Mode of action of triadimefon in Ustilago avenae

Triadimefon [1-(4-chlorophenoxy)-3,3-dimethyl-(1,2,4-triazol-1-yl)-2-butanone], 1.5–2.0 μ/ml, inhibited the multiplication of sporidia of Ustilago avenae more strongly than it did the increase of dry weight. The treated sporidia appeared swollen, multicellular, and branched. At concentrations of 1.5–100 μg of triadimefon/ml, the oxidation of glucose was not affected. Increase in dry weight and synthesis of protein, RNA, and DNA were inhibited slightly, whereas cell division was acutely arrested. After an incubation period of 9.5 hr, microscopic studies revealed that daughter cells of the treated sporidia also contained one nucleus. In sporidia treated for 6 hr with triadimefon, both the total lipid content and its composition of fatty acids were not appreciably altered. The treated cells, however, differed from control cells by a higher content of free fatty acids. Triadimefon markedly interfered in sterol biosynthesis in Ustilago avenae. Gas chromatographic (glc) analysis and [¹⁴C]acetate incorporation studies indicated that ergosterol biosynthesis was almost completely inhibited by triadimefon; on the other hand, sterol compounds representing precursors of ergosterol (probably 4,4-dimethyl and C-4-methyl sterols) accumulated in treated sporidia. As the results indicate, the inhibition of conversion of immediate sterol precursors to ergosterol may be regarded as the primary target for the action of triadimefon in Ustilago avenae.     

Physiological responses of corn (Zea mays) to AC 243,997 in combination with valine,leucine, and isoleucine

Various physiological processes were measured in corn after treatment with AC 243,997. Neutral sugar levels in leaves increased 39% over the control 24 hr after application of AC 243,997. Protein synthesis, measured by [¹⁴C]leucine and [¹⁴C]cystine incorporation, and lipid synthesis were not inhibited 24 hr after application of 150 μM of AC 243,997, while respiration and RNA synthesis were inhibited 32 and 15%, respectively. DNA synthesis was severely inhibited (70–90%) by 150 μM of the herbicide 24 hr after application. The inhibition of DNA synthesis by AC 243,997 did not begin until 5 to 7 hr after application. Although protein synthesis rates were apparently unaffected by AC 243,997, the level of the soluble proteins decreased 40% while free amino acid levels increased 32% 24 hr after application of the herbicide. An exogenous supply of valine, leucine, and isoleucine to corn prevented the inhibition of growth and reversed the inhibition of DNA synthesis caused by AC 243,997. All three amino acids at a concentration of 1 mM were needed to provide maximum protection. The results support the hypothesis that AC 243,997 kills plants by interfering with the biosynthesis of valine, leucine, and isoleucine.     

Differential effect of diclofop-methyl on fatty acid biosynthesis in leaves of sensitive and tolerant plant species

The herbicide diclofop-methyl caused an early and pronounced inhibition of the incorporation of [¹⁴C]acetate into leaf lipids of the sensitive plant species maize (Zea may L.), wild oat (Avena fatua L.), and barnyardgrass (Echinochloa crus-galli L.). With an EC₅₀ value of approximately 10^?7M inhibition was already apparent 0.5–4 hr after herbicide application. The fatty acid biosynthesis of tolerant bean (Phaseolus vulgaris L.), sugar beet (Beta vulgaris L.), and soybean (Glycine max L.) was not affected, with one exception [wheat (Triticum aestivum L.) belongs to the more tolerant species]; the inhibition of fatty acid biosynthesis, however, was in the same order of magnitude as in sensitive plants. More detailed studies showed that in wheat a recovery from inhibition of fatty acid biosynthesis occurred. Four days after herbicide application (0.18 kg diclofop-methyl/ha) in wheat normal fatty acid biosynthesis was restored, whereas in sensitive maize a 60% inhibition was maintained over the whole experimental period (8 days). The results support the view that tolerance of wheat to diclofop-methyl is based on its inactivation in leaves, whereas the tolerance of dicotyledonous species may probably lie at the level of the site of action of diclofop-methyl. In experiments with intact leaves, the inhibition of fatty acid biosynthesis resulted in an enhanced flow of [¹⁴C]acetate into organic acids and amino acids. This effect, however, was not always reproducible in experiments with leaf pieces or isolated root tips.     

