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The organophosphorus pesticides of this review were discovered in 1936 during the search for a replacement for nicotine for cockroach control. The basic biochemical characteristics of RBC AChE and BChE were determined in the 1940s. The mechanism of inhibition of both enzymes and other serine esterases was known in the 1940s and, in general, defined in the 1950s. In 1949, the death of a parathion mixer-loader dictated blood enzyme monitoring to prevent acute illness from organophosphorus pesticide intoxication. However, many of the chemical and biochemical steps for serine enzyme inhibition by OP compounds remain unknown today. The possible mechanisms of this inhibition are presented kinetically beginning with simple (by comparison) Michaelis-Menten substrate enzyme interaction kinetics. As complicated as the inhibition kinetics appear here, PBPK model kinetics will be more complex. The determination of inter- and intraindividual variation in RBC ChE and BChE was recognized early as critical knowledge for a blood esterase monitoring program. Because of the relatively constant production of RBCs, variation in RBC AChE was determined by about 1970. The source of plasma (or serum) BChE was shown to be the liver in the 1960s with the change in BChE phenotype to the donor in liver transplant patients. BChE activity was more variable than RBC AChE, and only in the 1990s have BChE individual variation questions been answered. We have reviewed the chemistry, metabolism, and toxicity of organophosphorus insecticides along with their inhibitory action toward tissue acetyl- and butyrylcholinesterases. On the basis of the review, a monitoring program for individuals mixing-loading and applying OP pesticides for commercial applicators was recommended. Approximately 41 OPs are currently registered for use by USEPA in the United States. Under agricultural working conditions, OPs primarily are absorbed through the skin. Liver P-450 isozymes catalyze the desulfurization of phosphorothioates and phosphorodithioates (e.g., parathion and azinphosmethyl, respectively) to the more toxic oxons (P = O(S to O)). In some cases, P-450 isozymes catalyze the oxidative cleavage of P-O-aryl bonds (e.g., parathion, methyl parathion, fenitrothion, and diazinon) to form inactive water-soluble alkyl phosphates and aryl leaving groups that are readily conjugated with glucuronic or sulfuric acids and excreted. In addition to the P-450 isozymes, mammalian tissues contain ('A' and 'B') esterases capable of reacting with OPs to produce hydrolysis products or phosphorylated enzymes. 'A'-esterases hydrolyze OPs (i.e., oxons), while 'B'-esterases with serine at the active center are inhibited by OPs. OPs possessing carboxylesters, such as malathion and isofenphos, are hydrolyzed by the direct action of 'B'-esterases (i.e., carboxylesterase, CaE). Metabolic pathways shown for isofenphos, parathion, and malathion define the order in which these reactions occur, while Michaelis-Menten kinetics define reaction parameters (Vmax, K(m)) for the enzymes and substrates involved, and rates of inhibition of 'B'-esterases (kis, bimolecular rate constants) by OPs and their oxons. OPs exert their insecticidal action by their ability to inhibit AChE at the cholinergic synapse, resulting in the accumulation of acetylcholine. The extent to which AChE or other 'B'-esterases are inhibited in workers is dependent upon the rate the OP pesticide is activated (i.e., oxon formation), metabolized to nontoxic products by tissue enzymes, its affinity for AChE and other 'B'-esterases, and esterase concentrations in tissues. Rapid recovery of OP BChE inhibition may be related to reactivation of inhibited forms. AChE, BChE, and CaE appear to function in vivo as scavengers, protecting workers against the inhibition of AChE at synapses. Species sensitivity to OPs varies widely and results in part from binding affinities (Ka) and rates of phosphorylation (kp) rather than rates of activation and detoxif

