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PubMedDate not supplied

Hepatic biotransformation of parathion: role of cytochrome P450 in NADPH- and NADH-mediated microsomal oxidation in vitro.

In vitro studies have established that cytochrome P450 (P450) is deactivated by the electrophilic sulfur atom released during the enzymic oxidation of parathion to paraoxon. However, in vivo studies in rats have been unable to demonstrate significant P450 loss. This study evaluated the possibility that there may be alternate pathways of parathion biotransformation in liver, other than those mediated by P450 and supported by NADPH. Initial experiments confirmed that parathion administration did not decrease microsomal P450 or testosterone hydroxylation activities. Subsequent in vitro experiments identified an NADH-dependent pathway of parathion biotransformation, and MS was used to confirm that paraoxon and 4-nitrophenol were the products of both the NADH- and NADPH-dependent reactions. The Michaelis constants of the NADH-dependent formation of paraoxon and 4-nitrophenol (26 +/- 6 microM and 53 +/- 10 microM, respectively) were approximately 3-fold greater than those for the NADPH-supported reactions (9 +/- 1 microM and 18 +/- 3 microM, respectively). Induction of male rats with phenobarbital and dexamethasone, but not beta-naphthoflavone, produced similar increases in the rates of NADH- and NADPH-mediated parathion metabolism. Rates of NADH- and NADPH-dependent metabolism were highly correlated in linear relationships. An anti-NADPH-cytochrome P450 reductase (NADPH-P450 reductase) antibody partially inhibited microsomal parathion oxidation mediated by either cofactor, and the P450 inhibitor clotrimazole was similarly effective against the NADH- and NADPH-supported oxidation of parathion. Finally, a reconstituted system containing P450 2B1, NADPH-P450 reductase, and phospholipid supported parathion oxidation mediated by NADH.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1982

Behavior of parathion in apple juice processed into cider and vinegar.

Apple juice, fortified with 25 ppm (ug/g) of parathion, was processed into cider and vinegar. After the initial fermentation period of 12 days, the supernatant cider contained 7.4 ppm of parathion while the level in the sedimented lees was 88 ppm. Sorption to the sedimented matter was the main pathway for parathion residue reduction in the cider. Levels of aminoparathion and 4-nitrophenol, the only metabolites of parathion detected as confirmed by thin-layer chromatography, were 0.19 and 1.2 ppm, respectively, in the cider. The 56-day-old finished cider prior to bottling contained 2.2 ppm parathion, 0.15 ppm aminoparathion and 1.3 ppm 4-nitrophenol. Storage of the cider at 24, 12, 4 and -20 degrees C resulted in further reduction in the parathion levels. After one year, samples stored at 24 degrees C contained only 2.5% of the initial level added to the juice. Samples stored at the three other temperatures contained about 5% of the original level. Vinegar formed after 57 days of fermentation contained 5.1 ppm parathion, while the residue level in the lees was 76 ppm. Aminoparathion and 4-nitrophenol levels were 0.23 and 1.2 ppm, respectively in the vinegar. Storage of the vinegar at 24 degrees C for one year resulted in a gradual decline in the parathion level and at the end of the storage period, the remaining residue represented about 6% of the initial 24 ppm added to the juice.

Fermentation↗
PubMed2001

Inhibition of cholinesterase enzymes following a single dermal dose of chlorpyrifos and methyl parathion, alone and in combination, in pregnant rats.