On the antifungal mode of action of tridemorph

Tridemorph (2,6-dimethyl-N-tridecylmorpholine) was active against representative of nearly all taxonomic groups of fungi; gram-positive bacteria were also sensitive although gram-negative were not. Tridemorph, 3–10 μg/ml, inhibited the multiplication of sporidia of Ustilago maydis more strongly than the increase of dry weight. The treated sporidia appeared swollen, multicellular, and sometimes branched. Unsaturated lipophilic compounds like α-tocopherol and trilinolein alleviated the toxicity of tridemorph to Botrytis allii and U. maydis. Protein and RNA syntheses were inhibited slightly. DNA synthesis was rather strongly affected already after 2 hr. Lipid synthesis was first inhibited but later stimulated. At an early stage (2 hr) treated cells differed already from control cells by a higher content of free fatty acids. Tridemorph also inhibited sterol biosynthesis. The antimicrobial spectrum, the characteristic morphology of treated cells of U. maydis, the observations on cross-resistance, the alleviating effect of unsaturated lipophilic compounds, and the alterations in neutral lipid pattern suggest strong similarity of the mode of action of tridemorph with that of the known inhibitors of sterol biosynthesis.     

Cytochrome P-450 in insects. 6. Age dependency and phenobarbital induction of cytochrome P-450, P-450 reductase,and monooxygenase activities in susceptible and resistant strains of Musca domestica

Development and phenobarbital (PB) induction of microsomal cytochrome P-450, cytochrome P-450 reductase, two epoxidation, and two O-demethylation activities were examined in chronologically timed populations of insecticide-susceptible (NAIDM) and -resistant (Rutgers) house flies. Measurements of these enzymes started with the pharate adult stage and ended 5 days following eclosion. Untreated insects of both strains had comparable reductase levels, whereas cytochrome P-450 and associated monooxygenase activities were 1.5-fold or more higher in Rutgers. Maximum induction, as well as toxicity, occurred at a lower PB concentration in NAIDM than Rutgers. The drug caused consistently higher increases in enzymes and activities within 12 hr of starting treatment in both strains. When PB was withdrawn from treated flies (both strains) 48 hr after treatment began, specific activities (product min^?1 mg protein^?1) in all enzymes returned to control values in 24 hr while metabolic capacity (product min^?1 insect^?1) achieved control values within 48 hr. The changes in turnover numbers (pmol product min^?1 pmol P-450^?1), in conjunction with the differences in the monooxygenation of the four substrates, suggest that PB treatment induced both a quantitative and qualitative change in NAIDM monooxygenation but only a quantitative change in Rutgers monooxygenation.     

Analogy in the mode of action of fluotrimazole and clotrimazole in Ustilago avenae

