage such as, serum amylase, lipase, aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP) and lactate dehydrogenase (LDH) activities as well as serum glucose levels, hepatic glycogen content and glucose-6-phosphatase (G6Pase) activity were determined. EB pre-treatment was effective in reducing serum amylase, lipase, AST, ALT, ALP, and LDH activities, protecting against pancreatic and hepatic damage. EB reduced hyperglycemia and increased hepatic glycogen content in animals exposed to DI. In the in vitro assays, EB (150 μM) or insulin (IN 10 μM, positive control) was incubated with either skeletal muscle or hepatic tissue with the aim of measuring glucose uptake, glycogen synthesis and glycogen breakdown. EB increased the glucose uptake in skeletal muscle, stimulated hepatic glycogen synthesis and inhibited glycogen breakdown in a similar way to IN. In conclusion, EB, possibly through its insulin-mimetic action, protected against pancreatic and hepatic damage caused by DI in rats.

Pharmacology and toxicology of cholinesterase inhibitors: uses and misuses of a common mechanism of action

Cholinesterase inhibitors have been used in the treatment of human diseases, the control of insect pests, and more notoriously as chemical warfare agents and weapons of terrorism. Most uses of cholinesterase inhibitors are based on a common mechanism of action initiated by inhibition of acetylcholinesterase (AChE). Extensive inhibition of this enzyme leads to accumulation of the neurotransmitter acetylcholine and enhanced stimulation of postsynaptic cholinergic receptors. This action is beneficial in cases where a reduction in cholinergic transmission contributes to clinical symptoms, e.g., low muscle tone in the autoimmune disorder myasthenia gravis due to loss of nicotinic receptors. Under normal conditions, however, extensive inhibition of AChE leads to excess synaptic acetylcholine levels, over-stimulation of cholinergic receptors, alteration of postsynaptic cell function and consequent signs of cholinergic toxicity. This biochemical cascade forms the basis for the use of anticholinesterase insecticides in pest control as well as for nerve agents in chemical warfare. Paradoxically, the short-acting cholinesterase inhibitor pyridostigmine, an important therapeutic agent in the treatment of myasthenia gravis, was used during the Persian Gulf War to prevent the long-term clinical consequences of possible organophosphate nerve agent exposure. As shown in the attacks in Matsumoto and Tokyo, these same nerve agents can be effectively used to inflict urban terror. Cholinesterase inhibitors thus share a common mechanism of pharmacological or toxicological action, ultimately modifying cholinergic signaling through disruption of acetylcholine degradation. While the use of cholinesterase inhibitors relies on their interaction with AChE, a variety of reports indicate that a number of cholinesterase inhibitors have additional sites of action that may have pharmacologic or toxicologic relevance. A variety of esterase and non-esterase enzymes, neurotransmitter receptors and elements of cell signaling pathways are targeted by some anticholinesterases. In some cases, these actions may occur at concentrations/dosages below those affecting cholinergic transmission. Studies of interactive toxicity of binary mixtures of common organophosphorus insecticides indicate that non-cholinesterase targets may be important in cumulative toxicity. Exposure to multiple anticholinesterases having selective effects on other macromolecules could confound the assumption of additivity in cumulative risk assessment. Knowledge of such selective additional targets may aid, however, in the optimization of strategies for poisoning therapy and in the further elucidation of mechanisms of toxicity for this class of compounds.

Hair analysis used to assess chronic exposure to the organophosphate diazinon: a model study with rabbits

The main purpose of the present study was to determine whether hair analysis would be a suitable method to assess chronic exposure of rabbits to the pesticide diazinon. A controlled study was designed, in which white rabbits of the New Zealand variety were systemically exposed to two dosage levels (15 mg/kg per day and 8 mg/kg per day) of the pesticide, through their drinking water, for a period of 4 months. Hair samples from the back of the rabbits were removed before commencing the experiment and at the end of the dosing period. Parallel experiments with spiked hair were carried out in order to design a simple and efficient method of extraction of diazinon from hair. The hair was pulverized in a ball mill homogenizer, incubated in methanol at 37 degrees C overnight, liquid-liquid extracted with ethyl acetate and measured by chromatography techniques (GC-NPD and GC-MS) for confirmation. The concentration of the diazinon in the hair of the exposed animals ranged from 0.11 to 0.26 ng/mg hair. It was concluded that there is a relationship between the administered dose and the detected pesticide concentration in hair. Finally, it seems that hair analysis may be used to investigate chronic exposure to the pesticide.

Kynurenine formamidase: determination of primary structure and modeling-based prediction of tertiary structure and catalytic triad

Kynurenine formamidase (KFase) (EC 3.5.1.9) hydrolyzes N-formyl-L-kynurenine, an obligatory step in the conversion of tryptophan to nicotinic acid. Low KFase activity in chicken embryos, from inhibition by organophosphorus insecticides and their metabolites such as diazoxon, leads to marked developmental abnormalities. While KFase was purportedly isolated previously, the structure and residues important for catalysis and inhibition were not established. KFase was isolated here from mouse liver cytosol by (NH4)2SO4 precipitation and three FPLC steps (resulting in 221-fold increase in specific activity for N-formyl-L-kynurenine hydrolysis) followed by conversion to [3H]diethylphosphoryl-KFase and finally isolation by C4 reverse-phase high-performance liquid chromatography. Determination of tryptic fragment amino acid sequences and cDNA cloning produced a new 305-amino-acid protein sequence. Although an amidase by function, the primary structure of KFase lacks the amidase signature sequence and is more similar to esterases and lipases. Sequence profile analysis indicates KFase is related to the esterase/lipase/thioesterase family containing the conserved active-site serine sequence GXSXG. The alpha/beta-hydrolase fold is suggested for KFase by its primary sequence and predicted secondary conformation. A three-dimensional model based on the structures of homologous carboxylesterase EST2 and brefeldin A esterase implicates Ser162, Asp247 and His279 as the active site triad.