Pregnant Sprague-Dawley rats (14-18 d of gestation) were treated with either a single dermal subclinical dose of 30 mg/kg (15% of dermal LD50) chlorpyrifos (O,O-diethyl-O-[3,5,6-trichloro-2-pyridinyl] phosphorothioate) or a single dermal subclinical dose of 10 mg/kg (15% of dermal LD50) methyl parathion (O,O-dimethyl O-4-nitrophenyl phosphorothioate) or the two in combination. Chlorpyrifos inhibited maternal and fetal brain acetylcholinesterase (AChE) activity within 24 h of dosing, (48% and 67% of control activity, respectively). Following application of methyl parathion, peak inhibition of maternal and fetal brain AChE activity occurred at 48 h and 24 h after dosing (17% and 48% of control activity, respectively). A combination of chlorpyrifos and methyl parathion produced peak inhibition of maternal and fetal brain AChE activity at 24 h postdosing (35% and 73% of control activity, respectively). Maternal and fetal brain AChE activity recovered to various degrees of percentage of control 96 h after dosing. Application of methyl parathion or chlorpyrifos alone or in combination significantly inhibited maternal plasma butyrylcholinesterase (BuChE) activity. No significant inhibition of fetal plasma BuChE activity was detected. Peak inhibition of maternal liver BuChE occurred 24 h after application of methyl parathion or chlorpyrifos alone or in combination (64%, 80%, and 61% of control activity, respectively). Significant inhibition of placental AChE occurred within 24 h after application of methyl parathion or chlorpyrifos alone or in combination. The results suggest that methyl parathion and chlorpyrifos, alone or in combination, were rapidly distributed in maternal and fetal tissues, resulting in rapid inhibition of cholinesterase enzyme activities. The lower inhibitory effect of the combination could be due to competition between chlorpyrifos and methyl parathion for cytochrome P-450 enzymes, resulting in inhibition of the formation of the potent cholinesterase inhibitor oxon forms. The faster recovery of fetal plasma BuChE is attributed to the de novo synthesis of cholinesterase by fetal tissues compared to maternal tissues.

Acetylcholinesterase↗
PubMed2006

A model for linking the effects of parathion in soil to its degradation and bioavailability kinetics.

Parathion is an insecticide of a group of highly toxic organophosphorus compounds. To investigate the dissipation and toxicological impact of parathion [O,O-diethyl O-(4-nitrophenyl) phosphorothioate] and its highly toxic metabolite, paraoxon, soil laboratory experiments were conducted in columns during a 19-d experiment under variably saturated conditions. Water and pesticide transport, sorption, and biodegradation of parathion were measured in three soil pools (soluble phase, weakly and strongly sorbed phases) using C-labeled pesticide. The effects of parathion and its metabolite on the mobility of soil nematodes were observed and then modeled with an effective variable, which combined pesticide concentration and time of application. Results showed that parathion was highly sorbed and slowly degraded to a mixture of metabolites. The parent compound and its metabolites remained located in the top 0.06-m soil layer. A kinetic model describing the sorption, biodegradation, and allocation into different soil pools of parathion and its metabolites was coupled with heat and water transport equations to predict the fate of parathion in soil. Simulated results were in agreement with experimental data, showing that the products remained in the upper soil layers even in the case of long-term (11-mo) simulation. The strongly sorbed fraction may be regarded as a pesticide reservoir that regularly provides pesticide to the weakly sorbed phase, and then, liquid phase, respectively. From both modeling and observations, no major toxicological damage of parathion and paraoxon to soil nematodes was found, although some effects on nematodes were possible, but at the soil surface only (0.01- and 0.02-m depth).

Animals↗
PubMed1992

Methyl parathion induced regional alterations in the regulatory proteins during critical stage of central nervous system development in albino rat pups.

Sublethal doses of methyl parathion (O,O-dimethyl-O-nitrophenyl- thiophosphate) injected intraperitoneally to 15 and 21 day old rat pups induced regional alterations in the central nervous system (CNS) in the levels of total RNA, total proteins, modulatory protein Calmodulin (CaM), in the activity levels of membrane bound enzyme Ca(2+)-ATPase and phospholipids. Levels of RNA and total proteins increased considerably in 15 days old methyl parathion treated (MPT) rat pups. Contrary to this the RNA and total protein content exhibited remarkable decrease in 21 day old methyl parathion treated animals. Calmodulin level showed an increase in cerebral cortex and brain stem and decrease in cerebellum and spinal cord in 15 day old methyl parathion treated rat pups. Whereas the level of Calmodulin decreased in cerebral cortex and cerebellum and increased in brain stem and spinal cord in 21 day old methyl parathion treated rat pups. Activity levels of calcium dependent ATPase showed significant inhibition in all the regions of Central Nervous System (CNS) of 15 and 21 day old methyl parathion treated rat pups. Phospholipids showed a general increase in all the regions of Central Nervous System on methyl parathion exposure. In the light of these observations, it has been suggested that the molecular regulatory mechanisms involving Ca2+/CaM are rendered inefficient due to toxic impact of methyl parathion.

