J. Scott Boone

Role of detoxication pathways in acute toxicity levels of phosphorothionate insecticides in the rat

Phosphorothionate insecticides and their active oxon metabolites can be detoxified by a variety of hepatic mechanisms. Cytochrome P450-mediated dearylation activity was higher in males than in females. While dearylation was induced by phenobarbital in both sexes, it was induced by beta-naphthoflavone in females only. Detoxication of oxons in the presence of EDTA was inducible by phenobarbital, was higher in males than in females, and paralleled aliesterase activity. In vitro Ca(++)-dependent A-esterase-mediated hydrolysis of chlorpyrifos-oxon but not of paraoxon occurred at biologically relevant nM concentrations. This hydrolysis was also inducible by phenobarbital. Glutathione-mediated conjugation did not appear to be relevant to the disposition of the phosphorothionates studied here. Hepatic detoxication via dearylation, aliesterase phosphorylation and A-esterase-mediated hydrolysis (for some organophosphates) all appear to be relevant reactions in the attenuation of acute toxicity.

Biochemical factors contributing to toxicity differences among chlorpyrifos, parathion, and methyl parathion in mosquitofish (Gambusia affinis)

Abstract Biochemical systems in the mosquitofish were assayed in order to determine the factors contributing to the differences in toxicity among chlorpyrifos, parathion, and methyl parathion. In mosquitofish, chlorpyrifos was more toxic than parathion followed by methyl parathion. The brain cholinesterase (ChE) was more sensitive to chlorpyrifos-oxon than paraoxon followed by methyl paraoxon, assessed by the I50: 50nM for chlorpyrifos-oxon, 270nM for paraoxon, and 8400nM for methyl paraoxon. The muscle ChE was also more sensitive to chlorpyrifos-oxon than paraoxon followed by methyl paraoxon and was more sensitive to each compound than brain ChE; the I50s were: 6nM for chlorpyrifos-oxon, 60nM for paraoxon, and 540nM for methyl paraoxon. The hepatic aliesterases (AliE) were also more sensitive to chlorpyrifos-oxon than paraoxon followed by methyl paraoxon; the I50s were: InM for chlorpyrifos-oxon, 40nM for paraoxon, and 900nM for methyl paraoxon. Methyl parathion was activated by P450-mediated desulfuration in liver microsomes to the greatest extent followed by chlorpyrifos and parathion. Chlorpyrifos was detoxified by P450-mediated dearylation in liver microsomes more than parathion followed by methyl parathion. Hepatic A-esterases were capable of detoxifying chlorpyrifos-oxon, but had no significant effect on paraoxon or methyl paraoxon. The ChE sensitivity to oxon inhibition appears to reflect the toxicity levels of the parent insecticides, while metabolic factors are not predictions of toxicity levels.

Biomarkers as Predictors in Health and Ecological Risk Assessment

Biomarkers are measurable biological parameters that change in response to xenobiotic exposure and other environmental or physiological stressors, and can be indices of toxicant exposure or effects. If the biomarkers are sufficiently specific and well characterized, they can have great utility in the risk assessment process by providing an indication of the degree of exposure of humans or animals in natural populations to a specific xenobiotic or class of xenobiotics. Most biomarkers are effective as indices of exposure, but adequate information is rarely available on the appropriate dose-response curves to have well-described biomarkers of effect that can be widely applicable to additional populations. Specific examples of acetylcholinest-erase inhibition following exposure to organophosphorus insecticides are cited from experiments in both mammals (rats) and fish. These experiments have indicated that the degree of inhibition can be readily influenced by endogenous (e.g., age) and exogenous (e.g., chemical exposures) factors, and that the degree of inhibition is not readily correlated with toxicological effects. Caution is urged, therefore, in an attempt to utilize biomarkers in the risk assessment process until more complete documentation is available on the specificity, sensitivity, and time course of changes, and on the impact of multiple exposures or the time of exposures.