Fluotrimazole [BUE 0620; 1-(3-trifluoromethyltriphenyl) 1,2,4-triazole] (20 μg/ml of nutrient solution) and clotrimazole [Bay b 5097; bisphenyl(2-chlorophenyl)-1-imidazolyl methane] (5 μg/ml) did not inhibit dry weight increase and only slightly reduced multiplication of sporidia of Ustilago avenae during the first doubling period (about 4 hr). After 8 hr, both fluotrimazole and clotrimazole more strongly inhibited sporidia multiplication than dry weight increase. As a consequence of treatment with both fungicides the usually single-celled sporidia appear swollen, multicellular, and branched. Both chemicals at a concentration range of 5–100 μg/ml did not affect oxidation of glucose. The effect of fluotrimazole and clotrimazole on protein, DNA, and RNA synthesis was similar to that on dry weight. Following a 6-hr incubation period total lipid synthesis was quantitatively unaffected by both chemicals. As the analysis of major fatty acids of total lipids revealed fluotrimazole substantially induced the synthesis of 20:4 carbon fatty acids, while in clotrimazole-treated sporidia the pattern of fatty acids did not differ from that of control sporidia. Fluotrimazole and clotrimazole produced a higher quantity of free fatty acids in sporidia of U. avenae. Gas-liquid chromatographic analysis of sterol fractions in treated and control sporidia (6 hr) indicated that both fluotrimazole and clotrimazole seriously inhibited ergosterol biosynthesis and concomitantly caused an accumulation of immediate ergosterol precursors which represent C-4-methyl and 4,4-dimethyl sterols. Incorporation of [¹⁴C]acetate for 2 hr into various lipid fractions of sporidia of U. avenae also revealed that radioactivity in C-4-desmethyl sterols in both fluotrimazole- and clotrimazole-treated sporidia was drastically reduced, while the radioactivity of C-4-methyl and 4,4-dimethyl sterols distinctly increased. The data suggest that fluotrimazole and clotrimazole are specific inhibitors of the oxidative demethylation of the C-14-methyl group during ergosterol biosynthesis in U. avenae.     

Antifungal mode of action of imazalil

Imazalil had no effect on the initial growth of mycelia of Penicillium italicum (for 10 hr) or Aspergillus nidulans (for 2 hr). In P. italicum during this period neither respiration nor cell permeability was affected, but uptake of [³²P]phosphate, [¹⁴C]leucine, or [¹⁴C]uridine was partially inhibited. The initial (5 hr) inhibition of substrate uptake coincided with a 50% reduction in ergosterol content. Within 0.5 hr, incorporation of [¹⁴C]acetate into C-4-desmethyl sterols was strongly inhibited in mycelia of A. nidulans treated with 0.5 μg/ml of imazalil. However, radioactivity in C-4-methyl and dimethyl sterols exceeded that of control cultures. Concentrations of imazalil as low as 0.005 μg/ml caused short-term (1 hr) declines of incorporation into desmethyl sterols and increases into the C-4-methyl and dimethyl sterols. Incorporation into phospholipids, triglycerides, and free fatty acids was not affected. These data suggest that the primary antifungal action of imazalil is inhibition of demethylation in the biosynthesis of ergosterol.     

Mechanism of resistance to fenarimol in Aspergillus nidulans

Mycelial uptake of [¹⁴C]fenarimol (10 μg/ml) by 20 fenarimol-resistant mutants of Aspergillus nidulans was compared with uptake by wild-type strain 003. Uptake of the fungicide during the initial 10 min of incubation was significantly lower in all mutant strains than in the wild-type strain indicating that resistance is related with reduced uptake. Upon prolonged incubation a gradual decrease of accumulated radioactivity in the wild-type strain was observed. A few mutants displayed resistance to unrelated chemicals such as p-fluorophenylalanine or d-serine; this phenomenon appeared not to be due to a decreased uptake of the corresponding natural amino acids. Incorporation of [³H]adenine and [¹⁴C]leucine by mycelium of mutant M193 was hardly inhibited after 5 hr of incubation with the fungicide, whereas a distinct effect was found with the wild-type strain. At this time also fungitoxicity to the wild-type strain became apparent. Probably, this effect is indirectly caused by inhibition of ergosterol biosynthesis. Mycelium of mutant M193 incorporated [¹⁴C]acetate slightly less effectively than the wild-type strain. After 2 hr of incubation with this radiochemical leakage of [¹⁴C]acetate metabolites from mycelium of the mutant strain was observed. This indicates that resistance might be correlated with increased excretion of fungal metabolites, which in turn may be related with reduced fitness of fenarimol-resistant mutants.     

