Inhibitory kinetics of quercetin on phenoloxidase from loopworm

Phenoloxidase (PO; monophenol or dihydroxyphenolalanine: oxygen oxidoreductase; EC. 1.14.18.1 and also known as tyrosinase) is one of the key enzymes in the insect moulting process, so screening based on discovery of inhibitors of the enzyme might ultimately provide clues for enhancing the control of insect pests. In the present investigation, the inactivation of phenoloxidase from loopworm Semiothisa cinerearia Bremer et Grea (Homoptera: Geometridae) by quercetin were studied. The results showed that low concentration to 180 αmol/L quercetin could inhibit PO activity, and the IC50 (inhibition concentration showing 50% of the maximum inhibition) was estimated to be 111.5 αmol/L. In addition, quercetin was proven to be a competitive inhibitor, and the equilibrium constant for the inhibitor binding was determined. The results also showed that inactivation of the enzyme by quercetin was a slow, reversible reaction with fractional remaining activity, and the microscopic reaction rate constants of the inhibitor with the enzyme were determined.