Animals↗
PubMed1986

Cimetidine enhances and phenobarbital decreases parathion toxicity.

Parathion toxicity has been attributed to its metabolic product paraoxon which is formed in the mammal liver through the multiple oxidase enzymes. These are induced by barbiturates and inhibited by SKF 525 A and cimetidine. We assessed the effects of these drugs on the acute toxicity of parathion in rats by measuring the rate of survival at 24 h after the administration of the oral LD50 of parathion to four groups of rats: control and pretreated with the aforementioned drugs. Additional rats of these groups were used to measure the capability of liver isolated microsomes to transform p-nitroanisole to p-nitrophenol. In the control and cimetidine groups we also measured the transformation of parathion to paraoxon and p-nitrophenol by the liver microsomes. Phenobarbital increased the survival 100% whereas cimetidine and SKF 525 A dramatically potentiated parathion toxicity. Phenobarbital increased the formation of p-nitrophenol but cimetidine and SKF-525 A produced the opposite effect. Paraoxon and p-nitrophenol from parathion were decreased by cimetidine. Our results strongly suggest that parathion itself is largely responsible of its toxicity and the inhibition of its metabolism is harmful rather than beneficial.

Animals↗
PubMed1993

Differential modulation of organophosphate-sensitive muscarinic receptors in rat brain by parathion and chlorpyrifos.

We previously reported similar levels of brain cholinesterase inhibition but marked differences in toxicity following acute maximum tolerated doses of the organophosphate pesticides parathion and chlorpyrifos. Because extensive acetylcholinesterase inhibition often induces compensatory changes in cholinergic receptor populations, we compared the effects of parathion and chlorpyrifos on brain muscarinic receptors. Adult male rats were treated with vehicle or the maximum tolerated dose of parathion (18 mg/kg, sc) or chlorpyrifos (279 mg/kg, sc) and observed for signs of acute toxicity. Similarly treated animals were sacrificed at 2, 7, or 14 days after treatment for measurement of cholinesterase activity and binding to the nonselective muscarinic antagonist [3H]quinuclidinyl benzilate, the M2-preferential antagonist [3H]AFDX-384, and the high-affinity agonist [3H]cis-methyldioxolane. More acute toxicity was noted after parathion treatment. Both insecticides caused similar levels (> 85%) of maximal cholinesterase inhibition and reductions (up to 55%) in atropine-sensitive quinuclidinyl benzilate binding (i.e., total muscarinic receptors) and [3H]AFDX-384 binding in cortex and striatum. Parathion also reduced, whereas chlorpyrifos increased, total muscarinic receptor binding and [3H]AFDX-384 binding in the cerebellum. When tissues were preincubated with paraoxon (10 microM), radiolabeling of a subset of quinuclidinyl benzilate binding sites was blocked and the apparent densities of these organophosphate-sensitive receptors in all three tissues were decreased (16% maximal) by parathion but increased (up to 37%) by chlorpyrifos. Similarly, parathion decreased whereas chlorpyrifos increased [3H]cis-methyldioxolane binding sites in all three brain regions. We propose that differential modulation of these organophosphate-sensitive muscarinic receptors contributes to differences in acute toxicity following exposure to these pesticides.

Animals↗
PubMed2003

Effects of chronic dermal exposure to nonlethal doses of methyl parathion on brain regional acetylcholinesterase and muscarinic cholinergic receptors in female rats.