Effect of alachlor on Avena seedlings: Inhibition of growth and interaction with gibberellic acid and indoleacetic acid

The growth of Avena seedlings grown in sand was found to be inhibited by alachlor with the time of onset of inhibition after treatment being a function of herbicide concentration. There was a 12 hr lag period following a subirrigation with 2.5 × 10^?4M alachlor before growth inhibition could be detected. This lag period may be due to uptake and translocation of alachlor from the roots to the site of inhibition or to the exhaustion of certain growth-limiting substance(s) whose biosynthesis is inhibited by alachlor. Additions of gibberellic acid by subirrigation simultaneously with alachlor or after alachlor treatment did not prevent growth inhibition. However, treatment with 10^?3M gibberellic acid 24 hr prior to alachlor treatment overcame the alachlor inhibition. On the other hand, in contrast to gibberellic acid, indoleacetic acid did not prevent inhibition by alachlor.     

Cholinesterase inhibition by methamidophos and its subsequent reactivation

Methamidophos (O,S-dimethylphosphoramidothioate, Monitor) is an organophosphorus, cholinesterase-inhibiting insecticide. The rate constant (k_i) for inhibiting rat plasma cholinesterase (ChE) was 1.57 ± 0.03 × 10³M^?1 min^?1, for rat erythrocyte ChE was 8.86 ± 1.10 × 10³M^?1 min^?1, and for rat brain ChE was 6.58 ± 0.42 M^?1 min^?1. Brain and plasma cholinesterases spontaneously recovered from over 90% inhibition at 30 min to 50% inhibition in 4 and 14 hr, respectively. Pralidoxime increased the rate of reactivation in vitro. In vivo, rats poisoned with methamidophos exhibited signs of cholinergic stimulation. The LD₅₀ of ip methamidophos in male rats was 15 ± 0.7 mg/kg. Pralidoxime (60 mg/kg) and atropine (10 mg/kg) given with the methamidophos increased the LD₅₀ to 52 ± 4.9 mg/kg and 60 ± 0.4 mg/kg, respectively. In rats given 12.5 mg methamidophos (an LD₂₀), ChE activity was depressed 95 ± 12.5% in plasma, 92 ± 0.6% in stomach, and 88 ± 1% in brain at 1 hr after injection. At 48 hr after injection ChE activity had returned to 60% or more of control values in each of the tissues. Administration of a single dose of 60 mg/kg of pralidoxime along with methamidophos did not increase ChE activities at the times and places it was measured.     

Rapid effects of a thiocarbamate herbicide and its dichloroacetamide protectant on macromolecular syntheses and glutathione levels in maize cell cultures

The rapid effects of the thiocarbamate herbicide S-ethyl dipropyl thiocarbamate (EPTC) and the herbicide protectant N,N-diallyl-2,2-dichloroacetamide (DDCA) on macromolecular syntheses and glutathione (GSH) levels in maize cell cultures were studied to determine whether stimulation of GSH could be the primary mechanism of action of DDCA. EPTC (0.5 and 1 mM) reduced incorporation of radioactive precursors within 1 hr after treatment, and affected incorporation of [³H]acetate into lipids more than incorporation of [³H]adenosine into acid-precipitable nucleic acids, or [¹⁴C]protein hydrolysate into protein. [¹⁴C]EPTC was rapidly biotransformed within 8 hr by maize cell suspensions. Measureable decreases in GSH levels following treatment with 1 mM EPTC occurred after 15 hr. DDCA stimulated incorporation of [³H]acetate into lipids within 4 hr but did not affect incorporation of [¹⁴C]protein hydrolysate into protein or [³H]adenosine incorporation into nucleic acids. Measureable increases in GSH following DDCA treatment began after 12 hr. Treatment with EPTC and DDCA in combination inhibited incorporation of [³H]acetate into lipids less than EPTC given alone. Increases in GSH levels could be observed following pretreatments with glutathione precursors, but no protectant activity could be detected, in contrast to treatments with DDCA. It is suggested that DDCA has an initial rapid effect on lipid metabolism followed by a slower effect involving increases in cellular GSH.     