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The chemistry, biochemistry, toxicity, and industrial use of monoethanolamine (MEA), diethanolamine (DEA), and triethanolamine (TEA) are reviewed. The dual function groups, amino and hydroxyl, make them useful in cutting fluids and as intermediates in the production of surfactants, soaps, salts, corrosion control inhibitors, and in pharmaceutical and miscellaneous applications. In 1995, the annual U.S. production capacity for ethanolamines was 447,727 metric tons. The principal route of exposure is through skin, with some exposure occurring by inhalation of vapor and aerosols. MEA, DEA, and TEA in water penetrate rat skin at the rate of 2.9 x 10(-3), 4.36 x 10(-3) and 18 x 10(-3) cm/hr, respectively. MEA, DEA, and TEA are water-soluble ammonia derivatives, with pHs of 9-11 in water and pHa values of 9.3, 8.8, and 7.7, respectively. They are irritating to the skin, eyes, and respiratory tract, with MEA being the worst irritant, followed by DEA and TEA. The acute oral LD50s are 2.74 g/kg for MEA, 1.82 g/kg for DEA, and 2.34 g/kg for TEA (of bw), with most deaths occurring within 4 d of administration. MEA is present in nature as a nitrogenous base in phospholipids. These lipids, composed of glycerol, two fatty acid esters, phosphoric acid, and MEA, are the building blocks of biomembranes in animals. MEA is methylated to form choline, another important nitrogenous base in phospholipids and an essential vitamin. The rat dietary choline requirement is 10 mg kg-1 d-1; 30-d oral administration of MEA (160-2670 mg kg-1 d-1) to rats produced "altered" liver and kidney weights in animals ingesting 640 mg kg-1 d-1 or greater. Death occurred at dosages of 1280 mg kg-1 d-1. No treatment-related effects were noted in dogs administered as much as 22 mg kg-1 d-1 for 2 yr. DEA is not metabolized or readily eliminated from the liver or kidneys. At high tissue concentrations, DEA substitutes for MEA in phospholipids and is methylated to form phospholipids composed of N-methyl and N, N-dimethyl DEA. Dietary intake of DEA by rats for 13 wk at levels greater than 90 mg kg-1 d-1 resulted in degenerative changes in renal tubular epithelial cells and fatty degeneration of the liver. Similar effects were noted in drinking water studies. The findings are believed to be due to alterations in the structure and function of biomembranes brought about by the incorporation of DEA and methylated DEA in headgroups. TEA is not metabolized in the liver or incorporated into phospholipids. TEA, however, is readily eliminated in urine. Repeated oral administration to rats (7 d/wk, 24 wk) at dose levels up to and including 1600 mg kg-1 d-1 produced histopathological changes restricted to kidney and liver. Lesions in the liver consisted of cloudy swelling and occasional fatty changes, while cloudy swelling of the convoluted tubules and loop of Henle were observed in kidneys. Chronic administration (2 yr) of TEA in drinking water (0, 1%, or 2% w/v; 525 and 1100 mg kg-1 d-1 in males and 910 and 1970 mg kg-1 d-1 in females) depressed body and kidney weights in F-344 rats. Histopathological findings consisted of an "acceleration of so-called chronic nephropathy" commonly found in the kidneys of aging F-344 rats. In B6C3F1 mice, chronic administration of TEA in drinking water (0, 1%, or 2%) produced no significant change in terminal body weights between treated and control animals or gross pathological changes. TEA was not considered to be carcinogenic. Systemic effects in rats chronically administered TEA dermally (0, 32, 64, or 125 mg kg-1 d-1 in males; 0, 63, 125, or 250 mg kg-1 d-1 in females) 5 d/wk for 2 yr were primarily limited to hyperplasia of renal tubular epithelium and small microscopic adenomas. In a companion mouse dermal study, the most significant change was associated with nonneoplastic changes in livers of male mice consistent with chronic bacterial hepatitis.

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Industry continues to promote the use of sound science in regulatory decision making. The use of decision support methodologies for hazard identification/risk assessment of toxic substances such as models employing physiologically based pharmacokinetics (PBPK) and structure-activity relationships (SAR) are recommended. A collaborative program involving government and industry is needed to further the use of these decision support methodologies. The formation of federal and state working groups is recommended. Organizations such as the Halogenated Solvents Industrial Alliance (HSIA), American Industrial Health Council (AIHC), and the Chemical Manufacturers Association (CMA) are suitable industry representatives.

