Inhibition kinetics of acid and alkaline phosphatases by atrazine and methomyl pesticides
The main objective of this work was to investigate the kinetic characteristics of acid and alkaline phosphatases isolated from different sources and to study the effects of the herbicide atrazine and insecticide methomyl on the activity and kinetic properties of the enzymes. Acid phosphatase (ACP) was isolated from the tomato plant (Solanum lycopersicum L. var. lycopersicum); alkaline phosphatase (ALP) was isolated from two sources, including mature earthworms (Aporrectodea caliginosa) and larvae of the Egyptian cotton leafworm (Spodoptera littoralis). The specific activities of the enzymes were 33.31, 5.56 and 0.72 mmol substrate hydrolyzed per minute per milligram protein for plant ACP, earthworms ALP and cotton leafworm ALP, respectively. The inhibition kinetics indicated that atrazine and methomyl caused competitive–non-competitive inhibition of the enzymes. The relationships between estimates of Km and Vmax calculated from the Michaelis–Menten equation have been explored. The extent of the inhibition was different, as estimated by the values of the inhibition constant Ki that were found to be 3.34 × 10−3, 1.12 × 10−2 and 1.07 × 10−2 mM for plant ACP, earthworms ALP and cotton leafworm ALP, respectively, with methomyl. In the case of atrazine, Ki were found to be 8.99 × 10−3, 3.55 × 10−2 and 1.36 × 10−2 mM for plant ACP, earthworms ALP and cotton leafworm ALP, respectively.