Acephate toxicity,metabolism, and anticholinesterase activity in Heliothis virescens (F.) and Anthonomus grandis grandis (Boheman)

The toxicity (72 hr) of acephate and methamidophos to fourth-instar larvae of the tobacco budworm, Heliothis virescens (F.), was nearly equivalent. In contrast, toxicity (72 hr) of methamidophos to adult boll weevils, Anthonomus grandis grandis (Boheman), was substantially greater than that of acephate. The internal accumulation of acephate was greater for A. grandis grandis than for H. virescens at 24 and 48 hr post-treatment, as was excretion. Acephate was metabolized to methamidophos both in vivo and in vitro by H. virescens but not by A. grandis grandis. In vitro acetylcholinesterase inhibition by methamidophos was greater than that of acephate, but less than that of paraoxon for H. virescens, A. grandis grandis, and the electric eel. Treatment of H. virescens larvae with acephate resulted in increased in vivo acetylcholinesterase inhibition between 24 and 72 hr post-treatment, which was associated with a large increase in mortality. H. virescens treated with methamidophos showed greater mortality and greater acetylcholinesterase inhibition at earlier time periods than those treated with acephate. However, by 72 hr post-treatment, in vivo acetylcholinesterase inhibition by LD₅₀ doses of acephate and methamidophos were approximately equivalent. These results indicate that, for H. virescens, toxicity of acephate is directly related to its metabolism to methamidophos and subsequent acetylcholinesterase inhibition. Likewise, the differential toxicity of acephate and methamidophos to A. grandis grandis adults appears to be due to their inability to metabolize acephate to methamidophos.     

Why does DDT toxicity change after a blood meal in adult female Culex pipiens?

The toxicity of topically applied DDT to adult female anautogenous mosquitoes (Culex pipiens L.) showed dramatic variations in blood-fed insects. It decreased very rapidly about twofold to a minimum at 24 hr after a blood meal, then increased within 72 hr back to values typical of non-blood fed insects. A comparison of the metabolism of [¹⁴C]DDT in vivo revealed an increase in DDT dehydrochlorination to DDE at 72 hr after a blood meal, but this increase was not responsible for the variations in DDT toxicity at 24 hr. Changes in penetration rates were not observed and changes in the distribution of DDT could likewise not be related to the short period of decreased toxicity of DDT. Fenvalerate and trans-permethrin, two pyrethroid insecticides which are believed to have a mode of action similar to that of DDT, were also significantly less toxic 24 hr after a blood meal. By contrast, the cyclodiene insecticide aldrin and the carbamate insecticide propoxur were not less toxic 24 hr after a blood meal. The results suggest that after a blood meal an unidentified and transient change in C. pipiens specifically decreases DDT/pyrethroid toxicity. A hypothesis concerning this transient change is advanced. The results illustrate the difficulties in explaining physiological changes in insecticide toxicity.     

Modification of fatty acids in eggplant affects its resistance to Verticilliumdahliae

Expression of the yeast Δ-9 desaturase gene in eggplant increased its levels of 16:1, 18:1, and 16:3 fatty acids and enhanced its resistance to Verticillium dahliae. Eggplants responded to V. dahliae inoculation with transitory upsurges in phospholipase A₂and lipoxygenase activities, as well as levels of free fatty acids and lipid peroxides. The upsurges of certain fatty acids, in particular the 16:1 and 16:3 fatty acids, were more pronounced in the transgenic plants. cis -Δ9 16:1 was found to directly inhibit the growth of V. dahliae. The results suggest that increasing the production of 16:1 in plants could be an approach to enhance their resistance to fungal diseases     

Kinetic properties of the inhibition of juvenile hormone esterase by two trifluoromethylketones and O-ethyl,S-phenyl phosphoramidothioate