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A knowledge of the methods used to obtain partition coefficients, Vmax, and Km values, and the use of allometric relationships is essential to understanding their role in physiologically based pharmacokinetic (PBPK) models. Vial equilibration methods for obtaining the partition coefficients of volatile and nonvolatile compounds were presented using the results from studies with p-chlorobenzotrifluoride (PCBTF) and isofenphos, respectively. Partition coefficients for volatile and nonvolatile compounds from published studies were included. Several published in vivo inhalation (gas uptake) studies and in vitro enzyme studies were presented to demonstrate several methods for obtaining Vmax and Km values. Allometric equations used in PBPK models for body weight scaling of respiration and cardiac rates between species were presented along with equations for within species body weight scaling of Vmax.

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The rate of metabolism of [14C]isofenphos (IFP) to isofenphos oxon (IFP-oxon), des N-isofenphos (d-N-IFP), and des N-isofenphos oxon (d-N-IFP-oxon) by rat, guinea pig, monkey, dog, and human liver microsomal P-450 enzymes was studied to obtain Vmax and Km values for Michaelis-Menten kinetics. The monkey had the highest Vmax value for the conversion of IFP to IFP-oxon (desulfuration), 162 nmol isofenphos hr-1 per 1.3 nanomoles P-450, followed by guinea pig (98), rat (66), dog (43), and human (14). The Km values for the desulfuration of isofenphos were 19.2, 7.4, 14.1, 23.3, and 18.4 microM, respectively, for the monkey, guinea pig, rat, dog, and human. The Vmax values for the dealkylation process (conversion of IFP to d-N-IFP) were 64.6, 17.2, 9.7, and 7.3 nmol isofenphos hr-1 per 1.3 nanomoles P-450 for the monkey, rat, dog, and human, respectively. For the dealkylation process, monkey had the highest Km value, 16.3 microM IFP, followed by human (11.2), rat (9.9), and dog (9.3). The rate of metabolism of IFP-oxon and d-N-IFP to d-N-IFP-oxon were separately studied. The Vmax and Km values obtained in this study for animal and human liver P-450 enzymes will be used to develop a PB-PK/PB-PD model to predict the fate and toxicity of isofenphos in animals and man.

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An outbreak of dermatitis cases among 198 orange pickers employed by a Tulare County, California, packinghouse was investigated. Dermatitis was contracted by 114 (58%) of the 198 workers exposed when Omite-CR-treated fields were harvested. The dermatitis occurred predominantly in the exposed areas of the neck and chest. A dose-response association with dermatitis was suggested for Omite-CR exposure, but not for Carzol, Omite-CR + Carzol, or other pesticides. Because no violations of pesticide preharvest intervals or application rates were found, it appears that residue degradation was not given adequate consideration in the registration of Omite-CR, thus compromising the safety of the worker.

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Chlordimeform (Cdf) is readily absorbed through the skin of pesticide applicators. It also enters the body when breathed into the lungs. Cdf can be found in the urine of workers who handle or are involved in applying pesticide products which contain the chemical, even when they are wearing special protective clothing and respirators. Because of the adverse health effects seen in heavily exposed workers and the cancerous tumors seen in mice, special precautions are necessary when handling products containing Cdf to reduce the risk of adverse health effects. Because Cdf is known to enter the body very easily, with serious injury potential, workers handling pesticide products containing Cdf in California are required to have medical supervision including a blood test at the beginning and end of each application season and monthly testing of urine for possible cellular change and pesticide residues during the time Cdf exposure takes place. This program is designed to keep worker exposure as low as possible.

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Dermal dose-ChE response (toxicity) and percutaneous absorption of parathion and carbaryl were studied in the rat. Parathion was as toxic to young animals as it was to adults of the same sex and more toxic to adult females than to adult males. Carbaryl was nontoxic at administered dosages. In each percutaneous absorption study, [14C]parathion or [14C]carbaryl was applied to back skin (12.5-13.8 cm2) at 43.5 to 48 micrograms/cm2. Thirty-six rats were treated and three were killed at time intervals between 0.5 and 168 hr. The area of the skin which had been treated, plasma, heart, liver, kidneys, urine, feces, and the remaining carcass were analyzed for 14C. Recovery studies indicated that adult male and female rats, respectively, absorbed 59.2 and 57.0% of the applied parathion, while adult males absorbed 57.7% of the applied carbaryl. Parathion was lost from skin (t1/2, 28.6 hr) more rapidly than carbaryl (t1/2, 40.6 hr). Approximately 1.4 and 5.8%, respectively, of the applied parathion and carbaryl penetrated the skin within 1 hr and was available for absorption. Parathion was absorbed through skin of adult male and female rats, respectively, at rates of 0.33 and 0.49 micrograms hr-1 cm-2. Carbaryl was absorbed by male rats at the rate of 0.18 micrograms hr-1 cm-2. The half-lives for absorption of parathion by blood ranged between 0.38 and 2.1 hr, while elimination half-lives ranged between 28.6 and 39.5 hr. Carbaryl absorption and elimination half-lives were 1.26 and 67 hr, respectively.

