Abstract: The effects of Kojic acid on phenoloxidase (PO) of Plutella xylostella were studied, when which had been partially purified by 40% saturated [(NH4)2SO4] and Sephadex G-100 gel filtration. Kojic acid showed inhibitory effets on both monophenolase and o-diphenolase activity of the PO. The inhibitor concentrations leading to 50% (I50) activity lost were estimated to be 0.07 mmol?L-1 for monophenolase activity and 1 mmol?L-1 for diphenolase, respectively. Kojic acid can also prolong the lag time of PO for oxidation of L-tyrosine. The inhibitory kinetics were analyzed by Lineweaver-Burk plots and kojic acid was found to be a reversible competitive inhibitor with the Ki of 0.47 mmol?L-1. The ability of kojic acid to inhibit the enzyme activity may be associated with its ability to chelate copper at the active site. In addition, the iron ion was found to shorten the lag time obviously, but have no important effect on the monophenolase activity.

Key words: Kojic acid, Plutella xylostella, Phenoloxidase, Inhibitor, Inhibition kinetics