Chemistry of Organophosphorus Insecticides

Publisher Summary This chapter introduces and summarizes the chemistry of organophosphorus insecticides. A classification scheme has been presented into which all commercially important OP insecticides fit, based on the central phosphorus atom and the four atoms immediately surrounding it. OP insecticides may be considered to be derivatives of phosphoric acid (H3PO4) or phosphonic acid (H3PO3) in which all H atoms are replaced by organic moieties. Thus, phosphates are compounds in which the P atom is surrounded by four O atoms. In many OPs, one or more of the oxygen atoms are replaced by sulfur and/or nitrogen. For phosphoric acid derivatives, the O, S, and N atoms can be arranged in 20 different configurations. Initially, elemental phosphorus is converted into P2S5 by reaction with sulfur or into PCl3 by direct chlorination. Trialklyl phosphites are particularly useful in the preparation of dialkyl vinyl phosphates from α-chloroaldehydes and ketones. For OPs with three different substituents on the P atom, it is necessary to begin with P(:O) Cl3 (from intermediate 3) or P(:S) Cl3 (from intermediate 4). Dialkyl phosphates with phenolic or heterocyclic leaving groups are easily prepared from the phenol or heterocyclic alcohol and the dialkyl phosphorochloridate. Unfortunately, the only phosphorochloridates readily available commercially are the dimethyl and the diethyl, the former being quite unstable. OP insecticides, when kept cool, dark, and anhydrous, are usually quite stable. Exposure to heat, light (especially ultraviolet), and/or water, however, may lead to chemical alterations. The three primary reactions involving the phosphorus atom and those immediately surrounding it are hydrolysis, oxidation, and rearrangement.

Assessing transferable residues from intermittent exposure to flea control collars containing the organophosphate insecticide chlorpyrifos.

Children can be exposed to pesticides from numerous residential sources such as carpet, house dust, toys and clothing from treated homes, and flea control remedies on pets. In the present studies, 48 pet dogs (24 in each of two studies) of different breeds and weights were treated with over-the-counter flea collars containing chlorpyrifos (CP), an organophosphorus insecticide. Transferable insecticide residues were quantified on cotton gloves used to rub the dogs for 5 min and on cotton tee shirts worn by a child (Study 2 only). First morning urine samples were also obtained from adults and children in both studies for metabolite (3,5,6-trichloro-2-pyridinol) quantification. Blood samples were obtained from treated dogs in Study 1 and plasma cholinesterase (ChE) activity was monitored. Transferable residues on gloves for all compounds were highest near the neck of the dogs and were lowest in areas most distant from the neck. Rubbing samples (over the collar) at two weeks post-collar application contained 447±57 μg CP/glove while samples from the fur of the back contained 8±2 μg CP/glove. In Study 2, cotton tee shirts worn by children at 15 days post-collar application for 4 h showed CP levels of 134±66 ng/g shirt. There were significant differences between adults and children in the levels of urinary metabolites with children generally having higher urinary levels of metabolites than adults (grand mean±SE; 11.6±1.1 and 7.9±0.74 ng/mg creatinine for children and adults, respectively, compared to 9.4±0.8 and 6.9±0.5 ng/mg creatinine before collar placement). Therefore, there was little evidence that the use of this flea collar contributed to enhanced CP exposure of either children or adults.

Assessing intermittent pesticide exposure from flea control collars containing the organophosphorus insecticide tetrachlorvinphos.

Fleas are a persistent problem for pets that require implementation of control measures. Consequently, pesticide use by homeowners for flea control is common and may increase pesticide exposure for adults and children. Fifty-five pet dogs (23 in study 1; 22 in study 2) of different breeds and weights were treated with over-the-counter flea collars containing tetrachlorvinphos (TCVP). During study 1, fur of treated dogs was monitored for transferable TCVP residues using cotton gloves to pet the dogs during 5-min rubbings post-collar application. Plasma cholinesterase (ChE) activity was also measured in treated dogs. Average amounts of TCVP transferred from the fur of the neck (rubbing over the collar) and from the back to gloves at 3 days post-collar application were 23,700±2100 and 260±50 μg/glove, respectively. No inhibition of plasma ChE was observed. During study 2, transferable TCVP residues to cotton gloves were monitored during 5-min rubbings post-collar application. Transferable residues were also monitored on cotton tee shirts worn by children and in the first morning urine samples obtained from adults and children. Average amounts of TCVP transferred to gloves at 5 days post-collar application from the neck (over the collar) and from the back were 22,400±2900 and 80±20 μg/glove, respectively. Tee shirts worn by children on days 7–11 contained 1.8±0.8 μg TCVP/g shirt. No significant differences were observed between adults and children in urinary 2,4,5-trichloromandelic acid (TCMA) levels; however, all TCMA residues (adults and children) were significantly greater than pretreatment concentrations (α=0.05). The lack of ChE inhibition in dogs and the low acute toxicity level of TCVP (rat oral LD50 of 4–5 g/kg) strongly suggest that TCVP is rapidly detoxified and excreted and therefore poses a very low toxicological risk, despite these high residues.

