Esterase inhibition and reactivation in relation to piperonyl butoxide-phosphorothionate interactions

The effect of piperonyl butoxide on the acute toxicity of phosphorothionate insecticides was studied in male mice. One hour after piperonyl butoxide (400 mg/kg), the toxicity of the dimethyl phosphorothionates, methyl parathion and Guthion, was antagonized, whereas the toxicity of their respective diethyl homologs, parathion and Ethyl Guthion, was potentiated. Piperonyl butoxide did not appreciably alter the toxicity of the oxygen analogs of these compounds. Pretreatment with SKF 525-A (50 mg/kg) modified the toxicity of the phosphorothionates in a manner qualitatively similar to piperonyl butoxide pretreatment. Plasma concentrations of all four insecticides were increased three- to sevenfold in piperonyl butoxide-pretreated mice. This increase may result in a greater total oxon formation; however, reactivation in vitro of esterases inhibited in vivo was 5 to 10 times more rapid following methyl parathion or Guthion challenge than after their diethyl homologs. Although a greater total oxon formation-cholinesterase inhibition is possible for both dimethyl and diethyl phosphorothionates following piperonyl butoxide pretreatment, rapid reactivation of inhibited nerve tissue cholinesterases after dimethyl phosphorothionate challenge appears to compensate for further inhibition occurring at a decreased rate. The net result would be a reduction in dimethyl phosphorothionate toxicity. In contrast, slow reactivation of inhibited nerve tissue cholinesterases following diethyl phosphorothionate challenge appears unable to compensate for increased oxon formation-cholinesterase inhibition. The net result is a potentiation of the toxicity of the diethyl-substituted compounds.     

Effect of piperonyl butoxide on the metabolism of dimethyl and diethyl phosphorothionate insecticides

The effect of piperonyl butoxide on the acute toxicity of phosphorothionate insecticides was studied in male mice. One hour after piperonyl butoxide (400 mg/kg), the toxicity of the dimethyl phosphorothionates, methyl parathion and Guthion, was antagonized, whereas the toxicity of their respective diethyl homologs, parathion and Ethyl Guthion, was potentiated. Piperonyl butoxide did not appreciably alter the toxicity of the oxygen analogs of these compounds. Pretreatment with SKF 525-A (50 mg/kg) modified the toxicity of the phosphorothionates in a manner qualitatively similar to piperonyl butoxide pretreatment. Plasma concentrations of all four insecticides were increased three- to sevenfold in piperonyl butoxide-pretreated mice. This increase may result in a greater total oxon formation; however, reactivation in vitro of esterases inhibited in vivo was 5 to 10 times more rapid following methyl parathion or Guthion challenge than after their diethyl homologs. Although a greater total oxon formation-cholinesterase inhibition is possible for both dimethyl and diethyl phosphorothionates following piperonyl butoxide pretreatment, rapid reactivation of inhibited nerve tissue cholinesterases after dimethyl phosphorothionate challenge appears to compensate for further inhibition occurring at a decreased rate. The net result would be a reduction in dimethyl phosphorothionate toxicity. In contrast, slow reactivation of inhibited nerve tissue cholinesterases following diethyl phosphorothionate challenge appears unable to compensate for increased oxon formation-cholinesterase inhibition. The net result is a potentiation of the toxicity of the diethyl-substituted compounds.     

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.     

Activation of parathion and guthion by mammalian, avian, and piscine liver homogenates and cell fractions

The production and accumulation of potent anticholinesterase metabolites when parathion [o,o-diethyl o-(4-nitrophenyl) phosphorothionate] and Guthion [o,o,-dimethyl s-(4 oxo-1,2,3-benzotriazin-3(4H)-ylmethyl) phosphorodithioate] were incubated with liver homogenates or cell fractions from 3 mammalian, 2 avian, and 2 fish species was studied. Comparison of activation of the phosphorothionates by whole liver homogenates without added cofactors and in the presence of NAD, NADP, or NADP plus glucose 6-phosphate showed that for all species maximum activation was obtained in the presence of NAPD plus glucose 6-phosphate, although in a few cases NAD alone was equally effective. For all species half or more of the phosphorothionate activation activity of whole liver homogenates was recovered in the 8500 g supernatant fractions, but because of species differences in the relative activity of different fractions, it appears that whole homogenates are most useful for comparative studies. Activity was present in the washed microsomal fraction of all species, except flounder liver, when reduced NADP was added. There appeared to be some species differences in the stability of the phosphorothionate activating system when homogenates were stored at different temperatures. Tests on whole liver homogenates from several animals of each species yielded the following general order of activation: mammals > birds > fish.     