Some inhibition kinetic properties and in vivo inhibition of the plasma juvenile hormone esterase from the cabbage looper (Trichoplusia ni Hübner) by one phosphoramidothioate and two trifluoromethylketones were examined. O-ethyl,S-phenyl phosphoramidothioate was shown to react irreversibly with the enzyme in a time-dependent manner, and the inhibition reaction can be factored into a reversible step with a dissociation constant, K_d, of 4.55 × 10^?5M followed by a phosphorylation step with a rate constant, k₂, of 1.98 min^?1. The phosphorylated enzyme did not show spontaneous recovery after 48 hr of dialysis. On the other hand, the two trifluoromethylketones were shown to act as reversible inhibitors, as their inhibited enzyme was regenerated completely after dialysis. However, 1,1,1,-trifluoro-3-thiooctylpropan-2-one, in contrast to 1,1,1-trifluorotetradecan-2-one, showed progressive time-dependent inhibition, and its reaction with the enzyme followed characteristic bimolecular second-order kinetics with a rate constant, k_i, of 3.37 × 10⁷M^?1 min^?1. The in vivo inhibition data of topically treated larvae at equimolar amounts of the tested compounds indicated rapid penetration, and the stability of the inhibition was higher for the phosphoramidothioate than for the trifluoromethylketones. The relationship of the mechanism of inhibition and the in vivo inhibition of these compounds to the understanding of the interactions between juvenile hormone and juvenile hormone esterase is discussed.     

Effects of dimethazone (FMC 57020) on chloroplast development: 1. Ultrastructural effects in cowpea (Vigna unguiculata L.) primary leaves

The effects of dimethazone [FMC 57020; 2-(2-chlorophenyl)methyl-4,4-dimethyl-3-isoxalidinone] on the growth and ultrastructure of cowpea (Vigna unguiculata L.) were examined. Seeds were imbibed in 0.5 mM dimethazone for 1 day and grown for 4 to 5 subsequent days in darkness without the herbicide. The herbicide stunted etiolated hypocotyl growth and retarded greening under 150 μmol · m⁻² · sec⁻¹ white light. No effects of dimethazone on the in vivo absorption spectrum of the etiolated primary leaf was detected. The herbicide caused some reduction and disorganization of prothylakoids in etiplasts. After 3 hr of white light chlorophyll accumulation was greatly reduced in treated leaves and ultrastructural development of the chloroplasts of herbicide-treated tissues appeared to be retarded. Pronounced thylakoid disruption was noticed in some cells after 12 hr, was more common after 24 hr, and was found in all cells by 72 hr. Maximally affected plastids lacked thylakoids, had irregular envelopes, and contained numerous vesicles.     

Induction of glutathione S-transterase by fumigants in larvae of the Khapra beetle,Trogoderma granarium (E.)

The effect of fumigants on glutathione and glutathione S-transferase in the Khapra beetle larvae (Trogoderma granarium) was studied by fumigating for 1, 3, and 5 hr with a dose causing 100% mortality at 24 hr of exposure. Glutathione and glutathione S-transferase were assayed in the cytosol at 1, 3, and 5 hr of exposure. Time-dependent depletion of glutathione was seen for all fumigants except carbon tetrachloride and phosphine. The depletion was maximum (60–70%) in the cases of methyl bromide, methyl iodide, and acrylonitrile, and least (20–30%) in the cases of ethylene dibromide and ethylene oxide. The order of glutathione depletion by various fumigants at 5 hr exposure was methyl iodide > methyl bromide = acrylonitrile > ethylene dichloride > ethylene oxide > ethylene dibromide. Glutathione S-transferase was induced by all fumigants except ethylene dibromide, methyl bromide being more potent than methyl iodide. The enzyme induction ranged from 186% by acrylonitrile to 40% by carbon tetrachloride. Mortality above 10% correlated well with the degree of GSH depletion (r = 0.729) whereas the latter did not correlate with the transferase induction.     

