ABSTRACT
124Dimethoate and chlorpyrifos are pesticides frequently used for pest control in agricultural fields, especially in Malaysia. These pesticides may reach the surrounding freshwater bodies by direct application or through draining of agricultural watersheds, overspray, and runoff from crop fields after rain. The mode of action of dimethoate and chlorpyrifos are through inhibition of esterase activity. There were two objectives for this study, firstly to determine the inhibitor concentration to inhibit 50% enzyme activity (IC 50) and secondly to determine the constant rate of inhibition (k i). The esterase was extracted from the muscles of the tilapia (Oreochromis niloticus) and was stored at - 80 o C. There were four optimum parameters determined: pH of buffer, substrate concentration, temperature, and incubation time. Total protein and enzyme activity were determined by Lowry and van Leeuwen methods, respectively. This study showed the optimum condition to be at pH 7 of phosphate buffer with 12.0 × 10−4 M of α‐naphthyl acetate at 25 o C for 30 minutes. The enzyme activity marked as 3.29 × 10 - 7 μ m o l / m g / min $ 3.29 \times 10^{ - 7} \mu mol/mg/ {\text{min }} $ https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315365961/49eb7077-e83c-41bc-a926-8bb706f03c7b/content/inline-math9_1.tif"/> and IC 50 for both inhibitors was 1 × 10−4 M. Besides that, the k i values for dimethoate and chlorpyrifos were marked as 4.796 and 0.834, respectively. Therefore, from the values obtained, dimethoate and chlorpyrifos can be indicated as highly toxic since it can inhibit esterase activity at very low concentration. However, from the k i values, it was showed that chlorpyrifos was more potent compared with dimethoate as the constant rate value was low enough to indicate the high affinity between the enzyme‐inhibitor complexes. Furthermore, the information in this study can be coupled with variety of management strategies available such as integrated pest management system to assure the effective use of these pesticides.