Metabolic activation of the organophosphorus insecticides chlorpyrifos and fenitrothion by perfused rat liver.

The present study was undertaken to characterize the metabolic activation of the organophosphorus insecticides chlorpyrifos [O,O-diethyl O-(3,5,6-trichloro-2-pyridyl) phosphorothionate] and fenitrothion [O,O-dimethyl O-(3-methyl-p-nitrophenyl) phosphorothionate] by intact rat liver. Single-pass perfusions of rat livers with chlorpyrifos or fenitrothion to steady state conditions resulted in the appearance of their corresponding oxygen analogs in effluent. In addition, detoxification of chlorpyrifos oxon [O,O-diethyl O-(3,5,6-trichloro-2-pyridyl) phosphate] or fenitrooxon [O,O-dimethyl O-(3-methyl-p-nitrophenyl) phosphate] by rat blood did not proceed at a rate rapid enough to prevent passage of at least some of these chemicals from liver to extrahepatic tissues, suggesting that hepatic biotransformation of chlorpyrifos and fenitrothion by rat liver results in their net activation. Although male rat livers produced more chlorpyrifos oxon and fenitrooxon from chlorpyrifos and fenitrothion, respectively, than did livers from female rats, the acute toxicities of chlorpyrifos and fenitrothion were greater in females than in males. Therefore, differences in hepatic activation of chlorpyrifos and fenitrothion in males and females cannot account for the sex differences in their acute toxicities in the rat. Finally, S-methyl glutathione and S-p-nitrophenyl glutathione were not detected in effluent or bile of livers perfused with fenitrothion, suggesting that glutathione-mediated biotransformation of this insecticide does not occur to any significant degree in intact liver.     

Kinetic Analyses of the Microsomal Biotransformation of the Phosphorothioate Insecticides Chlorpyrifos and Parathion

Kinetic Analyses of the Microsomal Biotransformation of thePhosphorothioate Insecticides Chlorphyrifos and Parathion. Sultatos,L.G. and Murphy, S.D. (1983). Fundam. and Appl. Toxicol. 3:16-21.Chlorpyrifos [0,0-diethyl-0-(3,5,6-trichloro-2-pyridyl) phosphorothioate]was metabolized to chlorpyrifos oxon [0,0-diethyl-0-(3,5,6-trichloro-2-pyridyl)phosphate] and to 3,5,6-trichloro-2-pyridinol by mouse hepaticmicrosomes. Formation of both chlorpyrifos oxon and 3,5,6-trichloro-2-pyridinolrequired NADPH, and was inhibited by carbon monoxide. Kineticanalyses using direct linear plots determined the appKm's forformation of chlorpyrifos oxon and 3,5,6-trichloro-2-pyridinolto be 20.9 ± 3.3 µM and 16.1 ± 3.4 µMrespectively, while the appVmax's for the same reactions were3.9 ± 0.2 nmols/100 mg liver/min and 8.1 ± 0.3nmols/100 mg liver/min respectively. Incubation of parathion[0,0-diethyl-0-(4-nitrophenyl) phosphorothioate] with mousehepatic microsomes produced paraoxon [0,0-diethyl-0-(4-nitrophenyl)phosphate] and p-nitrophenol. The appKm's for the formationof paraoxon and p-nitrophenol were 29.6 ± 4.2 µMand 26.5 ± 3.8 µM respectively, with appVmax'sof 5.8 ± 0.6 nmols/100 mg liver/min and 6.7 ±0.5 nmols/100 mg liver/min, respectively. Incubation of bothparathion and chlorpyrifos at various concentrations with mousehepatic microsomes resulted in inhibition of production of paraoxon,p-nitrophenol, chlorpyrifos oxon, and 3,5,6-trichloro-2-pyridinol,which was characteristic of mixed type inhibition. This complexkinetic behavior could arise as a result of competitive interactionsof parathion and chlorpyrifos with multiple forms of microsomalcytochrome P-450.     

