Abstract: The inhibition to monophenolase and odiphenolase activity of phenoloxidase (PO) in Spodoptera exigua (Lepidoptera: Noctuidae) by quercetin was studied. The results showed that the I₅₀ were estimated to be 0.079 mmol/L for monophenolase activity and 0.087 mmol/L for diphenolase, respectively. Quercetin extended the lag time of the enzyme for oxidation of Ltyrosine. In 0.079 mmol/L the natural source compound resulted in the lag time extension from 134 s to 330 s; in 0158 mmol/L the compound resulted in the lag time extension from 134 s to 440 s. The results of inhibition kinetics analyzed by LineweaverBurk plots indicated that quercetin was a competitive inhibitor for the oxidation of LDOPA, and the inhibition constant was determined to be 33.16 mmol/L. The bathochromic shift was observed with quercetin (367－435 nm) by adding excess Cu²⁺[KG-*3], and this shift was not observed by adding excess Ca²⁺or Mg²⁺[KG-*3].

Key words: Spodoptera exigua, phenoloxidase, quercetin, inhibitory mechanism, metal ion