A comparative study of acetylcholinesterase and general-esterase activity assays using different substrates, in vitro and in vivo exposures and model organisms

The Potential Effect of Insulin on AChE and Its Interactions with Rivastigmine In Vitro

There is no definite cure for Alzheimer’s disease (AD) due to its multifactorial origin. Drugs that inhibit acetylcholinesterase (AChE), such as rivastigmine, are promising symptomatic treatments for AD. Emerging evidence suggests that insulin therapy can hinder several aspects of AD pathology. Insulin has been shown to modify the activity of AChE, but it is still unknown how insulin and AChE interact. Combination therapy, which targets several features of the disease based on existing medications, can provide a worthy therapy option for AD management. However, to date, no studies have examined the potential interaction of insulin with AChE and/or rivastigmine in vitro. In the present study, we employed the Response Surface Methodology (RSM) as an in vitro assessment to investigate the effect of insulin on both AChE activity and rivastigmine inhibitory action using a common spectrophotometric assay for cholinesterase activity, Ellman’s method. Our results showed that insulin, even at high concentrations, has an insignificant effect on both the activity of AChE and rivastigmine’s inhibitory action. The variance of our data is near zero, which means that the dispersion is negligible. However, to improve our understanding of the possible interaction of insulin and rivastigmine, or its target AChE, more in silico modelling and in vivo studies are needed.

Using TKTD Models in Combination with In Vivo Enzyme Inhibition Assays to Investigate the Mechanisms behind Synergistic Interactions across Two Species

The aim of this study is to compare the azole synergy across an insect, Chironomus riparius, and a crustacean species, Daphnia magna. We use a combination of in vivo measurements of cytochrome P450 monooxygenase (CYP) biotransformation potential and toxicokinetic (TK) and toxicodynamic (TD) modeling to understand the mechanism behind the synergy of two azole fungicides: the imidazole prochloraz and the triazole propiconazole on the pyrethroid insecticide α-cypermethrin. For both species, the synergistic effect of prochloraz was well-described by its effect on in vivo CYP activity, which corresponded to the biotransformation rate of the TK model parameterized on the survival data of the mixture experiment. For propiconazole, however, there were 100-fold and 50-fold differences between the 50% effect concentration of in vivo CYP activity and the modeled biotransformation rate for C. riparius and D. magna, respectively. Propiconazole, therefore, seems to induce synergy through a mechanism that cannot be quantified solely by the CYP activity assay used in this study in either of the two species. We discuss the differences between prochloraz and propiconazole as synergists across the two species in the light of the type and time dynamics of affected biotransformation processes.

A Smartphone Camera Colorimetric Assay of Acetylcholinesterase and Butyrylcholinesterase Activity

Acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) can serve as biochemical markers of various pathologies like liver disfunction and poisonings by nerve agents. Ellman’s assay is the standard spectrophotometric method to measure cholinesterase activity in clinical laboratories. The authors present a new colorimetric test to assess AChE and BChE activity in biological samples using chromogenic reagents, treated 3D-printed measuring pads and a smartphone camera as a signal detector. Multiwell pads treated with reagent substrates 2,6-dichlorophenolindophenyl acetate, indoxylacetate, ethoxyresorufin and methoxyresorufin were prepared and tested for AChE and BChE. In the experiments, 3D-printed pads containing indoxylacetate as a chromogenic substrate were optimal for analytical purposes. The best results were achieved using the red (R) channel, where the limit of detection was 4.05 µkat/mL for BChE and 4.38 µkat/mL for AChE using a 40 µL sample and a 60 min assay. The major advantage of this method is its overall simplicity, as samples are applied directly without any specific treatment or added reagents. The assay was also validated to the standard Ellman’s assay using human plasma samples. In conclusion, this smartphone camera-based colorimetric assay appears to have practical applicability and to be a suitable method for point-of-care testing because it does not require specific manipulation, additional education of staff or use of sophisticated analytical instruments.

Can Organophosphates and Carbamates Cause Synergisms by Inhibiting Esterases Responsible for Biotransformation of Pyrethroids?

Hydrolysis catalyzed by general esterases (GEs) is the most efficient route for hydrolyzation of pyrethroid insecticides. Organophosphate (OP) and carbamate (CB) insecticides are known to inhibit GEs in addition to acetylcholinesterase (AChE), which is their main target. We hypothesize that synergies can be induced by OPs and CBs when mixed with pyrethroids, due to their inhibition of GE-dependent detoxification of pyrethroids. To test this hypothesis, we conducted mixture toxicity experiments with Daphnia magna using α-cypermethrin (α-cyp) in combination with the noninsecticidal OP tetraisopropyl pyrophosphoramide (iso-OMPA) and five AChE inhibitors diazinon, chlorpyrifos, chlorfenviphos, parathion, and aldicarb. In addition, the in vivo GE activity inhibition was measured for all compounds. Up to 10-fold synergy was found between α-cyp and iso-OMPA, and the degree of synergy correlated linearly with the inhibition of the GE activity. No synergy, however, was found in any of the insecticide mixtures nor was the GE activity inhibited within the nonlethal concentration range tested. It was concluded that the effect of the insecticides on AChE occurred at lower concentrations than their effect on GEs, making the daphnids become immobilized before any synergistic effects on mortality could be observed. The implications of the findings are discussed from a risk assessment perspective.