Effect of carbon disulfide on the anticholinesterase action of several organophosphorus insecticides in mice

Effect of carbon disulfide (CS2) on toxic action of 11 organophosphorus (OP) insecticides were examined by determining the plasma cholinesterase activity in mice. CS2 pretreatment potentiated the anticholinesterase action of parathion and EPN, but suppressed that of dimethoate and diazinon. CS2 had no significant effect or a slightly suppressive effect on the other compounds. Some of these effects were contrasted with the reported alteration of the toxicity following phenobarbital pretreatment. CS2 administration suppressed both detoxification and activation of parathion and EPN by liver microsomes in vitro, as measured by p-nitrophenol production and cholinesterase inhibition, respectively. Causal relationship between the in vitro and in vivo observations, however, remains to be clarified.     

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.     

Microsomal biotransformation of chlorpyrifos, parathion and fenthion in rainbow trout (Oncorhynchus mykiss) and coho salmon (Oncorhynchus kisutch): mechanistic insights into interspecific differences in toxicity

Rainbow trout often serve as a surrogate species evaluating xenobiotic toxicity in cold-water species including other salmonids of the same genus, which are listed as threatened or endangered. Biotransformation tends to show species-specific patterns that influence susceptibility to xenobiotic toxicity, particularly organophosphate insecticides (OPs). To evaluate the contribution of biotransformation in the mechanism of toxicity of three organophosphate (phosphorothionate) insecticides, (chlorpyrifos, parathion and fenthion), microsomal bioactivation and detoxification pathways were measured in gills, liver and olfactory tissues in juvenile rainbow trout (Oncorhynchus mykiss) and compared to juvenile coho salmon (Oncorhynchus kisutch). Consistent with species differences in acute toxicity, significantly higher chlorpyrifos bioactivation was found in liver microsomes of rainbow trout (up to 2-fold) when compared with coho salmon. Although bioactivation to the oxon was observed, the catalytic efficiency towards chlorpyrifos dearylation (detoxification) was significantly higher in liver for both species (1.82 and 0.79 for trout and salmon, respectively) when compared to desulfuration (bioactivation). Bioactivation of parathion to paraoxon was significantly higher (up to 2.2-fold) than detoxification to p-nitrophenol in all tissues of both species with rates of conversion in rainbow trout, again significantly higher than coho salmon. Production of fenoxon and fenthion sulfoxides from fenthion was detected only in liver and gills of both species with activities in rainbow trout significantly higher than coho salmon. NADPH-dependent cleavage of fenthion was observed in all tissues, and was the only activity detected in olfactory tissues. These results indicate rainbow trout are more sensitive than coho salmon to the acute toxicity of OP pesticides because trout have higher catalytic rates of oxon formation. Thus, rainbow trout may serve as a conservative surrogate species for the evaluation of OP pesticides in coho salmon.     

Degradation by rat tissues in vitro of organophosphorus esters which inhibit cholinesterase

Hydrolytic "A"-esterase activities of various tissues of rat (plasma, liver, kidney, brain and intestinal mucosa) against selected OP esters of diverse structure as potential substrates (paraoxon, di-n-propyl paraoxon, di-n-butyl paraoxon, chlorpyrifos oxon, di-(4-phenyl butyl) phosphorofluoridate and the chiral isomers of ethyl 4-nitrophenyl phenylphosphonate) were studied. We have developed a sensitive and widely applicable assay depending on measuring decline in residual inhibitory power of any chosen OP against horse serum cholinesterase: for seven compounds examined so far I50s against BuChE ranged from 0.07 to 70 nM, and it is easy to monitor loss of OP starting from an initial 25 microM concentration. Progressive destruction rates were always highest in liver and plasma with activity sometimes detectable in kidney, brain but not in intestinal mucosa, but the ratios of activity between tissues differed for different substrates. At 25 microM/37 degrees/pH 7.2 hydrolysis rates ranged from 8500 nmol/min/g liver for di-(4-phenylbutyl) phosphorofluoridate down to 0.8 nmol/min for the butyl analogue of paraoxon; the rate for L(-) isomer of EPN oxon (23 nmol/min/g liver) was greater than 2x that for the D(+) isomer and for paraoxon. From our data we conclude that several OP hydrolases exist whose identity may be further characterised by use of selective substrates.     