Chronic diazinon exposure: pathologies of spleen, thymus, blood cells, and lymph nodes are modulated by dietary protein or lipid in the mouse

Little is known about the immunotoxicity of the organophosphate pesticide, diazinon. This study aims at detailing the pathologies in the thymus, spleen, blood cells, and lymph nodes (brachial, mesenteric, and hind quarter gluteal nodes) during chronic oral exposure (300 mg diazinonkg-1 food for 45 days), and explore the combined toxicity with excess dietary protein (40%) or lipid (20% corn oil). Animals were allowed to recover on normal food for 2 weeks. All experimental treatments caused organ pathologies, including necrotic degeneration of the trabeculae (spleen and thymus), hyperplasia of the cortex and medulla (thymus and lymph nodes), hyperplasia of white and red pulp (spleen), and sometimes haemorrhage (all tissues). Blood smears often showed crenated/hypochromic red cells and vacuolated white cells with abnormal nuclei. The severity of lesions during exposure was generally in the following order: lipid<protein<diazinon alone<protein plus diazinon<lipid plus diazinon. Post-exposure recovery was limited, especially in the thymus and for lipid treatments. Quantitative image analysis revealed treatment and organ-specific changes in the proportions of fixed lymphocytes, PAS-positive carbohydrates, DNA, and protein staining. Histochemical changes were greatest after exposure. We conclude that the immunotoxicity of diazinon is exacerbated by excess dietary protein or lipid. The limited recovery and post-exposure histochemical changes imply deleterious effects on metabolism with oxidative stress after exposure.

Parathion, a cholinesterase-inhibiting plaguicide induces changes in tertiary villi of placenta of women exposed: a scanning electron microscopy study

The objective of this work was to describe the anatomy of placentas from women who were at risk of exposure to parathion during their pregnancy, when examined with the light and scanning electron microscopes. Twenty term placentas were analyzed; 10 from women living in an agricultural area, who were at risk of exposure to parathion during their pregnancy, and 10 from women living in an urban area, not expressly exposed to pesticides. Each sample was examined with both light and scanning electron microscopes. Cholinesterase activity was significantly reduced in blood from women of the exposed group. In some placentas of women exposed to parathion, recent microinfarctions, microcalcifications and increased deposition of fibrinoid material were seen, along with a larger proportion of atypical characteristics of villi, such as bullous and balloon-like formations with nonhomogeneous surface, and other areas devoid of microvilli. These observations suggest that in chronic exposure to pesticides, the rate of atypical characteristics of placental villi increases, which could be related to changes in the fetus biology. In this study, one newborn from the exposed group showed intrauterine growth retardation and another one, some signs of hypoxia.

Structural requirements for altering the L-tryptophan metabolism in mice by organophosphorous and methylcarbamate insecticides

This study defined structural requirements for organophosphorous and methylcarbamate insecticides for altering the L-kynurenine pathway of L-tryptophan metabolism in mice. Kynurenine formamidase inhibition by organophosphorous acid triesters and methylcarbamates is the proposed primary event resulting in increase in xanthurenic acid urinary excretion and plasma L-kynurenine. Alteration of the L-kynurenine pathway occurred with compounds that inhibited liver kynurenine formamidase by more than 80%. Pyrimidinyl phosphorothioates followed by crotonamide phosphates were the most potent compounds that changed L-tryptophan metabolism, i.e., pirimiphos-ethyl (20 mg/kg) inhibited liver kynurenine formamidase by 99%, and increased xanthurenic acid urinary excretion and plasma L-kynurenine by 576 +/- 195 and 330 +/- 44%, respectively. Replacement of sulphur by oxygen in the phosphorothioate diazinon reduced in vivo liver kynurenine formamidase inhibition. Consequently, xanthurenic acid urinary excretion and plasma L-kynurenine were not elevated. Atropine, cycloheximide, 2-PAM and phenylmethylsulfonyl fluoride did not alleviate diazinon-altered L-tryptophan metabolism. Because of the potential of the majority of organophosphorous acid triesters and methylcarbamates to inhibit kynurenine formamidase, this novel noncholinergic mechanism warrants consideration in assessment of organophosphorous and methylcarbamate toxicity in occupational and accidental exposures.

Assay of tryptophan 2,3-dioxygenase using liver slices and high-performance liquid chromatography

Liver tryptophan 2,3-dioxygenase (TDO) activity was determined by high-performance liquid chromatography. The enzyme activity was expressed as the sum of N-formyl-L-kynurenine (FK) and L-kynurenine (KYN) produced from L-tryptophan (TRY) by liver slices. FK and KYN were detected spectrophotometrically at 254 nm after their separation on a reversed-phase C18 column. KYN formation proceeded according to zero-order kinetics for at least 4 h with 15 mM TRY at 37 degrees C. The apparent Michaelis constant was 1.2 mM TRY with a maximum velocity of 59 pmol min-1 mg-1 wet weight. The method was applied for TDO assay in mice treated with the organophosphorus acid triester diazinon. Kynurenine formamidase inhibition by diazinon resulted in reduced KYN formation, FK accumulation, and moderate TDO increase.