Phenoloxidase, which is also known as tyrosinase, is an enzyme responsible for a cascade of reactions in which the end product of the spontaneous nonenzymatic reactions that follow is melanin. Insect phenoloxidase has been considered to play several key roles during metamorphosis such as cuticular tanning and sclerotization, wound healing and defense against foreign pathogens. In many species of insects, the haemolymph phenoloxidases have been found as proenzymes (prophenoloxidases) which can be activated by endogenous activators. Most of the biochemical studies recently carried out on the regulation system of insect phenoloxidases have extensively been focused on the activation mechanisms of the proenzyme. However, no detailed biochemical study on the regulation of the fate of the active form of this enzyme had been reported until the recent finding of the presence of an endogenous phenoloxidase inhibitor in the housefly. The identification of endogenous phenoloxidase inhibitor will provide a new tool for understanding not only the regulatory mechanisms of phenoloxidase activity, but also the structure-function of insect phenoloxidases.
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