The role of liver microsomal enzymes in the metabolism of parathion

The involvement of microsomal enzymes in the metabolism of the organophosphorus insecticide diethyl p-nitrophenyl phosphorothionate (parathion) by rat liver was studied. The effect of parathion on the activity of NADPH- and NADH-cyto-chrome c reductases in hepatic microsomes isolated from untreated and phenobarbital pretreated rats was determined.The results demonstrate that both NADH- and NADPH-linked (microsomal) electron transport components are involved in the oxidative metabolism of parathion. Phenobarbital pretreatment increased the rate of parathion metabolism.The activity of microsomal NADH-cytochrome c reductase was significantly higher than that of NADPH-cytochrome c reductase. Parathion had an inhibitory effect on both enzymes when applied directly to the microsomal suspension.     

The joint neurotoxic action of inhaled methyl butyl ketone vapor and dermally applied O-ethyl O-4-nitrophenyl phenylphosphonothioate in hens: potentiating effect

The neurotoxic action of inhaled technical grade methyl butyl ketone and dermally applied (O-ethyl O-4-nitrophenyl phenylphosphonothioate (EPN) was studied. Three groups of five hens each were treated 5 days/week for 90 days with a dermal dose of 1.0 mg/kg of EPN (85%) on the unprotected back of the neck. These groups were exposed simultaneously to 10, 50, or 100 ppm of technical methyl butyl ketone (MBK; methyl n-butyl ketone:methyl isobutyl ketone, 7:3) in inhalation chambers. A fourth group was treated only with the dose of EPN and a fifth group with only 100 ppm MBK. The control consisted of a group of five hens treated with a dose of 0.1 ml acetone. Treatment was followed by a 30-day observation period. Simultaneous exposure to EPN and MBK greatly enhanced the neurotoxicity produced when compared to the neurotoxicity produced by either chemical when applied alone. Continued exposure to EPN and MBK resulted in earlier onset and more severe signs of neurotoxicity than exposure to either individual compound. The severity and characteristics of histopathologic lesions in hens given the same daily dermal dose of EPN in combination with inhaled MBK depended on the MBK concentration. Histopathologic changes were more severe and prevalent in the 100 ppm MBK:1 mg/kg EPN group than in the others. In this group, Wallerian-type degeneration was seen along with paranodal axonal swellings. The morphology and distribution of these lesions were characteristic of those induced by MBK. In the 50 ppm MBK:1 mg/kg EPN group axonal swelling was evident but not clearly identifiable as paranodal. Hens treated with 10 ppm MBK:1 mg/kg EPN had minimal lesions with low incidence of axonal swellings. These were not as large as those seen in MBK neurotoxicity, but instead resembled the histopathologic lesions caused by EPN. The results indicate that the combined treatment gave a value for neurotoxicity coefficient which was two times the additive neurotoxic effect of each treatment alone. Pretreatment with three daily ip doses of 5 mmol/kg technical grade MBK or methyl n-butyl ketone (MnBK), equally increased chicken hepatic microsomal cytochrome P-450 content. Also, hepatic microsomes from MBK-treated hens metabolized [14C]EPN in vitro to [14C]EPN oxon to a much greater extent than those from control hens. These results suggest that MBK potentiates the neurotoxic effect of EPN, at least in part, by increasing the metabolic activation of EPN to the more neurotoxic metabolite EPN oxon.(ABSTRACT TRUNCATED AT 400 WORDS)     