The in vivo and in vitro effects of methyl parathion, a phosphorothionate insecticide, on cholinergic neurotransmitter systems in the brain of rats were investigated. Three groups of adult female rats received 0, 0.1, or 1.0 mg/kg methyl parathion via dermal exposure for 95 days. Exposure to 0.1 mg/kg methyl parathion produced inhibition of AChE in the caudate-putamen and thalamic nuclei, whereas 1.0 mg/kg resulted in inhibition of AChE in most brain regions. The same doses of methyl parathion had no effect on [(3)H]QNB binding to muscarinic receptors in the brain regions examined. The in vitro study demonstrated that methyl parathion causes preferential inhibition of AChE and [(3)H]QNB binding in specific brain regions. As an inhibitor of AChE, methyl paraoxon was 1,000-fold more potent than was methyl parathion. Similarly, methyl paraoxon showed brain region-specific inhibition of the enzyme. Generally, the brain stem was highly sensitive to organophosphate-induced inhibition of AChE activity and [(3)H]QNB binding. Because central respiratory neurons gather in the brain stem, preferential effects there and in other brain regions may underlie lethal toxicity of methyl parathion and other organophosphates.

Acetylcholinesterase↗
PubMed1977

Airborne and surface residues of parathion and its conversion products in a treated plum orchard environment.

Airborne pesticide residues were collected both within and downwind from a parathion-treated plum orchard by high volume sampling through XAD-4 macroreticular resin. Levels of paraoxon in excess of 100 ng/m3 were found in orchard air, along with parathion, during the early days of two 21-day sampling studies. Paraoxon:parathion ratios in the orchard air were relatively constant, averaging ca. 0.5 for days 1 to 21 following treatment. Likely sources of airborne paraoxon include vaporization and dislodgement from soil and leaf surfaces, and chemical conversion of parathion in the air. Support for the latter came from observation of an increased paraoxon:parathion ration in air samples collected downwind from the orchard. Atmospheric conversion of parathion to paraoxon, accelerated by sunlight, was indicated by both field and laboratory studies. Overall dissipation of parathion from the orchard air, soil, and leaf tissue proceeded to a considerable extent through breakdown to paraozon under the dry climatic conditions of these studies. Eventual conversion to the relatively stable breakdown product, p-nitrophenol, was indicated from analysis of air in the orchard vicinity.

Air↗
PubMed2006

Biodegradation of methyl parathion and p-nitrophenol: evidence for the presence of a p-nitrophenol 2-hydroxylase in a Gram-negative Serratia sp. strain DS001.

A soil bacterium capable of utilizing methyl parathion as sole carbon and energy source was isolated by selective enrichment on minimal medium containing methyl parathion. The strain was identified as belonging to the genus Serratia based on a phylogram constructed using the complete sequence of the 16S rRNA. Serratia sp. strain DS001 utilized methyl parathion, p-nitrophenol, 4-nitrocatechol, and 1,2,4-benzenetriol as sole carbon and energy sources but could not grow using hydroquinone as a source of carbon. p-Nitrophenol and dimethylthiophosphoric acid were found to be the major degradation products of methyl parathion. Growth on p-nitrophenol led to release of stoichiometric amounts of nitrite and to the formation of 4-nitrocatechol and benzenetriol. When these catabolic intermediates of p-nitrophenol were added to resting cells of Serratia sp. strain DS001 oxygen consumption was detected whereas no oxygen consumption was apparent when hydroquinone was added to the resting cells suggesting that it is not part of the p-nitrophenol degradation pathway. Key enzymes involved in degradation of methyl parathion and in conversion of p-nitrophenol to 4-nitrocatechol, namely parathion hydrolase and p-nitrophenol hydroxylase component "A" were detected in the proteomes of the methyl parathion and p-nitrophenol grown cultures, respectively. These studies report for the first time the existence of a p-nitrophenol hydroxylase component "A", typically found in Gram-positive bacteria, in a Gram-negative strain of the genus Serratia.

Bacterial Proteins↗
PubMed1986

The effects of phenobarbital pretreatment on the metabolism and acute toxicity of the pesticide parathion in the mouse.

Single-pass perfusion of mouse livers in situ with the phosphorothioate pesticide parathion resulted in formation of the cholinesterase inhibitor paraoxon (PO), p-nitrophenol (PNP), p-nitrophenyl sulfate (PNPS), and p-nitrophenyl glucuronide (PNPG). Daily pretreatment of mice with phenobarbital (80 mg/kg, ip) for 4 days induced hepatic cytochrome P-450 content, as well as oxidative activation and oxidative detoxification of parathion, as measured in vitro. However, phenobarbital pretreatment did not alter production of PO from parathion in mouse livers perfused in situ, although it increased production of PNP, PNPS, and PNPG. Additionally, phenobarbital pretreatment antagonized the acute toxicity of parathion in mice. These results indicate that phenobarbital pretreatment clearly induces that form(s) of cytochrome P-450 catalyzing conversion of parathion to PO. Yet increased amounts of PO do not exit perfused livers from phenobarbital pretreated mice. Instead, the enhanced detoxification of parathion to PNP, PNPS, and PNPG likely results in the observed antagonism of parathion's acute toxicity.