Effects of triazoles on fungi: IV. Growth and lipids of Cercospora arachidicola and Cercosporidium personatum

The effects of propiconazole (a sterol C-14 demethylation inhibitor) on the growth and lipid content of Cercospora arachidicola and Cercosporidium personatum were examined in vitro using gravimetric, chromatographic, and colorimetric techniques. The lipid content and composition of both species were very similar. C_16:0, C_18:1, and C_18:2 were the principal fatty acids of the major acyl lipids, ergosterol (ergosta-5,7,22-trienol) was the principal sterol, and free fatty acids comprised a large portion (ca. 30%) of lipid. Cercospora and Cercosporidium were both very sensitive to the inhibitor; 0.10 to 0.15 μg propiconzole/ml was required for an average of approximately 50% growth inhibition among isolates on a mycelial dry weight basis. Changes in lipid composition were similar in both species grown in media containing the inhibitor. The total sterol content was twofold higher than that in the corresponding controls, which was due to the accumulation of ergosterol precursors (e.g., 24-methylene dihydrolanosterol). The free fatty acid content of treated mycelia was lower than that of the controls, and the degree of unsaturation of the lipids was higher, particularly in phosphatidylcholine. Also, the ratio of saturated to unsaturated fatty acids was less in the polar lipid of inhibitor-treated mycelium than in controls.     

Characterization of the mode of action of the experimental herbicide LS 82–556: [(S)3-N-(Methylbenzyl)carbamoyl-5-propionyl-2,6-lutidine]

The herbicidal activity of the pyridine derivative LS 82–556 [(S)3-N-(methylbenzyl)carbamoyl-5-propionyl-2,6-lutidine] has been studied on green or etiolated seedlings of cucumber (Cucumis sativus L. cv Vert Long Maraîcher), wheat (Triticum aestivum L. cv Fidel), and maize (Zea mays L. cv Monclair). The symptoms were strictly light dependent, and consisted of bleaching, wilting, and desiccation of leaves and stems. In green seedlings pretreated for 24 hr in the dark with 10 μM LS 82–556, increases in ethane evolution and thiobarbituric acid-reacting material content could be detected after a 3-hr exposure to high light intensity (400 μE m⁻² sec⁻¹ photosynthetically active radiation). Similar increases were also detected in etiolated seedlings. Electron microscope observations of treated tissues exposed to light for 6 hr or more revealed structural damage at the level of cellular membranes (chloroplast envelope, tonoplast, and plasma membrane). In cotyledons or leaves grown in the light, activity of LS 82–556 required the presence of chloroplastic pigments. All the above results suggest that as for diphenyl ether herbicides, the mechanism of action of LS 82–556 involves photooxidative reactions leading to the degradation of fatty acids. A further analogy between the two types of herbicide action comes from their similar requirement for an operating photosynthetic electron flow when they are administered to green tissues.     

Distribution and excretion of [14CH3S]methamidophos after intravenous administration of a toxic dose and the relationship with anticholinesterase activity

The tissue distribution and excretion of [¹⁴CH₃S]methamidophos was followed in female Sprague-Dawley rats after intravenous injection at a toxic, but nonlethal, dose (8 mg/kg). Radiolabel was rapidly distributed to all tissues at approximately equal concentrations. Peak tissue levels were achieved within 1–10 min except in the central and peripheral nervous system where peak levels (40 nmol/g) were found between 20 and 60 min, corresponding to peak signs of toxicity. Within 24 hr of dosing, 47% of the radioactivity was recovered in the urine and 34% as ¹⁴CO₂ with <5% in the feces over 7 days. Cholinesterase (ChE) inhibition was measured in erythrocytes, plasma, and various regions of the central nervous system (CNS) at selected times after administration of methamidophos at 8 mg/kg. The degree of acetylcholinesterase (AChE) inhibition in the three CNS regions was similar, reaching a minimum of 15–20% of control values at 30–60 min, when toxicity was most severe. The degree of erythrocyte AChE inhibition was less than that of the CNS although the time course was similar. Plasma ChE inhibition was more rapid than that of the CNS or erythrocytes and reactivation was slower. When similar concentrations of methamidophos to those found in vivo were incubated with CNS homogenates, plasma, or erythrocytes in vitro (5 × 10^?5M) a similar degree of inhibition occurred over the same time course. It is, therefore, concluded that the cholinergic toxicity produced by methamidophos is a result of the in vivo stability of this compound combined with its entry into the nervous system in sufficiently high concentrations to inhibit AChE.