Methyl parathion activation by a partially purified rat brain fraction

Organophosphorus pesticides are one of the most commonly used insecticide classes. They act through a potent inhibition of acetylcholinesterase (AChE). Many of them must undergo transformation into the corresponding oxon analogs to inhibit AChE. This study showed that a brain tissue subfraction transformed methyl parathion (O,O-dimethyl O-p-nitrophenyl phosphorothioate) in vitro. Methyl parathion activation was assayed by solvent extraction of the products followed by HPLC and GC-MS analyses and, indirectly, by the inhibition of AChE present in the incubation mixture. The lack of impairment of AChE after 2 h of incubation of the brain subfraction with methyl parathion and, alternatively, with NADPH, CO, SKF 525-A, piperonyl butoxide or nitrogen indicated that this brain subfraction transformed methyl parathion without the involvement of a mixed-function oxidative pathway.The results from HPLC analysis did not show a peak corresponding to methyl paraoxon (O,O-dimethyl O-p-nitrophenylphosphate), but showed the production of an unidentified peak which eluted nearby standard methyl parathion (retention times of 10.65 and 8.86 min, respectively). GC-MS analysis suggested that the unidentified product could be a methyl parathion isomer.     

In vitro sensitivity of cholinesterases and [3H]oxotremorine-M binding in heart and brain of adult and aging rats to organophosphorus anticholinesterases

Organophosphorus (OP) insecticides elicit toxicity via acetylcholinesterase inhibition, allowing acetylcholine accumulation and excessive stimulation of cholinergic receptors. Some OP insecticides bind to additional macromolecules including butyrylcholinesterase and cholinergic receptors. While neurotoxicity from OP anticholinesterases has been extensively studied, effects on cardiac function have received less attention. We compared the in vitro sensitivity of acetylcholinesterase, butyrylcholinesterase and [³H]oxotremorine-M binding to muscarinic receptors in the cortex and heart of adult (3 months) and aging (18 months) rats to chlorpyrifos, methyl parathion and their active metabolites chlorpyrifos oxon and methyl paraoxon. Using selective inhibitors, the great majority of cholinesterase in brain was defined as acetylcholinesterase, while butyrylcholinesterase was the major cholinesterase in heart, regardless of age. In the heart, butyrylcholinesterase was markedly more sensitive than acetylcholinesterase to inhibition by chlorpyrifos oxon, and butyrylcholinesterase in tissues from aging rats was more sensitive than enzyme from adults, possibly due to differences in A-esterase mediated detoxification. Relatively similar differences were noted in brain. In contrast, acetylcholinesterase was more sensitive than butyrylcholinesterase to methyl paraoxon in both heart and brain, but no age-related differences were noted. Both oxons displaced [³H]oxotremorine-M binding in heart and brain of both age groups in a concentration-dependent manner. Chlorpyrifos had no effect but methyl parathion was a potent displacer of binding in heart and brain of both age groups. Such OP and age-related differences in interactions with cholinergic macromolecules may be important because of potential for environmental exposures to insecticides as well as the use of anticholinesterases in age-related neurological disorders.     

The interaction of the phosphorothioate insecticides chlorpyrifos and parathion and their oxygen analogues with bovine serum albumin

The distribution and subsequent toxicity of hazardous chemicals can be influenced by their interactions with plasma proteins. In the present study reversible binding of the phosphorothioate insecticides chlorpyrifos and parathion to fatty acid-free bovine serum albumin (BSA) was examined using the technique of equilibrium dialysis. Computer analyses of the binding data revealed that chlorpyrifos and parathion each bound reversibly to a single class of binding sites on BSA, with apparent KD values of 3.4 +/- 0.1 and 11.1 +/- 0.3 microM, respectively. Additionally, the maximal number of binding sites for each insecticide per molecule of BSA was one. Displacement studies using both chlorpyrifos and parathion indicated that each was a competitive inhibitor of the other's binding, suggesting that they were bound to the same site. Incubation of chlorpyrifos oxon or paraoxon with a 1% solution of BSA resulted in limited, EDTA-insensitive formation of 3,5,6-trichloro-2-pyridinol or p-nitrophenol, respectively. Pretreatment of BSA with 5 mM paraoxon, chlorpyrifos oxon, or 1 mM diisopropylfluorophosphate did not alter this activity, suggesting that these reactions resulted from an esterase-like capacity of BSA, and not from phosphorylation of BSA by these oxons.     