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The percutaneous absorption of 14C-phenoxy ring labeled triadimefon was studied in adult and young male and female Sprague-Dawley rats. Triadimefon was applied (41.1 to 46.4 micrograms/cm2) in 0.2 ml of acetone to areas comprising 3% of the body surface (7.0 to 14.5 cm2). Thirty-six animals were treated at the initiation of each study. Groups of three animals were subsequently killed at 1, 4, 8, 12, 24, 48, 72, 96, 120, 144, 168, and 192 hr after treatment. Skin from the treated area as well as blood, heart, liver, kidneys, remaining carcass, urine, and feces were analyzed for 14C by scintillation counting techniques. Based on 14C counts, triadimefon was lost more rapidly from the skin of young animals (t 1/2, 20 to 25 hr) than from the skin of adult animals (t 1/2, 29 to 53 hr). Recovery studies indicated that adult males, adult females, young males, and young females, respectively, absorbed 53, 82, 57, and 52% of the dose. The rest of the dose based on material balance was presumably lost by evaporation. Approximately 2.5 to 3.9% of the dose penetrated the skin in one hour and was available for absorption. The rate of entry triadimefon into blood was 2 to 2.5 times faster for young than that observed in adult animals. Elimination of it from blood was faster in the case of the young animals. Triadimefon was absorbed through the skins of the adult male, adult female, young male, and young female rats, respectively, at rates of 0.20, 0.50, 0.58, and 0.48 micrograms/hr/cm2 of skin.

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The effectiveness of currently used pesticide mixing-loading and application equipment in preventing workers from being exposed to cholinesterase (ChE) inhibiting pesticides was measured. Blood samples from workers involved in the application of pesticides in Monterey and Imperial counties of California were analyzed for ChE activity. The analyses indicated that only the mean red cell activity of mixer-loaders was significantly less than controls. Seasonal variations in ChE activity were noted for workers in Imperial County between January and August. These variations were apparently related to the toxicity of the pesticides used. A prototype, closed-transfer system used in Imperial County did not substantially reduce pesticide residues in the air around mixing-loading sites or prevent a reduction in ChE activity over that of open-transfer systems. This was attributed to inadequate training in the use of the new equipment on the part of the mixer-loaders and improperly maintained equipment. A protytype closed-transfer system used in Monterey County appeared to give some protection to mixer-loader applicators during the application season; however, the blood ChE activities of two mixer-loaders using another prototype closed system in Monterey County were severely depressed.

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Blood cholinesterase (CHE) activities and urinary dialkyl phosphate levels of five mixer-loaders and four mixer-loader applicators, using a closed-transfer system in conjunction with mixing-loading and application equipment, were monitored over a period of 18 weeks. Airborne pesticide residues in the breathing zone during mixing-loading and the transfer of concentrated liquid pesticide from their original container to mix and spray tanks were determined along with airborne residues during ground application. Blood ChE activities of the majority of the workers increased slightly during the study with increased use of toxic organophosphates and carbamates. Urinary dialkyl phosphate levels varied between 0.02 and 2.4 ppm. During the study, the blood ChE activities of two mixer-loaders decreased and dialkyl phosphate levels of 2.4 ppm were found in the urine of one worker. An investigation indicated that the workers had failed to use the provided closed-transfer system. Airborne residues from liquid pesticides during closed transfer and mixing-loading averaged 5.8 microgram/m3, while residues from dusty powders averaged 152 microgram/m3. Airborne residues during ground application averaged 3.7 microgram/m3 during the workday. Mevinphos residues on cloth patches averaged 0.2 microgram/cm2.

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