Abstract
Background:
Determination of normal whole blood cholinesterase (WBChE) activity is required in the assessment of exposure to organophosphate and carbamate pesticides in humans and animals.

Aim:
Using a modified electrometric method to comparatively measure normal WBChE activities in humans, sheep, goats, and cattle with their true ChE components by inhibiting pseudo ChE activity in vitro using quinidine.

Methods:
The WBChE activities in humans, sheep, goats, and cattle were measured by a modified electrometric method using 7.1% acetylcholine iodide as a substrate in the reaction mixture. The mixture also contained whole blood samples (0.2 mL), distilled water (3 mL), and the pH 8.1 barbital-phosphate buffer (3 mL). The incubation period was 20 minutes in humans and 30 minutes in ruminants at 37 ºC. Whole blood samples (8-10/species) were subjected to in vitro pseudo ChE inhibition using 50 µL of 1% quinidine sulfate with the objective to estimate the remaining true ChE in the blood.

Results:
The overall WBChE activity in humans (1.049 ± 0.411 Δ pH/20 min) was found to be significantly higher than that observed in the sheep (0.595 ± 0.324 Δ pH/30 min), goats (0.432 ± 0.226 Δ pH/30 min) and cattle (0.693 ± 0.328 Δ pH/30 min). The comparative median WBChE activities in humans and ruminants were 1.12, 0.565, 0.395, and 0.665, respectively. True ChE activity of humans was found to be significantly higher than that of ruminants, whereas the pseudo ChE of humans was found to be significantly higher than that of the sheep and goats. Within each species, the pseudo ChE activity in the whole blood was consistently low (19%-26%) and significantly below the corresponding value of true ChE (74%-81%).

Conclusion:
The study presents a comparative modified electrometric measurement of normal WBChE activities between humans and ruminants. The results obtained demonstrate that human blood exhibits higher WBChE, pseudo ChE, and true ChE activities compared to those of ruminants. A comparative determination of WBChE activity using a simple modified electrometric method in humans and farm animals could serve as a valuable tool for collectively assessing cross-species exposure to pesticides.

Key words: Acetylcholinesterase; Pseudocholinesterase; Sheep; Goat; Cattle.