The absorption, distribution, excretion, and metabolism of a single oral dose of O-ethyl O-4-nitrophenyl phenylphosphonothioate in hens

The disposition and metabolism of a single oral 10 mg/kg (LD50) of uniformly phenyl-labeled [14C]EPN (O-ethyl O-4-nitrophenyl [14C]phenylphosphonothioate) were studied in adult hens. The birds were protected from acute toxicity with atropine sulfate. Three treated hens were killed at each time interval (days): 0.5, 2, 4, 8, 12. Radioactivity was adsorbed from the gastrointestinal tract and distributed in all tissues. Most of the dose was excreted in the combined urinary-fecal excreta (74%). Only traces of the radioactivity (0.2%) were detected in expired CO2. Most of the excreted radioactive materials were identified as phenylphosphonic acid (PPA), O-ethyl phenylphosphonic acid (EPPA), and O-ethyl phenylphosphonothioc acid (EPPTA). Radioactivity in tissues reached a peak of 11.8% in 12 days. The highest concentration of radioactivity was present in the liver followed by bile, kidney, adipose tissue, and muscle. EPN was the major compound identified in brain, spinal cord, sciatic nerve, kidney, and plasma. Most of the radioactivity in the liver was identified as EPPA followed by EPPTA and PPA. Kinetic studies showed that EPN disappeared exponentially from tissues. The half-life of the elimination of EPN from plasma was 16.5 days corresponding to a constant rate value of 0.04 day-1. Relative residence (RR) of EPN relative to plasma was shortest in liver and longest in adipose tissue followed by sciatic nerve and spinal cord.     

Each lipase has a unique sensitivity profile for organophosphorus inhibitors.

Lipases sensitive to organophosphorus (OP) inhibitors play critical roles in cell regulation, nutrition, and disease, but little is known on the toxicological aspects in mammals. To help fill this gap, six lipases or lipase-like proteins are assayed for OP sensitivity in vitro under standard conditions (25 degrees C, 15 min incubation). Postheparin serum lipase, lipoprotein lipase (LPL) (two sources), pancreatic lipase, monoacylglycerol (MAG) lipase, cholesterol esterase, and KIAA1363 are considered with 32 OP pesticides and related compounds. Postheparin lipolytic activity in rat serum is inhibited by 14 OPs, including chlorpyrifos oxon (IC50 50-97 nM). LPL (bovine milk and Pseudomonas) generally is less inhibited by the insecticides or activated oxons, but the milk enzyme is very sensitive to six fluorophosphonates and benzodioxaphosphorin oxides (IC50 7-20 nM). Porcine pancreatic lipase is very sensitive to dioctyl 4-nitrophenyl phosphate (IC50 8 nM), MAG lipase of mouse brain to O-4-nitrophenyl methyldodecylphosphinate (IC50 0.6 nM), and cholesterol esterase (bovine pancreas) to all of the classes of OPs tested (IC50 < 10 nM for 17 compounds). KIAA1363 is sensitive to numerous OPs, including two O-4-nitrophenyl compounds (IC50 3-4 nM). In an overview, inhibition of 28 serine hydrolases (including lipases) by eight OPs (chlorpyrifos oxon, diazoxon, paraoxon, dichlorvos, and four nonpesticides) showed that brain acetylcholinesterase is usually less sensitive than butyrylcholinesterase, liver esterase, cholesterol esterase, and KIAA1363. In general, each lipase (like each serine hydrolase) has a different spectrum of OP sensitivity, and individual OPs have unique ranking of potency for inhibition of serine hydrolases.     