CHAPTER 44 – Chemistry of Organophosphorus Insecticides

This chapter reviews chemistry of organophosphorus (OP) insecticides for toxicologist and others who desire a basic knowledge of this important class of toxicants. OP chemistry apparently began with the esterification of alcohols to phosphoric acid. OP insecticides may be considered to be derivatives of phosphoric acid or phosphonic acid in which all H atoms are replaced by organic moieties. OP insecticides; when kept cool, dark, and anhydrous; are usually quite stable. However, exposure to heat, light (especially ultraviolet), and/or water, may lead to chemical alterations. The three primary reactions involving the phosphorus atom and those immediately surrounding it are hydrolysis, oxidation, and rearrangement. The class of organophosphorus insecticides contains a diverse array of structures, all united by the presence of a pentavalent phosphorus atom with three singly bonded constituents and a coordinate covalent bond to either sulfur or oxygen.

CHAPTER 45 – The Metabolism of Organophosphorus Insecticides

The organophosphorus insecticides are metabolically highly labile. This metabolic lability, along with their general lack of extreme lipophilicity, prevents their bioaccumulation. A variety of oxidation, reduction, hydrolysis, and conjugation reactions are possible within the group of organophosphorus insecticides. The mechanism of their acute toxicity is the inhibition of acetyl cholinesterase. Some of the organophosphorus insecticides are active anticholinesterases, and any metabolism is, therefore, a detoxication. The potency of the organophosphorus insecticides or their active metabolites as inhibitors of target brain acetyl cholinesterase does not correspond to the acute toxicity levels, indicating that metabolism and disposition are of great significance in determining the overall acute toxicity level of these insecticides. Many of the insecticides, however, are not active anticholinesterases in their parent form and require bioactivation in order to be effective anticholinesterases. The P450 mediated desulfuration reaction is responsible for the majority of these bioactivation. Most other routes of metabolism would be detoxication. Many of the insecticides, or the active metabolites of those insecticides requiring bioactivation, are potent anticholinesterases, and others are not.

Effects of Topical Phosmet on Fur Residue and Cholinesterase Activity of Dogs

Fleas, ticks, and mites are a major problem in many areas of the country for pet owners, and one treatment option involves the use of dips that contain pesticides. In the present study, dogs were dipped with a commercial phosmet (ImidanR) flea dip using the recommended guidelines for four consecutive treatments to determine the residues available for transfer to humans from the fur of the dogs. Twenty-four dogs of various breeds and weights were dipped, and each animals fur was sampled with cotton gloves by petting for 5 minutes in a 10″ × 4″ area along the upper back before dipping and at 4 hours, and 1, 3, 7, and 14 days after dipping. Over the 4 dippings the 4-hour samples had a geometric mean of 2653 μg, and the 1-, 3-, 7-, and 14- day samples had geometric means of 877, 316, 84, and 20 μg, respectively. The samples ranged (in μg) from 80 to 16,794 at 4 hours, 44 to 7028 at 1 day, 1 to 4897 at 3 days, 1 to 2691 at 7 days, and 0.3 to 835 at 14 days. The residues removed by the petting did increase with the subsequent dips, but this was probably due to handler experience. The increase is not attributed to accumulation since there was less than 2% of transferable residue on the dog at 14 days post application. There was no significant inhibition of the plasma cholinesterase in the dogs over the study, suggesting that there was either a very low level of dermal absorption of phosmet or there was rapid detoxication (supported by EPA R 825170-01-0).

Per- and polyfluoroalkyl substances in source and treated drinking waters of the United States

Contaminants of emerging concern (CECs), including per- and polyfluoroalkyl substances (PFAS), are of interest to regulators, water treatment utilities, the general public and scientists. This study measured 17 PFAS in source and treated water from 25 drinking water treatment plants (DWTPs) as part of a broader study of CECs in drinking water across the United States. PFAS were quantitatively detected in all 50 samples, with summed concentrations of the 17 PFAS ranging from <1 ng/L to 1102 ng/L. The median total PFAS concentration was 21.4 ng/L in the source water and 19.5 ng/L in the treated drinking water. Comparing the total PFAS concentration in source and treated water at each location, only five locations demonstrated statistically significant differences (i.e. P < 0.05) between the source and treated water. When the perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) concentrations in the treated drinking water are compared to the existing US Environmental Protection Agency’s PFOA and PFOS drinking water heath advisory of 70 ng/L for each chemical or their sum one DWTP exceeded the threshold. Six of the 25 DWTPs were along twolarge rivers. The DWTPswithin each of the river systems had specific PFAS profiles, with the three DWTPs from one river being dominated by PFOA, while three DWTPs on the second river were dominated by perfluorobutyric acid (PFBA).