Mechanism Of Selective Toxicity Research Articles

Metabolism of a sea lamprey pesticide by fish liver enzymes part B: method development and application in quantification of TFM metabolites formed in vivo.

The sea lamprey (Petromyzon marinus) is a destructive invasive species in the Great Lakes. Since the 1960s, tons of the lampricide 3-trifluoromethyl-4-nitrophenol (TFM) has been applied to selected tributaries each year to eliminate or reduce sea lamprey larval populations. Therefore, the environmental impact of TFM needs to be evaluated. However, the metabolism of TFM and its mechanism of selective toxicity in sea lamprey is not yet fully understood. Based upon our previous report on the identification, synthesis, and characterization of TFM metabolites observed in liver incubates from sea lamprey and non-target fishes, we now provide a robust assay for quantifying TFM and its metabolites in fish liver tissue. This method is important for assessing bioaccumulation of TFM in the ecosystems. The compounds purified in our previous report were used to develop and validate a quantitative ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) assay for TFM and TFM metabolites formed in vivo. Several sample preparation techniques were compared, and a protein precipitation method was selected. The unavailability of stable isotopic internal standards was overcome by using a matrix matching method. After a thorough validation, this method was applied to determine the concentrations of TFM and its metabolites in fish liver tissues from animals exposed to TFM, and in the comparison between dead animals and survivors. Seven of eight expected metabolites were observed, some for the first time in vivo. Our results indicate that in vivo nitroreduction, glucuronidation, sulfation, and glutathione conjugation are involved in TFM metabolism in sea lamprey.

Resmethrin, the first modern pyrethroid insecticide.

The discovery of resmethrin almost five decades ago was the seminal event in the development of pyrethroid insecticides as important pest management tools, the value of which endures to this day. This brief review considers the development of pyrethroids from the perspective of the discovery of resmethrin. I describe the pathway to the discovery of resmethrin and the unique properties that differentiated it from the pyrethrins and earlier synthetic pyrethroids is described. I also summarize information on metabolic fate and mechanisms of selective toxicity, first elucidated with resmethrin, that have shaped our understanding of pyrethroid toxicology since that time. Finally, I review the discovery pathway that led from resmethrin to the development of the first photostable, agriculturally useful pyrethroids that established the importance of this insecticide class.

Formic acid: A neurologically active, hydrolyzed metabolite of insecticidal formate esters

Formate esters have potential uses as insecticides in both grain fumigation and public health pest management. This research investigated the hydrolytic metabolism and neurological effects of formate ester compounds. Studies were conducted using Drosophila and houseflies as models for other dipteran pests of medical importance. Results indicated significant formic acid liberation from a broad range of formate esters, both in vivo and in vitro. Based on these initial observations, we subsequently investigated neurological effects of formic acid on the larval housefly nervous system. We found that formic acid caused significant neuroexcitation at concentrations lower than previously shown for inhibition of insect cytochrome c oxidase; however, this conclusion does not preclude that mitochondrial effects may also be occurring in non-nervous tissues. Finally, we investigated formate ester hydrolysis by A- and B-type esterases commonly considered in pesticide detoxification research; however, no significant interactions could be identified, suggesting that as yet unidentified carboxylesterases play roles in formate ester hydrolysis. These findings provide information on formate ester metabolism and modes of action, as well as rationale for further studies on formate ester neurotoxicology and mechanisms of selective toxicity.

Cytotoxicity of S-conjugates of the sevoflurane degradation product fluoromethyl-2,2-difluoro-1-(trifluoromethyl) vinyl ether (Compound A) in a human proximal tubular cell line

Fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (FDVE) is a fluorinated alkene formed by degradation of the volatile anesthetic sevoflurane in anesthesia machines. FDVE is nephrotoxic in rats but not humans. Rat FDVE nephrotoxicity is attributed to FDVE glutathione conjugation and bioactivation of subsequent FDVE-cysteine S-conjugates, in part by renal β-lyase. Although FDVE conjugation and metabolism occur in both rats and humans, the mechanism for selective toxicity in rats and lack of effect in humans is incompletely elucidated. This investigation measured FDVE S-conjugate cytotoxicity in cultured human proximal tubular HK-2 cells, and compared this with known cytotoxic S-conjugates. HK-2 cells were incubated with FDVE and its GSH, cysteine S-mercapturic acid, cysteine S-sulfoxide, and mercapturic acid sulfoxide conjugates (0.1–2.7 mM) for 24 h. Cytotoxicity was determined by lactate dehydrogenase (LDH) release, total LDH, and the ability of viable cells to reduce a tetrazolium-based compound (MTT). FDVE was cytotoxic only at concentrations ≥0.9 mM. No increase in LDH release was observed with either FDVE-GSH conjugate. The FDVE-cysteine conjugates S-(1,1-difluoro-2-fluoromethoxy-2-(trifluoromethyl) ethyl)- l-cysteine (DFEC) and (Z)- S-(1-fluoro-2-fluoromethoxy-2-(trifluoromethyl) vinyl)- l-cysteine ((Z)-FFVC) caused significant differences in LDH release and MTT reduction only at 2.7 mM; (Z)-FFVC was slightly more cytotoxic. Both S-(1,1-difluoro-2-fluoromethoxy-2-(trifluoromethyl) ethyl)- l-cysteine sulfoxide (DFEC-SO) and (Z)- N-acetyl- S-(1-fluoro-2-fluoromethoxy-2-(trifluoromethyl) vinyl)- l-cysteine sulfoxide ((Z)- N-Ac-FFVC-SO) caused slightly greater changes in LDH release or total LDH than the corresponding equimolar DFEC and (Z)- N-acetyl- S-(1-fluoro-2-fluoromethoxy-2-(trifluoromethyl) vinyl)- l-cysteine ((Z)- N-Ac-FFVC) conjugates. In contrast to FDVE S-conjugates, S-(1,2-dichlorovinyl)- l-cysteine was markedly cytotoxic, at concentrations as low as 0.1 mM. These results show that human proximal tubular cells are relatively resistant to FDVE and FDVE S-conjugate cytotoxicity. This may partially explain the lack of FDVE nephrotoxicity in humans.