Isocarboxazid metabolism in vitro by liver microsomal carboxylesterase of monkey

An amidase that hydrolyzes isocarboxazid, a monoamine oxidase inhibitor, was studied in the monkey. High activity was observed in the liver, moderate activity in the pancreas, and low activity in the kidney, intestine and lung. No activity was observed in the brain, spleen, heart, uterus, or serum. This enzyme was thought to be located mainly in the microsomal fraction of liver, since the subcellular distribution pattern was similar to that of glucose-6-phosphatase, a marker enzyme of the microsomal fraction. This amidase was stable at low temperature (?20°) for at least 1 month, and no cofactors were required. The enzyme was not inhibited by either the substrate or its metabolites. However, significant inhibition of the enzyme was produced by organophosphorous compounds such as parathion and O-ethylp-nitrophenyl phenylphosphorothioate (EPN). Both α-naphthylacetate and p-nitrophenyl acetate, substrates of carboxylesterase, competitively inhibited the activity of the isocarboxazid amidase. These results suggest that isocarboxazid amidase is also a carboxylesterase. A comparison between monkey and rat isocarboxazid amidases is also discussed in this paper.     

Biotransformation of the organophosphorus insecticides parathion and methyl parathion in male and female rat livers perfused in situ.

Although numerous previous reports have characterized the mammalian biotransformation of the organophosphorus insecticides parathion and methyl parathion, questions still remain regarding the toxicological significance of certain metabolic pathways in vivo. The present study utilized rat liver perfusions in order to better characterize the hepatic biotransformation of parathion and methyl parathion in intact liver. Single-pass liver perfusions with parathion and methyl parathion over a range of perfusate concentrations of 10-80 microM resulted in the appearance of paraoxon and methyl paraoxon, respectively, in effluent. Furthermore, rat blood did not have the capacity to prevent transport of paraoxon and methyl paraoxon to extrahepatic tissues, suggesting that oxon produced hepatically can distribute to extrahepatic tissues. In addition, striking sex differences were noted in the metabolite profile of parathion and methyl parathion in perfused livers. However, these differences could not account for the observation that females are more susceptible to parathion, but less susceptible to methyl parathion, compared to males. And finally, S-methyl glutathione or S-p-nitrophenyl glutathione could not be detected in effluent or bile of livers from either sex perfused with methyl parathion, suggesting that glutathione-dependent detoxification of this insecticide does not occur to any significant degree in intact rat liver.     

Delayed neurotoxicity of O-ethyl O-4-nitrophenyl phenylphosphonothioate: Subchronic (90 days) oral administration in hens

Delayed neurotoxicity in hens was produced following daily oral administration of 0.1, 0.5, 1.0, 2.5, 5.0, and 10.0 mg/kg of technical (85%) O-ethyl O-4-nitrophenyl phenylphosphonothioate (EPN) in gelatin capsules for 90 days. Daily, three groups of hens were given empty gelatin capsules, 10 mg/kg of tri-o-cresyl phosphate (TOCP), or 1 mg/kg of parathion (O,O-diethyl O-4-nitrophenyl phenylphosphorothioate) and served as gelatin capsule controls, positive controls, and negative controls, respectively. TOCP-Treated hens developed delayed neurotoxicity, and those given parathion showed leg weakness with subsequent recovery when the administration of this agent had stopped. The clinical condition of most ataxic hens deteriorated during the 30-day observation period following the end of the oral administration of EPN. Severity of the clinical condition depended on the size of the daily ingested dose, i.e., while hens given small doses showed only ataxia, those treated with large doses progressed to paralysis and died. Days of administration and “total administered dose” before onset of ataxia depended on the daily dose. Degeneration of myelin and axons in the spinal cord was the most consistent histologic change and was identical to that found in TOCP control hens. Only one hen showed sciatic nerve degeneration. Livers from two hens given the highest dose of EPN manifested a moderate degree of hemosiderosis. Plasma cholin esterase was significantly inhibited in all surviving hens given EPN or TOCP at the end of the observation period. A group of hens treated daily with 0.01 mg/kg of EPN showed no abnormality in gait or behavior, and its plasma cholinesterase activity was not significantly different from that of the control. Hens treated with parathion had plasma cholinesterase activity comparable to that of the control 30 days after the administration of the last dose.