In Vitro Inhibition of the Diphenolase Activity of Tyrosinase by Insecticides and Allelochemicals in Micromelalopha troglodyta (Lepidoptera: Notodontidae)

Tyrosinase is a copper enzyme and plays a key role in normal insect development. We studied the in vitro inhibitory effects of selected insecticides and allelochemicals on the diphenolase activity of tyrosinase in Micromelalopha troglodyta (Graeser) (Lepidoptera: Notodontidae). Two pyrethriods (cyfluthrin and deltamethrin) and 3 other insecticides (hexaflumuron, abamectin and imidacloprid) were the least inhibitory, whereas 5 organophosphates (triazophos, malathion, chlorpyrifos, omethoate and profenofos), 1 carbamate (methomyl), 4 pyrethriods (fenpropathrin, beta-cypermethrin, bifenthrin and lambda-cyhalothrin), 1 organochlorine (endosulfan), 2 allelochemicals (tannic acid and 2-tridecanone) and 4 other insecticides (emamectin benzoate, fipronil, acetamiprid and pyridaben) were moderately inhibitory. Three chemicals (quercetin, phenyl thiourea and phoxim) were the most potent inhibitors of the enzymes among all compounds tested and inhibited the diphenolase activity of tyrosinase in vitro in a dose-dependent manner. Furthermore, phenyl thiourea, phoxim and quercetin showed neither typical competitive nor noncompetitive binding to the substrate, with Ki of 0.13 μM, 49.30 μM and 37.71 μM, respectively.