Animals↗
PubMed1992

Biotransformation of the insecticide parathion by mouse brain.

The acute toxicity of organothiophosphate insecticides like parathion results from their metabolic activation by cytochromes P450. The present study is directed towards the characterization of cytochrome-P450-dependent metabolism of parathion by various mouse brain regions. Intraperitoneal administration of [35S]parathion to mice led to covalently bound [35S]sulfur in various tissues, indicating their capacity to oxidatively desulfurate this insecticide. Liver contained the greatest amount of covalently bound sulfur, and brain the least. Among individual brain regions the olfactory bulb and hypothalamus possessed the highest levels of sulfur binding when expressed on a per milligram tissue basis. However, when expressed on a per brain region basis, sulfur binding was greatest within the cortex as a result of the large mass of this region, compared to the hypothalamus and olfactory bulb. Incubation of the 78,000 x g fraction of mouse brain with parathion resulted in formation of p-nitrophenol, although paraoxon could not be detected. However, given the current understanding of parathion metabolism by cytochromes P450, and given that paraoxon can rapidly disappear through phosphorylation of serine hydroxyl groups, it is reasonable to assume that at least some paraoxon was formed. Production of p-nitrophenol required NADPH and was inhibited by carbon monoxide. In vitro incubations of parathion with the 78,000 x g fraction of mouse brain indicated that the hypothalamus and olfactory bulb had the greatest capacity to produce p-nitrophenol. These results demonstrate that various mouse brain regions possess different capacities to metabolize parathion.

Animals↗
PubMed2006

Investigation on the sonocatalytic degradation of parathion in the presence of nanometer rutile titanium dioxide (TiO2) catalyst.

The nanometer rutile titanium dioxide (TiO2) powder was adopted to act as the sonocatalyst after treatment of high-temperature activation and the ultrasound of low power was used as an irradiation source to induce heat-treated TiO2 powder performing sonocatalytic degradation of parathion. Although there are many factors influencing sonocatalytic degradation of parathion, the experimental results demonstrate that the optimal degradation condition of parathion can be obtained when the experimental conditions such as initial concentration of 50 mg/L parathion, addition amount of 1000 mg/L nanometer rutile TiO2, ultrasonic of 30-50 kHz frequency and 50 W output power, acidity of pH 10.0 and temperature of 20 degrees C are adopted. The degradation ratio of parathion surpassed 90% within 120 min ultrasonic irradiation in these optimal experiment conditions. The total degradation process of parathion has been monitored by UV-vis spectra and ion chromatography. At last, the parathions in aqueous solution are completely degraded and become some simple inorganic ions such as NO3(-), PO4(3-), SO4(2-), etc. In addition, the sonocatalytic activities of reused TiO2 catalysts were also studied and found to decline gradually along with the reused times. In this paper, the research on sonocatalytic degradation kinetics was also been performed and found to follow pseudo first-order reaction. All experiments indicated that the sonocatalytic method in the presence of nanometer rutile TiO2 powder was an advisable choice for the treatments of non- or low-transparent organic wastewaters in future.

Catalysis↗
PubMed2005

Interactive toxicity of chlorpyrifos and parathion in neonatal rats: role of esterases in exposure sequence-dependent toxicity.

