1 A pH METHOD FOR MEASURING BLOOD CHOLINESTERASE ACTIVITY IN GOATS M. M. H. Al-Jobory and F. K. Mohammad * * Department of Physiology-Division of Pharmacology and Toxicology, P.O. Box 11136, College of Veterinary Medicine, University of Mosul, Mosul, Iraq ABSTRACT A modified electrometric method was described and validated for measurement of plasma and erythrocyte cholinesterase activities in 6-18 months old goats of both sexes. A typical reaction mixture contained 3 ml distilled water, 3 ml barbital-phosphate buffer (pH 8.1), 0.2 ml plasma or erythrocytes and 0.1 ml acetylthiocholine iodide (7.5%) as a substrate. The mixture was incubated at 37 ο C for 40 minutes. The pH of the reaction mixture was determined by a pH meter before and after the incubation. The initial pH was measured before the substrate addition. The enzyme activity was expressed as delta pH/40 min. The coefficients of variation of the described method in measuring plasma and erythrocyte cholinesterase activities were 4 and 2%, respectively. Preliminary reference values (N=14) of the mean cholinesterase activity (delta pH/40 min) and 95% confidence interval in the plasma were 0.194 and 0.184-0.204, respectively, and those of the erythrocytes were 0.416 and 0.396-0.436, respectively. The % of pseudocholinesterase activity of the plasma cholinesterase was 63.5% as determined by quinidine sulfate inhibition. The organophosphorus insecticides dichlorvos and diazinon 0.5-4 μM and the carbamate insecticide carbaryl at 5-20 μM in the reaction mixture significantly inhibited plasma and erythrocyte cholinesterases in vitro. The results suggest that the described electrometric method is simple, precise and efficient in measuring blood cholinesterase activity in goats. INTRODUCTION Determination of erythrocyte and plasma cholinesterase (ChE) activity is used to monitor exposure to organophosphate (OP) or carbamate insecticides (Fairbrother et al., 1991; Wilson and Henderson, 1992; Wilson et al., 1995; 1998). One of the principle methods for measuring blood ChE activity is the electrometric method which is based on production of acetic acid that decreases the pH of the reaction mixture (Wills, 1972; Wilson, 1999). The original electrometric method of Michel (1949) is most commonly used in man (Wilson et al., 1995; 1998). However, the method is not directly applicable to samples of different animal species (Wills, 1972; Fairbrother et al., 1991; Wilson et al., 1998; Wilson, 1999). This is because of the inherent variations in blood ChE activities between different animal species (Mohammad and St. Omer, 1982; Wilson, 1999, Al-Qarawi and Ali, 2003), and the special need for different buffer compositions, reaction temperatures, incubation times and sample volumes (Callahan and Kruckenberg, 1967; Silvestri, 1977; Mohammad and St. Omer, 1982; Mohammad et al., 1997). Various modifications of the electrometric method are available for measuring blood ChE activity in animals (Wills, 1972; Silvestri, 1977; Mohammad and St. Omer, 1982; Mohammad et al., 1997; Wilson, 1999). These modifications include increasing sample volume, increasing or decreasing incubation time, increasing *Author for correspondence, E-mail: fouadmohammad@yahoo.com