Control of microbial growth in water-based metal-working fluids

The presence of bacteria and fungi in the metal-working fluid (MWF) of a large central tank (150 m 3) in an engineering factory was studied over 3 years with the aim of microbial prevention. During the first year it was possible to prevent the growth of bacteria and fungi by the use of formaldehyde (270 ppm) and sodium Omadine (60 ppm). During the subsequent years the antimicrobial capacity of alkanolamines was explored. In laboratory tests it was found that the antimicrobial activity was greatly enhanced at higher pH values. Butylethanolamine and dimethylamino-methyl-propanol had the highest antimicrobial potency of the alkanolamines, in vitro. Butylethanolamine in combination with a pH higher than 9 was also an efficient antimicrobial agent when used in MWF under workshop conditions. The enhanced antimicrobial activity of the alkanolamines at higher pH offers both a non-expensive way of microbial control and a mechanism for selective toxicity.

Pesticides and birds — mechanisms of selective toxicity

On present evidence certain pesticides tend to be more acutely toxic to birds than to mammals. This trend is marked with carbamates, less marked with organophosphates. There is also some evidence that birds have a greater capacity for bioaccumulation of persistent pesticides such as dieldrin and DDE than do mammals of similar size. Consideration is given to biochemical and physiological contrasts between the two groups which may be connected with these differences. Small liver size, the maintenance of relatively high body temperatures, the direct movement of blood to the kidneys, the process of egg laying and the release of urine into the cloaca are all special features of birds which may affect selective toxicity. There is also evidence that birds are deficient in certain enzymes which aid the detoxication of pesticides. Birds tend to have very low levels of ‘A’ esterase in comparison to mammals, and this may explain their high susceptibility to organophosphates such as pirimiphos-methyl and diazinon. Hepatic microsomal monooxygenase activity also tends to be low in birds and this may have far-reaching consequences. In some birds the relatively high susceptibility to carbamates and the tendency to bioaccumulate persistent organochlorine compounds may be due, at least in part, to a deficiency in this enzyme system. A case can be made for more detailed investigation of comparative detoxication mechanisms in birds and other vertebrates to aid the explanation and the prediction of patterns of selective toxicity.

Effects of exogenous polyamine and trypanocides on the DNA polymerase activities from Trypanosoma brucei brucei, mouse thymus and murine leukemia virus

The effects of exogenously added spermine on activated (gapped) DNA-directed and poly(dC) - (dG) 12–18-directed DNA synthesis were tested on the chromatographically separated DNA polymerase activities of Trypanosoma brucei brucei. Activated DNA-directed DNA synthesis by the Peak I (eluting from DNA-agarose at 0.15 M KCl) and Peak II (eluting at 0.3 M KCl) polymerase was consistently inhibited or stimulated, respectively, by exogenous spermine. Kinetic analysis revealed that inhibition of the Peak I enzyme with respect to template DNA occurred by a mixed mechanism, while a major factor in the stimulation of the Peak II enzyme by spermine appeared to be the polyamine-mediated reversal of ‘substrate inhibition’ by DNA at concentrations above 10 μg/ml. The apparent K m values of Peak I and Peak II DNA polymerases for activated DNA were determined to be 5 and 0.5 μg/ml, respectively. In contrast to the results observed with activated DNA, activation of Peak II-enzyme-catalyzed poly(dC)-directed DNA synthesis was similar at all template-primer concentrations. Peak I enzyme-catalyzed poly(dG) synthesis was either inhibited or slightly stimulated by spermine, depending upon the presence or absence of heteropolymeric DNA, respectively. Dose-dependent inhibition of DNA-directed DNA synthesis catalyzed by T. b. brucei DNA polymerases, murine thymus DNA polymerase α, and Rauscher murine leukemia virus reverse transcriptase by trypanocides was examined to determine a possible mechanism of selective toxicity by such agents. The drugs Antrycide (quinapyramine), pentamidine, imidocarb, Berenil (diminazene aceturate), WR-199-385 [2,5-bis(4-guanylphenyl)furan-2HCl] and isometamidium inhibited DNA polymerases of the eucaryotic cells at approximately the same degree, and at similar concentrations. The presence of spermine in reaction mixtures did not spare any drug inhibition. Stimulation of reverse transcriptase activity was observed in the presence of Antrycide and imidocarb, however, this could be negated by stimulatory amounts of spermine present in the reaction mixture. The results, obtained using an activated DNA-directed assay system, suggest that trypanosomal DNA polymerases are not the selective target of trypanocidal drugs currently available.