We previously reported that sequence of exposure to chlorpyrifos and parathion in adult rats can markedly influence toxic outcome. In the present study, we evaluated the interactive toxicity of chlorpyrifos (8 mg/kg, po) and parathion (0.5 mg/kg, po) in neonatal (7 days old) rats. Rats were exposed to the insecticides either concurrently or sequentially (separated by 4 h) and sacrificed at 4, 8, and 24 h after the first exposure for biochemical measurements (cholinesterase activity in brain, plasma, and diaphragm and carboxylesterase activity in plasma and liver). The concurrently-exposed group showed more cumulative lethality (15/24) than either of the sequential dosing groups. With sequential dosing, rats treated initially with chlorpyrifos prior to parathion (C/P) exhibited higher lethality (7/23) compared to those treated with parathion before chlorpyrifos (P/C; 1/24). At 8 h after initial dosing, brain cholinesterase inhibition was significantly greater in the C/P group (59%) compared to the P/C group (28%). Diaphragm and plasma cholinesterase activity also followed a relatively similar pattern of inhibition. Carboxylesterase inhibition in plasma and liver was relatively similar among the treatment groups across time-points. Similar sequence-dependent differences in brain cholinesterase inhibition were also noted with lower binary exposures to chlorpyrifos (2 mg/kg) and parathion (0.35 mg/kg). In vitro and ex vivo studies compared relative oxon detoxification of carboxylesterases (calcium-insensitive) and A-esterases (calcium-sensitive) in liver homogenates from untreated and insecticide pretreated rats. Using tissues from untreated rats, carboxylesterases detoxified both chlorpyrifos oxon and paraoxon, while A-esterases only detoxified chlorpyrifos oxon. With parathion pretreatment, A-esterases still detoxified chlorpyrifos oxon while liver from chlorpyrifos pretreated rats had little apparent effect on paraoxon. We conclude that while neonatal rats are less capable than adults at detoxifying many organophosphorus insecticides including chlorpyrifos and parathion, toxicant-selective differences in detoxification play a role in sequence-dependent toxicity in both neonatal and adult rats with these two insecticides.

Animals↗
PubMed2004

Methyl parathion interaction with human and bovine serum albumin.

Methyl parathion (MP; O,O-dimethyl O-p-nitrophenyl phosphorothioate) is an organophosphorous compound still largely used in agriculture and fish hatcheries. This pesticide is not quite selective and is potentially toxic for both vertebrates and invertebrates. Its mechanism of acute toxicity is the inhibition of the enzyme acetylcholinesterase in nervous tissue. Binding of pesticides to plasma proteins is one of many factors that influence their distribution and elimination. The free concentration available for toxic action can be effectively reduced for pesticides with high binding to plasma proteins, although the affinity of pesticides to plasma proteins is often lower than for the enzyme targets. Several different transport proteins exist in blood plasma, but albumin only is able to bind a wide diversity of xenobiotics reversibly with high affinity. It was already known that parathion (ethyl parathion) exhibits a high affinity to human and bovine serum albumins. We studied interactions of methyl parathion with these albumins by using fluorescence quenching techniques. We selectively excited the fluorescence of tryptophan residues with a 290 nm wavelength light, and observed quenching by titrating human and bovine serum albumin solutions with methyl parathion. Stern-Volmer graphs were plotted and quenching constants were estimated. Our results pointed to the formation of complexes of methyl parathion with albumins. Association constants at 25 degrees C were 3.07 x 10(4) (1.2 x 10(3))M(-1) for human serum albumin, and 1.96 x 10(4) (+/- 4.5 x 10(2))M(-1) for bovine serum albumin. At 37 degrees C, they were 1.08 x 10(4) (+/- 2.0 x 10(2))M(-1) for human serum albumin, and 8.16 x 10(3) (+/- 1.9 x 10(2))M(-1) for bovine serum albumin. Results also suggest that the primary binding site for methyl parathion on albumin is close to tryptophan residues 214 of human serum albumin and 212 of bovine serum albumin.

Animals↗
PubMed2006

Sonocatalytic degradation of methyl parathion in the presence of nanometer and ordinary anatase titanium dioxide catalysts and comparison of their sonocatalytic abilities.

