Protective effect induced by atropine, carbamates, and 2-pyridine aldoxime methoiodide Artemia salina larvae exposed to fonofos and phosphamidon.

The acute toxicity of fonofos and phosphamidon on three age classes of Artemia salina was evaluated. An increase in toxicity of these organophosphorous (OP) insecticides was found following longer development of A. salina. The effects of pretreatment with the nonselective muscarinic antagonist atropine, the two reversible acetylcholinesterease inhibitors physostigmine and pyridostigmine, and the cholinesterase-reactivating oxime 2-pyridine aldoxime methoiodide (2-PAM), as individual and combined pretreatments, on OP-induced lethality in 24 h Artemia were also investigated. The lethal action of both OP insecticides was prevented by pretreatment of 24 h Artemia with atropine and 2-PAM, while physostigmine proved ineffective against intoxication with both OP insecticides and pyridostigmine exhibited a low synergic effect. In both cases, the inhibitory effects of combinations of atropine (10(-5)M) plus 2-PAM were greater than those elicited by either drug alone, with the maximum protection afforded being 100%. Combined pretreatment of atropine (10(-5)M) plus physostigmine practically abolished the lethal effects induced by both insecticides. Pretreatment with 2-PAM (10(-6)M) plus physostigmine afforded maximal protection of 100% and 76% on the lethality induced by fonofos and phosphamidon, respectively. The data obtained suggest that the combination of atropine plus 2-PAM or physostigmine and the combined pretreatment of 2-PAM plus physostigmine are effective in the prevention of the lethal effects induced by fonofos and phosphamidon in A. salina larvae.

Inhibition of the human liver microsomal and human cytochrome P450 1A2 and 3A4 metabolism of estradiol by deployment-related and other chemicals.

Cytochromes P450 (P450s) are major catalysts in the metabolism of xenobiotics and endogenous substrates such as estradiol (E2). It has previously been shown that E2 is predominantly metabolized in humans by CYP1A2 and CYP3A4 with 2-hydroxyestradiol (2-OHE2) the major metabolite. This study examines effects of deployment-related and other chemicals on E2 metabolism by human liver microsomes (HLM) and individual P450 isoforms. Kinetic studies using HLM, CYP3A4, and CYP1A2 showed similar affinities (Km) for E2 with respect to 2-OHE2 production. Vmax and CLint values for HLM are 0.32 nmol/min/mg protein and 7.5 microl/min/mg protein; those for CYP3A4 are 6.9 nmol/min/nmol P450 and 291 microl/min/nmol P450; and those for CYP1A2 are 17.4 nmol/min/nmol P450 and 633 microl/min/nmol P450. Phenotyped HLM use showed that individuals with high levels of CYP1A2 and CYP3A4 have the greatest potential to metabolize E2. Preincubation of HLM with a variety of chemicals, including those used in military deployments, resulted in varying levels of inhibition of E2 metabolism. The greatest inhibition was observed with organophosphorus compounds, including chlorpyrifos and fonofos, with up to 80% inhibition for 2-OHE2 production. Carbaryl, a carbamate pesticide, and naphthalene, a jet fuel component, inhibited ca. 40% of E2 metabolism. Preincubation of CYP1A2 with chlorpyrifos, fonofos, carbaryl, or naphthalene resulted in 96, 59, 84, and 87% inhibition of E2 metabolism, respectively. Preincubation of CYP3A4 with chlorpyrifos, fonofos, deltamethrin, or permethrin resulted in 94, 87, 58, and 37% inhibition of E2 metabolism. Chlorpyrifos inhibition of E2 metabolism is shown to be irreversible.

Aquatic hazard assessment of the organophosphate insecticide fonofos.

This study determined the acute and chronic toxicity of the organophosphate insecticide fonofos to standard freshwater aquatic organisms under laboratory conditions. Fonofos was acutely toxic to bluegill (Lepomis macrochirus), Daphnia (D. magna), and midge (Chironomous riparius) at 5.3, 2.7, and 39 micrograms/L, respectively. Three fonofos formulations (technical, 94.8% A.I.; 20G, field granular 20% A.I.; and 4E, field liquid 4#/gal A.I.) exhibited similar acute toxicities to bluegill. Exposure to fonofos delayed reproduction and decreased the intrinsic rate of increase of Daphnia during 21-d chronic exposure at the lowest tested concentration (0.08 micrograms/L). The no observable effect concentration (NOEC) for Daphnia survival was 0.42 micrograms/L; 0% survival occurred at the lowest observable effect concentration (LOEC) of 1.45 micrograms/L. The NOEC for midge emergence was 3.42 micrograms/L; only 34% emergence occurred at the LOEC of 8.24 micrograms/L. Chronic 30-d exposure of juvenile bluegills decreased growth and survival at 5.65 micrograms/L (LOEC), but no effects occurred at 2.33 micrograms/L (NOEC). The relative hazard of fonofos to aquatic life is similar to other carbamate and organophosphate corn insecticides.

Comparative sensitivity of bovine and rodent acetylcholinesterase to in vitro inhibition by organophosphate insecticides.

Biochemical studies were conducted to compare the in vitro sensitivities of bovine and rodent brain and erythrocyte cholinesterases to inhibition by Dyfonate-oxon, paraoxon, and malaoxon. This comparison was done to determine if the reported greater sensitivity of cattle to Dyfonate might be explained by a greater sensitivity of the target enzyme, acetylcholinesterase, in cattle to inhibition by Dyfonate's toxic metabolite, Dyfonate-oxon. Studies were conducted with brain homogenates and lysed erythrocytes obtained from cows and from male and female rats. Additional studies were conducted with a commercially available sample of purified bovine erythrocyte acetylcholinesterase (ACHE). In all cases, the concentrations of organophosphates required to produce 50% inhibition (IC50) of enzyme activity were determined. Cow brain ACHE was 1.7 to 3.8 times more resistant to inhibition by Dyfonate-oxon, paraoxon, and malaoxon than was brain ACHE from male or female rats. For both species, paraoxon was 1.2 to 1.6 times more potent than Dyfonate-oxon and 3.8 to 6.9 times more potent than malaoxon. The bimolecular reaction rate constants (ki) were also determined for inhibition of brain ACHE of cows and male rats by the three organophosphates. In general, the ki data were in agreement with the IC50 data indicating that cow brain ACHE was less sensitive than rat brain ACHE to inhibition. Additional IC50 studies were conducted with lysed erythrocytes from cows and from male and female rats. Both quantitative and qualitative differences between species and among the organophosphates were in excellent agreement with the results of the brain ACHE studies. Also, in related studies with purified bovine erythrocyte ACHE, there was excellent agreement with the results of tests involving ACHE inhibition in erythrocyte lysates. This study demonstrated that, as an inhibitor of ACHE in vitro, Dyfonate-oxon was equal to or slightly lower in potency than paraoxon and more potent than malaoxon. In addition, the study demonstrated that, in general, ACHE from brain or erythrocytes of cows was less sensitive to in vitro inhibition by organophosphates than was that from male or female rats. Thus, the apparent greater susceptibility of cows to Dyfonate, in vivo, cannot be explained on the basis of an unusual target enzyme (ACHE) sensitivity to inhibition by Dyfonate-oxon.