The degradation of methyl parathion (O,O-dimethyl-O-(4-nitrophenyl)-phosphorothioate) using anatase titanium dioxide (TiO(2)) powder as heterogeneous sonocatalysts is reported. The influences of reaction parameters such as the species of TiO(2) sonocatalysts, methyl parathion concentrations, TiO(2) adding amount, pH, ultrasonic intensity, ultrasonic frequency and temperature have been investigated and the optimal conditions for eliminating methyl parathion have been identified. The efficiencies of sonocatalytic degradation in both nanometer and ordinary anatase systems are compared and the results indicate that the sonocatalytic activity of nanometer anatase TiO(2) powder is better than that of ordinary anatase TiO(2) powder. The primary degradation and the total mineralization of methyl parathion have been monitored by high performance liquid chromatography (HPLC) and UV-vis spectra, respectively. Methyl parathion got destroyed to some extent in both nanometer and ordinary anatase systems under ultrasonic irradiation. The kinetics for the degradation process of methyl parathion follows the first-order reaction. The degradation ratio of methyl parathion surpassed 90% within 50min in the optimal experiment conditions.

Catalysis↗
PubMed2000

Possible involvement of beta-adrenergic receptors in the enhancement of nocturnal pineal N-acetyltransferase activity due to parathion administration.

The purpose of the present study was to examine the effects of administration of sublethal doses of O,O-diethyl-O-p-nitrophenyl phosphorothioate (parathion) on serum epinephrine (EPI) and norepinephrine (NE), as well as on night-time rat pineal melatonin synthesis, both in the presence and absence of propranolol, a beta-adrenergic receptor antagonist. In the first experiment, two groups of adult albino rats were administered parathion orally (1.08 and 2.17 mg/kg/day; the total received by each animal was 6.5 and 13.0 mg/kg body weight over 6 days); another two groups received corn oil only. Animals were killed at 23:00 and 01:00 h by decapitation. Serum EPI was augmented at 01:00 h, but NE was increased at 01:00 and 23:00 h due to administration of the high dose of parathion (13 mg/kg). In the second experiment, two groups of adult male albino rats were administered parathion orally (13 mg/kg); another two groups received an intraperitoneal injection of propranolol (20 mg/kg body weight, 1 h before the lights were turned off). In addition, two groups were given a saline injection. Four hours after darkness onset, pineal N-acetyltransferase (NAT) activity as well as pineal and serum melatonin levels were measured. Parathion by itself significantly augmented nocturnal pineal NAT activity and serum melatonin levels in otherwise untreated rats; the insecticide was ineffective in reference to this enzyme when it was given in conjunction with the beta-adrenergic receptor antagonist propranolol. The augmentation of NAT activity by parathion also caused significant reduction in pineal serotonin (5-HT); again, this response was blocked by propranolol treatment. The results are consistent with the idea that parathion influences pineal 5-HT metabolism either at the level of the beta-adrenergic receptor or via the sympathetic innervation to the pineal gland.

Animals↗
PubMed1999

The role of metabolism in determining susceptibility to parathion toxicity in man.

Human liver microsomes (n = 16) activated parathion (O, O, diethyl O-p-nitrophenyl phosphorothioate, 20 and 200 microM) to paraoxon at a rate of 23.3-199.3 and 18.7-310.3 pmol/min per mg protein, respectively. p-Nitrophenol, was also formed, at 321.1-769.2 and 406.2-778.3 pmol/min per mg protein. This represented a 16-fold and 2-fold range in capacity to activate and detoxify parathion, respectively. Parathion was activated with an apparent Km of 9-16 microM (n = 3). The activation of parathion (200 microM) was positively correlated with nifedipine oxidation, indicating the involvement of CYP3A. Correlations were not significant with ethoxyresorufin-O-dealkylation (CYP1A1/2), pentoxyresorufin-O-dealkylation (CYP2B6), p-nitrophenol hydroxylation (CYP2E1), paraoxon hydrolysis (A-esterase) or phenylvalerate hydrolysis (B-esterase). Paraoxon formation from parathion was markedly reduced by CYP3A inhibitors. Experiments with EDTA indicated that A-esterase was not functionally important at low levels of paraoxon. Human P450s 3A4 and 3A5 expressed microsomes were the most efficient at biotransforming parathion to paraoxon, although P450s 1A1, 2B6 and 2C8 also catalysed the reaction. This study has determined wide interindividual variations in capacity to metabolise parathion, mainly by CYP3A, which may influence its manifest toxicity.

Cytochrome P-450 Enzyme System↗